Precipitation (Theory, Measurement and A nalysis): A Beginner Level Learning ow, ood in a majority of rivers in India, and to the groundwater as well. The term rainfall used in this chapter is synonymously to precipitation, unless specically stated. The precipitation study falls in scope of hydrometeorology subject. The lifting of moist air masses in the atmosphere leads to the cooling and condensation which results in precipitation of water vapor from the atmosphere in the form of rain, snow, hail, and sleet (Salas et al., 2014). Box 3.1 gives the favorable atmospheric conditions for precipitation to form. However, at a given place and time, the precipitation depends on a number of factors that includes wind, temperature, humidity and pressure di erence etc. Box 3.1: Favorable atmospheric conditions for precipitation (Subramanya 2013) (a) The atmosphere must have moisture (b) There must be su cient nuclei present to aid condensation (c) Weather conditions must be good for condensation of water vapour to take place d) The products of condensation must reach the earth Forms of precipitation: Rain, snow, drizzle, glaze, sleet and hail are some of the common forms of precipitation. Rain: Rainfall is the principal form of precipitation in India and it is the precipitations in the form of water droplets of sizes larger than 0.5 mm, whereas, the maximum size of a raindrop is about 6 mm. Drops larger 6 mm tends to break up into smaller sizes drops during falling from the clouds. Intensity based classication of rainfall is given in Table 3.1. Type of Rain Table 3.1 Types of rainfall and corresponding rainfall intensity Light rain Moderate rain Rainfall Intensity Heavy rain Trace to 2.2 mm/hr 2.5 mm/hr - 7.5 mm/hr > 7.5 mm/hr (Source: Subramanya, 2013) 31
Precipitation (Theory, Measurement and A nalysis): A Beginner Level Learning Snow:Ice crystals usually combine to form snowakes. Initial density of fresh snow is varying from 0.06 to 0.15 g/cm3with an average density of 0.1 g/cm3usually. Usually, Himalayan region in India receives snowfall. Drizzle: Drizzle (so small that they appear/feels like oating in the air) is a ne sprinkle of several small water droplets (size less than 0.5 mm) having intensity less than 1 mm/hr. Glaze: At freezing point around 0°C, when the rain or drizzle comes in contact with cold surface/ground, the water drops freezes and the resulted ice coating is called glaze or freezing rain. Sleet: This type of the precipitation (frozen raindrops of transparent grains) form when rain falls through air which is at subfreezing temperature. Hail: It is one of the devastating forms of precipitation and falls under extreme event. It is precipitation in the form of irregular pellets or lumps of ice (generally greater than 8 mm in size). Weather mechanism for precipitation In precipitation phenomenon, air masses lift by three main mechanisms. When warm air is lifted over cooler air by frontal passage (the zone where the warm and cold air masses meet is called a front), frontal lifting occurs which results in cyclonic or frontal storms. Whereas, in a warm front, warm air advances (usually slow rate of ascent) over a colder air mass causing precipitation which covers larger area (typically 300–500 km ahead of the front). In a cold front, advancing cold air pushes warm air vertically upward direction at a relatively steep slope. This leads to smaller precipitation areas in advance of the cold front. Generally precipitation rates are higher in advance of cold fronts as compare to advance of warm fronts. Due to orographic lifting, warm air rises (as it is forced over hills or mountains), resulting in precipitation called as orographic storms. This orographic precipitation is a major factor in most mountainous areas with characteristics of a high degree of spatial variability. In case of convective lifting, warm air rises (being less dense than the surrounding air), and the resulting precipitation events are called convective storms or, more commonly, thunderstorms (Salas et al. 2014). Precipitation scenario of India The Indian subcontinent have two major seasons and two transitional periods as: i) South-west monsoon (June—September) (Summer Monsoon) ii) Transition-I, post-monsoon (October—November) iii) Winter season (December—February) iv) Transition-II, Summer (Pre-monsoon). (March—May) The details of these seasons are given is following paragraphs. 32
Precipitation (Theory, Measurement and A nalysis): A Beginner Level Learning South-West Monsoon The south-west monsoon (popularly known as Monsoon) is the principal rainy season of India started from June. India receives nearly 75% of the total annual rainfall from this monsoon. It is the principal source of rain in India with July as the month which has maximum rain. However, it is to be noted that the monsoon is not a period of continuous rainfall, but during monsoon season the weather is generally remains cloudy with frequent spells of rainfall occurs throughout the region at di erent time and of di erent duration and intensity. Its originates in the Indian ocean and advance across the country in two branches: (i) the Arabian Sea branch (moves northwards from Kerala to Karnataka, Maharashtra and Gujarat), and (ii) the Bay of Bengal branch (rst covers the north-eastern regions of the country and turns westwards to advance into Bihar and UP). Around June end both the branches reach nearby areas (on spatial scale) of Delhi, the capital of India. The strength an intensity of monsoon winds increases from June to July and begin to weaken in September. The withdrawal of the monsoon, marked by a substantial rainfall activity, starts in September in the northern part of the country. The onset and withdrawal of the monsoon at various parts of the country are shown in Fig. 3.1 (a) and (b). (a) (b) Fig. 3.1 : (a) Normal dates of onset of Monsoon and (b) Normal dates of withdrawl of Mosoon (Subramanya, 2013) The south-west monsoon rainfall (amount) over the country is indicated in Fig. 3.2. As seen from this gure, the heavy rainfall areas are Assam and the north-eastern region with 200- 400 cm, west coast and Western Ghats with 200-300 cm, West Bengal with 120-160 cm, UP, Haryana and the Punjab with 100-120 cm of rainfall. The long-term average monsoon rainfall over the country is estimated as 87.7 cm. 33
Precipitation (Theory, Measurement and A nalysis): A Beginner Level Learning Fig. 3.2 : South-west Monsoonn rainfall (in cm) over the India and neighbourhood (Subramanya, 2013) Post-Monsoon At the retreating time of south-west monsoon, low-pressure areas are forms in the Bay of Bengal and a north-easterly ow of air that picks up moisture in the Bay of Bengal is formed. Striking of this air mass at the east coast causes rainfall in the southern peninsula, mostly Tamil Nadu, during October and November. During the post monsoon (especially in November), severe tropical cyclones form in the Bay of Bengal and the Arabian sea which strikes the coastal areas with intense rainfall damaging the life and property in the region. Winter Season In mid-December, the western disturbances of extra tropical origin travel eastwards across Afghanistan and Pakistan; and cause moderate to heavy rain and snowfall (about 25 cm) in the Himalayan region. During this winter season, some light rainfall also occurs in the northern plains regions of India. On the other hand; during these months, low-pressure areas are formed in the Bay of Bengal which cause 10-12 cm of rainfall in the southern parts of Tamil Nadu. 34
Precipitation (Theory, Measurement and A nalysis): A Beginner Level Learning Summer During summer (pre-monsoon), there is very little or no rainfall in India. However, thunderstorms occur mainly in Kerala, West Bengal and Assam due to convective cells during this duration. Some of the interesting statistics of rainfall over India are as enlisted in Box 3.2. Box 3.2: Interesting statistics of Indian Rainfall (Subramanya, 2013) Ø The average annual rainfall of Indiais 119 cm Ø Average annual rainfall varies from 10 cm in the western desert to 1100 cm in the north-east region Ø A few heavy spells of rain contribute nearly 90% of total rainfall Ø More than 50% rain occurs within 15 days and less than 100 hours in ayear Ø More than 80% of seasonal rainfall is produced in 10-20% rain events, each lasting 1-3 days Precipitation Measurement The total amount of precipitation reaching the ground during a particular period is expressed as the depth to which it would cover a horizontal projection of the Earth's surface. There is a critical need in hydrology for accurate measurement of all forms of precipitation with the primary aim is to obtain representative samples of the fall over the area to which the measurement refers (WMO 2008). Rainfall Rainfall is expressed in terms of the depth, which would stand on an area if all the rain were collected on it. Thus, 1 cm rainfall can be explained as 1 cm of rainfall over a catchment area of 1 km2 represents a volume of water equal to 104 m3. In the case of snowfall, an equivalent depth of water is used as the depth of precipitation. The device or instrument which collects and measure the precipitation is called as a rain gauge which consists of a cylindrical-vessel assembly kept in the open space on a complete horizontal platform to collect rain. In order to accurate representation of rainfall of a particular area, the perfect exposure conditions are enlisted in Box 3.3. However, these conditions are di cult to attain in practice because of the e ect of the wind. 35
Precipitation (Theory, Measurement and A nalysis): A Beginner Level Learning Box 3.3: Exposure conditions for rain gauge installation (Subramanya, 2013) Ø The ground must be level and in the open and the instrument must present a horizontal catch surface. Ø The gauge must be set as near the ground as possible to reduce wind e ects but it must be su ciently high to prevent splashing, ooding, etc. Ø The instrument must be surrounded by an open fenced area of at least 5.5 m x 5.5 m. No object should be nearerto the instrument than 30 m ortwice the height of the obstruction Interested reader may refer Chapter 3 in WMO 2008 document for further reading. Classification of rain gauges Rain gauges are classied into two categories as (i) non-recording rain gauges, and (ii) recording gauges. Non-recording Gauges It is manual type of record keeping of rainfall. In India the extensively used non-recording gauge is Symons' gauge. It consists of a circular collecting area of 12.7 cm (5.0 inch) diameter connected to a funnel. The rim of the collector is set in a horizontal plane at a height of 30.5 cm above the ground level. The funnel discharges the rainfall into a rainfall collector bottle. Both, the funnel and receiving vessel are housed in a metallic container of height 30.5 cm. Fig.3.3 shows the details of the installation. Water contained in the receiving vessel is measured by a suitably graduated measuring glass, with an accuracy up to 0.1 mm. Note that, the receiving bottle normally does not hold more than 10 cm of rain and in the case of heavy rainfall, more frequent measurements is needed for accuracy. The rainfall is measured every day at 8.30 am Indian Standard Time (IST) and is recorded as the rainfall of that day. (a) (b) Fig. 3.3 : Non-recording Rain gauge (Symons' Gauge) (a) Dimensions (Subramanya, 2013) and (b) Pictorial view of Rain guage with measuring ask 36
Precipitation (Theory, Measurement and A nalysis): A Beginner Level Learning In India, IMD is a nodal agency which looks after the standards of rainfall measurement instrumentation and various IS standards are available for installation of rain gauges and measurement of rainfall respectively. Recording Gauges Unlike non-recording raingauge, recording gauges create a continuous plot of rainfall against the time. By reading the plot one can get valuable data of rainfall i.e. amount, intensity and duration of rainfall which is very essential for any hydrological analysis of storms or extreme events. Information of some of the commonly used recording raingauges are given in following paragraphs. (i) Tipping-Bucket Type In tipping bucket type rain gauge ( 30.5 cm in size) rainfall fall into funnel of the gauge and descend to one of a pair of small buckets located into gauge internal assembly (Fig 3.4). The size of bucket is 0.25 mm and designed and balanced in such a way that when rainfall of amount 0.25 mm collects in one bucket, it tips and bucket empties after each tipping. In this way it continuously keeps tipping as and when the rain of 0.25 mm falls onto one bucket and brings the second or other one in position. Simultaneously this tipping of bucket sactivates electrically driven pen which trace/plot a record of collected rainfall amount at each time on the clockwork-driven chart. In the assembly, the water from the tipped bucket is collected in a storage can which further can be measured at regular intervals to provide the total amount of rainfall. This also serves as a cross-check. The tipping bucket gives data on the intensity of rainfall. (a) (b) (c) Fig. 3.4 : (a) Exterior part (b) Interior part and (c) Recorder of tipping bucket type rain gauge (Shrestha and Deb, 2015) 37
Precipitation (Theory, Measurement and A nalysis): A Beginner Level Learning (ii) Weighing-Bucket Type This type of rain gauge essentially consists of a rainfall receiving funnel and receiver bucket supported by a spring over a balance/weighing mechanism (Fig. 3.5). The vertical movement of the receiving bucket due to its increasing weight (weight of collected rainfall) activates the pen which traces the record/contents on a clockwork-driven chart. This plot gives the details of accumulated rainfall against the elapsed time, which is also known as the mass curve of rainfall. Fig. 3.5 : Weighing-Bucket type Rain gauge (Source: https://www.ques10.com/p/29580/rainfall-and-its-treatment-1/) (iii) Natural-Syphon type This type of recording raingauge is also known as oat-type gauge and its working is similar to weighing bucket type rain gauge. In India, this type of rain gauge is adopted as a standard recording type rain gauge. The rainfall collected in a funnel-shaped collector which leads into a oat chamber causing a oat to rise. When the collected rainfall passes through the oat chamber, the water level rises and the oat moves upward. This vertical movement of the oat within the chamber leads to the movement of a pen on the chart which trace/records the elevation of the oat on a rotating drum driven by a clockwork mechanism (Fig.3.6). A siphoning arrangement empties the oat chamber when the oat has reached a preset maximum level and this reects on chart as a sudden vertical drop line, which resets the pen to zero level as shown in Fig. 3.7. Indian Standard (IS: 5235-1969) contains the details of this type of rain gauge. 38
Precipitation (Theory, Measurement and A nalysis): A Beginner Level Learning (a) (b) Fig. 3.6 : Natural-Syphon type/oat-type rain gauge (a) Internal specication and details and (b) Pictorial view Fig. 3.7 : Recording from a Natural Syphon-type gauge (Subramanya, 2013) Fig. 3.7 shows a chart of syphon type rain gauge where a station receives rainfall of 53.8 mm in 30 h. This curve drawn using the rainfall data is known as mass curve of rainfall. Telemetering Rain gauges This is recording type of rain gauge which contains electronic units to transmit the collected/measured rainfall data to a base station/server at regular intervals or at user 39
Precipitation (Theory, Measurement and A nalysis): A Beginner Level Learning dened set-up. The tipping-bucket type rain gauge, is usually adopted for this purpose and this telemetering gauge are found to be very e ective in keeping the rainfall recordings from di cult mountainous terrain which are inaccessible. Radar Measurement of Rainfall The meteorological radar which operate with wavelengths ranging from 3 to 10 cm is a powerful instrument for measuring the areal extent (as much as 1,00,000 km2), location and movement of rain storms, usually with hydrological range of about 200 km (Fig. 3.8). Radar measurement of rainfall is based on the principle of reection of energy. The radar emits a regular succession of pulses of electromagnetic radiation in a narrow beam and radio waves in the microwave frequencies are reected by solid or liquid particles in the atmosphere and are determined by electromagnetic energy of radar pulses. The radar measurement is continuous in time and space and considered of having a good degree of accuracy. Fig. 3.8 : Radar measurement of rainfall (Source : https://news.ucar.edu/3993/radars-next-phase) Snowfall Measurement of snowfall is di erent from that of rainfall, due to characteristics of snow i.e. accumulation and melting. First it accumulates over a surface for some time and get melts and causes runo thereafter. Evaporation loss from the surface of accumulated snow is need to be considered while measuring the snow depth. In order to prepare seasonal and annual precipitation records, water equivalent of snowfall is included in the total precipitation amounts of a station. 40
Precipitation (Theory, Measurement and A nalysis): A Beginner Level Learning Depth of Snowfall A Snow board (which is placed horizontally on a previous accumulation of snow) is 40 cm side square boards used to collect snow samples and a graduated stick or sta is used to measure the snow depth at a particular place (Fig. 3.9). Averaging the several measurements in an area is considered as the depth of snow. After a snowfall event, the snow samples are cut o from the board and depth of snow and water equivalent of snow are derived and recorded. Fig.3.9 : Measurement of snow depth on snow board (Source : https://www.weatherworksinc.com/how-to-measure-snow) Water Equivalent of Snow Water equivalent of snow (the parameter essential for estimating and analyzing the stream ow and oods due to snowmelt runo in a basin) is the depth of water that would result in melting of a unit amount of snow. There are two ways to obtain water equivalent of snow as explained in following paragraphs. (i) Snow Gauges Snow gauges are vessel to catch solid precipitation as it falls in a specied sampling area. A large cylindrical receiver of 20.3 cm in diameter is used to collect the snow as it falls. The height of the cylinder range from 60 cm to several meters depending upon the usual snowfall amount of the area and the receiver is mounted on a tower to keep the rim of the 41
Precipitation (Theory, Measurement and A nalysis): A Beginner Level Learning gauge above the anticipated maximum depth of accumulated snowfall in the area. The collected snow in the cylinder is brought in to a warm room where by weighing or by volume measurements, the determination of water equivalent of snow is done and recorded. (ii) Snow Tubes Snow tubes (Fig. 3.10) which is a set of telescopic metal tubes are used to measure the water equivalent of accumulated snow. Normal tube size is of 40 mm diameter and may goes higher up to 90 mm diameter depending upon anticipated snowfall in a particular area. Snow tube is provided with cutter edge for easy penetration and extracting of core sample; the tube is driven into the snow deposit till it touches bottom of the snow deposit and then twisted and turned to cut a core. The core is then studied for its physical properties and then melted to obtain water equivalent of the snow core. Large numbers of samples are needed to obtain representative values of snow depth for a large coverage areaof snow deposit. Fig. 3.10 : Measurement of snowfall using snow tube (Source: https://www.mprnews.org/story/2019/03/08/how-do-experts-forecast-oods Analysis of Precipitation Rain gauge network density As compared to the areal extent of a storm, the catching area of a raingauge is very small; and therefore, it is obvious that to get a representative rainfall value/over an area/catchment, the number of raingauges should be su cient enough to have accurate 42
Precipitation (Theory, Measurement and A nalysis): A Beginner Level Learning representation/estimation of rainfall as maximum as possible. There are many considerations while selecting exact number of rain gauges over a particular area, among them are economic consideration, topography, accessibility, maintenance etc. The rain gauge network densities as recommended by World Meteorological Organization (WMO) are given in Box 3.4. Box 4: Rain gauge network densities as recommended by W MO (Subramanya2013) (a) In at regions of temperate, Mediterranean and tropical zones · Ideal-1 station for600-900 km2 · A cceptable-1 station for900-3000 km2 (b) In mountainous regions of temperate, Mediterranean and topical zones · Ideal-1 station for100-250 km2 · A cceptable-1 station for25-1000 km2 (c) In arid and polarzones: · 1 station for1500-10,000 km2 depending on the feasibility WMO recommends that among all rain gauges in particular area, 10 per cent of raingauge stations should be equipped with self-recording gauges to know the intensities of rainfall. For Indian conditions, the Indian Standard (IS: 4987-1968) recommends the following densities of rain gauge network as su cient. · In plains: 1 station per 520 km2; · In regions of average elevation of 1000 m: 1 station per 260-390 km2; and · In predominantly hilly areas with heavy rainfall: 1 station per 130 km2. Adequacy of rain gauge stations The optimal number of stations that should exist in the estimation of mean rainfall over an area is obtained by Eq. (1) provided that there are already presences of some rain gauge stations in a catchment. (1) where, N = optimal number of stations, ε = allowable degree of error in the estimate of the mean rainfall and Cv = coe cient of variation of the rainfall values at the existing m stations (in per cent). If there are m stations in the catchment each recording rainfall values P1, 43
Precipitation (Theory, Measurement and A nalysis): A Beginner Level Learning P2,….., Pm in a known time, the coe cient of variation Cv is calculated as: (2) where, (3) The term represents the expected error/standard error (in percentage) in the estimation of the mean Estimation of Missing Data The very essential step to use rainfall for any related studies is to rst check the data for its continuity and consistency. The missing data may be attributed to damage or fault in the raingauge during a period or absence of manual measurement in case non-recording type measurements. In order to estimate the missing data from a raingauge station, the performance of a group of neighboring stations including the one with missing data are considered. A commonly used procedure for estimating missing data of a station is given below. Procedure of missing data estimation The annual precipitation values are P1, P2, P3,.....Pm at neighboring M stations 1, 2, 3, ..., M respectively and it is required to nd the missing annual precipitation Px at a station X not included in the above M stations. Further, the normal annual precipitations N1, N2, …,Ni,.... at each of the above (M+1) stations, including station X, are known. There are two methods for estimation of missing data of precipitation as follows. (i) Arithmetic average method: If the normal annual precipitations at various stations are within about 10% of the normal annual precipitation at station X, then a simple arithmetic average procedure is followed to estimate as (4) 44
Precipitation (Theory, Measurement and A nalysis): A Beginner Level Learning (ii) Normal ratio method: If the normal annual precipitations vary considerably then Px is estimated by weighing the precipitation at various stations by the ratios of normal annual precipitations. This method gives as (5) 3.4 Test for Consistency of Record Inconsistency used to arise in the rainfall data if the conditions relevant to the recording of a raingauge station have undergone a signicant change during the period of observations. This inconsistency or inconsistent datacan be detected from the time the signicant change took place rainfall recordings. Some of the common causes for inconsistency of record are given in Box 3.5. Box 3.5: Causes of inconsistency in rainfall record (Subramanya2013) Ø Shifting of arain gauge station to anew location Ø The neighborhood of the station undergoing a marked change Ø Change in the ecosystem due to calamities such as forest res, landslides Ø Occurrence of observational errorfrom a certain date Double-mass curve technique is used for checking for inconsistency of a record, which is based on the principle that when each recorded data comes from the same parent population, they are consistent. Double mass curve technique is explained as follows. Consider there is problem station X whose annual (or monthly or seasonal mean) rainfall is available. Further, group of 5 to 10 base stations (whose average rainfall covering a long period is available) in the neighborhood of the problem station X is selected. The data of station X and of base stations is arranged in the reverse chronological order (i.e. the recent data record as the rst entry and the oldest record as the last entry in the list). The accumulated precipitation of the station X (i.e. ∑Px) and the accumulated values of the average of the group of base stations (i.e. ∑Pav) are calculated starting from the recent record and plotted on Y and X axis respectively as shown in Fig. 3.11.Abreak in the slope of the resulting plot indicates a change in the precipitation regime of station X. The precipitation values at station X beyond the period of change of regime (point 63 in Fig. 3.11) is corrected as (6) where Pcx = corrected precipitation at any time period t1 at station X Px= original recorded precipitation at time period T1 at station X Mc = corrected slope of the double-mass curve Ma= original slope of the double-mass curve 45
Precipitation (Theory, Measurement and A nalysis): A Beginner Level Learning Fig. 3.11 : Double mass curve (Subramanya 2013) The double mass curve technique is also used to check the consistency of a rainfall series. Presentation of Rainfall Data For interpretation and analysis of rainfall data (in hydrological, meteorological and allied studies), commonly used methods such as mass curve and hyetograph are given as follows: Mass Curve of Rainfall Mass curve of rainfall is dened as a plot of the accumulated precipitation versus time, plotted in chronological order (Fig. 3.12). As mentioned in above sections, records of oat- type and weighing bucket-type rain gauges are of this form. Mass curve gives the details of duration, magnitude and intensities of the storm. For non-recording raingauges, mass curves are prepared from a available records of the approximate beginning and end of a storm event. The mass curves of adjacent recording gauge stations are also taken as a guide in this case for maximum possible accuracy. Fig. 3.12 : Mass curve of rainfall (Subramanya 2013) 46
Precipitation (Theory, Measurement and A nalysis): A Beginner Level Learning Hyetograph Hyetograph is a plot of rainfall intensity (mm/hr or cm/hr) against the time. The hyetograph (Fig 3.13) is derived from the mass curve and is usually represented as a bar chart. Hyetograph is very important in the development of design storms to predict extreme oods. The area under a hyetograph represents the total precipitation received in an area under study during the observation period/duration. Fig. 3.13 : Hyetograph of a storm (Subramanya 2013) Mean Precipitation over an area Raingauges represent only point sampling of the areal distribution of a rainfall event. However, hydrological analysis requires knowledge of total rainfall over an area/catchment in order to understand the complete water budget. Therefore, to convert the point rainfall values into an average value of a catchment, the following three methods are in use: (i) Arithmetical-mean method, (ii) Thiessen-polygon method, and (iii) Isohyetal method. Arithmetical-Mean Method In case, where the recorded precipitation at various stations in a catchment shows little variation, the average precipitation over the catchment area is taken as the simple arithmetic mean of the total available station values. Thus, if P1, P2, Pi,.....Pn are the precipitation values in a given period in N stations within catchment, then the value of the mean precipitation over the catchment by the arithmetic-mean method is (7) 47
Precipitation (Theory, Measurement and A nalysis): A Beginner Level Learning Thiessen-Mean Method The weightage is given to each recorded rainfall of each station on the basis of an area closest to the station. The procedure of determining the weighing area is as follows: Fig. 3.14 : Thiessen polygon method (Subramanya 2013) Consider the catchment area as in Fig. 3.14 containing six rainfall recording stations. Out of six, three stations are outside of the catchment but fall under its neighbourhood. Draw the catchment area to a known scale and mark the positions of the six stations. Join the stations 1 to 6 such that they form a network of triangles. Then bisects each of the sides perpendicularly in such way that they form a polygon around each station. The outer limit of any polygon should be restricted to outer boundary of the catchment. For example, station 1, the bounding polygon area is abcd, for station 2, kade is taken as the bounding polygon and for station 3 edcgf and so on. These bounding polygons are called Thiessen polygons. Using planimeter or any suitable techniques calculate the areas of these six Thiessen polygons. Now suppose, P1 P2, ...,P6 are the rainfall amount recorded by the stations 1, 2,..., 6 respectively, and A1, A2, ..., A6 are the respective areas of the Thiessen polygons then the average rainfall over the catchment is given by (8) Thus, in general, for M stations, (9) The ratio is called the weightage factor for each station. 48
Precipitation (Theory, Measurement and A nalysis): A Beginner Level Learning In Thiessen-polygon method, some weightage is given to the various stations on a rational basis for calculating the average precipitation over an area and therefore it is superior to the arithmetic-average method. Isohyetal Method Isohyets are lines joining the points of equal rainfall magnitude. In the isohyetal method of calculating the average precipitation over an area, the catchment area is drawn to scale and the raingauge stations are marked over it. Each recorded rainfall values are then marked on respective stations. As like Thiessen-polygon method, the neighborhood stations outside the catchment are also considered while calculating the average precipitation over an area. The isohyets of various rainfall values are then drawn by considering point rain-falls as guides and interpolating between them by the eye (Fig. 3.15). Fig. 3.15 : Isohyets of a storm (Source: Subramanya 2013) Using planimeter, the area between two adjacent isohyets is measured. Same as Thiessen- polygon method, if the isohyets go out of catchment, the catchment boundary is used as the bounding line while calculating the area between two isohyets. Principally, the average rainfall indicated by two isohyets is assumed to be acting over that particular inter-isohyet area. Therefore, if P1, P2,…..Pn, are the values of isohyets and a1, a2, …. , an-1 are the inter- isohyet areas respectively, then the mean precipitation over the catchment of area A is given by (10) 49
Precipitation (Theory, Measurement and A nalysis): A Beginner Level Learning The Isohyet method is superior to the other two methods especially when the stations are large in number. Conclusion These are some preliminary precipitation analysis that are required at rst place for further complex studies on precipitation and its application in di erent hydro-meteorological and allied studies. Further complicated interpretation and analysis of precipitation which is out of the scope of present chapter and the GSDP training (keeping in view the qualication and background of the participants of the course); and therefore,is not included here. Interested readers may read enlisted references for further reading and knowledge. Declaration The above content is based on fourth edition of book on Engineering Hydrology by K. Subramanya. Unless otherwise stated, the parts in this chapter is taken from the mentioned book in addition to other sources and is not used for any commercial purpose except for educational one. Further, this book chapter was prepared for training (GSDP) purpose and therefore, is, out of the scope of Plagiarism Concept and any Conict of the Interest. References https://news.ucar.edu/3993/radars-next-phase https://www.mprnews.org/story/2019/03/08/how-do-experts-forecast-oods https://www.ques10.com/p/29580/rainfall-and-its-treatment-1/ https://www.weatherworksinc.com/how-to-measure-snow Salas J D, Govindaraju R S, Anderson M, Arabi M, France´s F, Suarez W, Lavado-Casimiro W S and Green T R (2014). Introduction to Hydrology,in Modern Water Resources Engineering (Handbook of Environmental Engineering,Volume 15), Wang and Yang (Eds.), DOI: 10.1007/978-1-62703-595-8_1 Shrestha S and Deb P (2015). Manual of Hydrology: Measurement and Analysis. Asian Institute of Technology. Subramanya K (2013). Engineering Hydrology. McGraw Hill Education (India) Private Limited,4thEdition. World Meteorological Organization (WMO) (2008). Guide to Hydrological Practices. Volume I: Hydrology – From Measurement to Hydrological Information,6th Edition. 50
4 STREAM FLOW MEASUREMENTS Jyothi Prasad Abstract Stream ow representing the runo phase of the hydrologic cycle is the most important basic data for hydrologic studies. The measurement of discharge in a stream forms an important branch of Hydrometry, the science and practice of water measurement. Documenting and monitoring stream ow is an integral part of developing water budgets, conducting loading calculations, evaluating the relationship between groundwater and surface water are critical in evaluating impacts from runo . Measurement of surface water ow is an important component of most water quality monitoring projects. Flooding, stream geomorphology and aquatic life support are all directly inuenced by stream ow and runo and streamow drive the generation, transport and delivery of many nonpoint source (NPS) pollutants. Calculation of pollutant loads requires knowledge of water ow. There are a number of methods to document stream ow, but the most typical methods for eld evaluations are discussed in this chapter. Keywords: Flow Measurements, Stream Gauging, Stream ow Background The assessment, management and control of water resources can only be e ective if there is access to accurate and continuous information on streamow, and this information can only be obtained satisfactorily from a network of stream gauging stations. Most commonly, stream ow is estimated by measuring water velocities at several locations across a channel and then integrating these velocities over the cross-sectional area of the stream. Several direct methods like area velocity method, dilution method,ultrasonic and electromagnetic methods and indirect methods such as slope area method, ow measuring hydraulic Jyothi Prasad, Ph.D. Department of Civil Engineering G B Pant University of agriculture and Technology, Pantnagar [email protected] Monitoring and A ssessment of Environmental Parameters Eds. V. A gnihotri, S. Rai, A . Tiwari, S. Mukherjee, K. Kumar, R. Joshi, GBPNIHE, A lmora, Uttarakhand, India ©GBPNIHE 2020 51
Stream Flow Measurements structures such as weirs, umes and gated structures etc, etc. are in vogue. The selection of a particular method depends on site conditions and funds availability. This chapter briey covers the measurement of stages and discharges. Measurement of Water Stage Discharge can often not be measured directly and continuously. Therefore, one measures the water stage (Figure 4.1). Figure 4.1 : Cross section of a river with gauge datum and water stage The simplest way to measure water stage is by using a sta gauge (Figure 4.2). A scale is installed so that a portion of it is always immersed in the water. This gauge may be a vertical scale attached to a bridge, pier or other structure that extends into the low-water channel of Figure 4.2 : Sta gauge 52
Stream Flow Measurements the stream. Such sta (manual) gauges are simple and inexpensive but must be read frequently. For continuous recording of stage, recording gauges such as oat type water stage gauges, bubbler gauges and ultrasonic sensors are used. Recording gauges Float-type water-stage gauges Float-type water-stage gauges (Figure 4.3) record the motion of a oat. The motion of the oat moves a pen across a long strip chart. Float-type water-stage recorders are generally installed in a shelter house and stilling well. The stilling well serves to protect the oat and counterweightcables from oating debris and suppresses uctuations from surface waves in the stream. Such gauges are appropriate for streams with narrow, incised gravel-bed Figure 4.3 : Float type recorder channels, so that the stilling well can be located close to the stream. For wide sandy channels, the stilling well must be located on the stream bank some distance away. Pipes connecting the stilling well to the stream are vulnerable to blockage by siltation. Bubbler gauges These type of gauges (Figure4.4) record the pressure required to maintain a small ow of gas from an orice submerged in the stream. The advantage of such gauge is that it does not need a costly stilling like that for oat-operated gauges. 53
Stream Flow Measurements Figure 4.4 : Bubbler gauge Ultrasonic sensors Ultrasonic sensors (Figure 4.5) measure the water levels either by contact or non-contact methods. The contact ultrasonic sensor is installed below the water surface; the non- contact-ultrasonic sensor is mounted above the surface. These sensors transmit acoustic pulses, which are reected to the sensor by the water surface. Figure 4.5 : A contact type ultrasonic sensor 54
Stream Flow Measurements Discharge Measurement Methods based on velocity measurements Discharge is the product of cross-sectional area of owing water and its velocity as shown in equation (1) (Reddy, 2011) Q=v*A (1) where Q = discharge [m3/s] v = velocity [m/s] A= cross-section of ow [m2] Velocity prole The water molecules in a stream travel at di erent speeds. These are subject to friction as they come into contact with the sides and bottom of the channel. Due to these frictional e ects, water ows fastest at the surface and centre of the channel (away from the immediate frictional inuences) as shown in Figure 4.6 and 4.7. Figure 4.6 : Typical velocity prole 55
Stream Flow Measurements Figure 4.7 : Velocity distribution in channel cross section (a) Measurement using Floats Rough measurements of discharge can be made by using oats. These should be substantially submerged and therefore una ected by wind. The travel time over a certain distance is measured. The velocity is computed dividing the travel distance by the corresponding time interval. Several oats should be used at intervals across the stream. A surface oat travels with a velocity which is about 1.2 times the mean velocity. The discharge is received by multiplying this mean velocity with the cross section area (Figure 4.8). Figure 4.8 : Measurement of ow velocity using oats 56
Stream Flow Measurements (b) Measurement using Current Meters More accurate measurements of the velocity prole of the stream cross section are made by a mechanical device, called current meter, consisting essentially of a rotating element which rotates due to the reaction of the stream current with an angular velocity proportional to the stream velocity. By the propeller-type current meter (Figure 4.9) a propeller is turning about a horizontal axis or cup type current meter series of conical cups mounted around a vertical axis (Figure 4.10). The revolutions per time interval are recorded. The relation between revolutions per second (N) of the current meter and the water velocity (v) is given by equation (2) v = a + b*N (2) Where, b = constant of proportionality a = starting velocity or velocity required to overcome mechanical friction. Some di erences in these constants exist as a result of manufacturing variations and accidental damage. Therefore, each current meter should be recalibrated periodically. Figure 4.9 : Propeller-type current meter Figure 4.10 : Cup-type current meter 57
Stream Flow Measurements A discharge measurement requires determination of enough point velocities in a river cross section to permit computation of an average velocity in the stream. The stream is divided into a number of vertical sections (Figure 4.11). The sum of cross-sectional area of each section multiplied by the average velocity of each section gives the total discharge as shown in equation (3) (3) Where, Q =total total discharge m = number of sections Ai = cross area of section i Vi= mean velocity of section i Qi = discharge in section i The number of velocity determinations is limited to those which can be made within a reasonable time. Especially if stage is changing rapidly, one should complete the measurement with a minimum change in water stage. Figure 4.11 : Procedure for a current meter measurement Usually there are national guidelines how the vertical sections should be chosen and in which depth the velocity measurements should be made to determine the mean velocity within the vertical prole. Subramanya K (2013) gives following guidelines for selection of vertical segments: 58
Stream Flow Measurements 1. The stream should have a well-dened cross-section which does not change in various seasons. 2. It should be easily accessible throughout the year. 3. The site should be in a straight, stable reach. 4. The gauging site should be free from backwater e ects in the channel. These guidelines are sought to ensure homogenous data sets. Two methods often used to determine the mean velocity over the vertical velocity prole are: Two-point measurement: The variation for most channels is such that the average of the velocities at two-tenths and eight–tenth depth below the surface equals the mean velocity in the vertical. One-point measurement: The velocity at 0.6 depths below the surface closely approximates the mean in the vertical. The adequacy of these assumptions for a particular stream can be tested by making a detailed vertical velocity traverse. Access to individual verticals of a section may be obtained by wading, if the water is shallow. At high water stages the meter must be lowered from an overhead support (bridges or cableway). Where no such overhead support is present, measurements may be made from a boat. This is less satisfactory due to the problems of maintaining position during a measurement and due to the changing of the velocity prole by the motion of the boat. Chemical dilution gauging (Subramanya, 2013; Reddy, 2011) Current-meter gauging is di cult and sometimes impossible like in boulder-strewn mountain torrents, very small streams, etc. In such situations chemical dilution gauging is more suitable. But it is restricted to those streams where mixing occurs readily. Tracer material (like salt, uorescent dye, radioactive material, or any easily measurable material does not present in the stream and not likely to be lost by chemical combination with materials in the stream) may be used. Complete mixing of the tracer in the ow and accurate determination of initial and nal concentrations are essential. The chemical method is based on continuity principle. Two methods viz, sudden injection method and constant injection methods are in vogue. Sudden injection method: Consider a tracer which does not react with the uid or boundary. Let Co be the small initial concentration of the tracer in the streamow. At section '1' a small quantity (volume V1) of high concentration C1of this tracer is added (Fig. 11). Let section '2' be su ciently far away on the downstream of section 1 so that the tracer mixes thoroughly with the uid due to the turbulent mixing process while passing through the reach. The concentration prole taken at section 2 is schematically shown in Figure4.12. 59
Figure 4.12 : Concentration Prole of Sudden Injection method The concentration will have a base value of C0, increases from time t1 to a peak value and gradually reaches the base value of C0 at time t2. The stream ow is assumed to be steady. By continuity of the tracer material M1 = mass of tracer added at section 1 = V1C1 (4) Neglecting the second term on the right-hand side as insignicantly small, (5) Thus the discharge Q in the stream can be estimated if for a known M1 the variation of C2 with time at section 2 and C0 are determined. This method is known as sudden injection or gulp orintegration method. Constant injection method: Another way of using the dilution principle is to inject the tracer of concentration C1 at a constant rate Qt at section 1. At section 2, the concentration gradually rises from the background value of C0 at time t1 to a constant value C2 (Figure 4.13). At the steady state, the continuity equation for the tracer is (6) (7) 60
Stream Flow Measurements Tracers The tracer should have ideally the following properties: 1. It should not be absorbed by the sediment, channel boundary and vegetation. It should not chemically react with any of the above surfaces and also should not be lost by evaporation. 2. It should be non-toxic. 3. It should be capable of being detected in a distinctive manner in small concentrations. 4. It should not be very expensive. The tracers used are of three main types. Figure 4.13 : Concentration Prole of Constant Rate Injection method 1. Chemicals (common salt and sodium dichromate are typical); 2. Fluorescent dyes (Rhodamine-WT and Sulpho-Rhodamine B Extra are typical); and 3. Radioactive materials (such as Bromine-82, Sodium-24 and Iodine-132). Common salt can be detected with an error of ±1% up to a concentration of 10 ppm. Sodium dichromate can be detected up to 0.2 ppm concentrations. Fluorescent dyes have the advantage that they can be detected at levels of tens of nanograms per litre (~ 1 in 1011) and hence require very small amounts of solution for injections. Radioactive tracers are detectable up to accuracies of tens of picocuries per litre (~ 1 in 1014) and therefore permit large-scale dilutions. However, they involve the use of very sophisticated instrument sand handling by trained personnel only. The availability of detection instrumentation, 61
Stream Flow Measurements environmental e ects of the tracer and overall cost of the operation are chief factors that decide the tracer to be used. Length of Reach The length of the reach between the dosing section and sampling section should be adequate to have complete mixing of the tracer with the ow. This length depends upon the geometric dimensions of the channel cross-section, discharge and turbulence levels. The value of length varies from about 1 km for a mountain stream carrying a discharge of about 1.0 m3/s to about 100 km for river in a plain with a discharge of about 300 m3/s. The mixing length becomes large for large rivers and is one of the major constraints of the dilution method. Measurement of Discharge by Hydraulic Structures Flumes and weirs Flow measurements on small streams often are made by weirs or umes (most frequently used gauging structures), notches, sluice gates etc (Ackers, 1978, U.S. Bureau of Reclamation, 2001). Flumes: Man –made channel with clearly specied shape and dimensions, which can be used for discharge measurement. Weir: Overow structure which is used for controlling upstream water level or for measuring discharge or for both (Bos, 1989). Weirs produce the critical relationship between stage and discharge by obstructing the ow; the gravity head over the crest of the weir can then be related to velocity and, because weirs have a rigid cross-section (either rectangular or triangular), the computation of discharge is simple (Figure 4.14 and 4.15). Figure 4.14 : Flow over a Sharp-Crested Rectangular and V-Notch weirs 62
Stream Flow Measurements Flow conditions in umes are more complex: a section of critical, high-velocity ow is promoted by a lateral smooth-walled constriction of the ow.The intention in both cases is to prevent inuences of the water level downstream of the installation on ow and water stage at the point of measurement. These measurement devices are commonly rated on the basis of laboratory calibration; however the rating should be checked in place with current meters. Large weirs are usually problematic because sediment deposition in the upstream part changes the discharge characteristics. Figure 4.15 : Typical weir and ume The Manning equation provides a quick mean of estimating the average velocity of streamow in situations where other forms of measurement are not possible (in situationswhere neither time nor need exists to dene stream velocity (and discharge) in greater detail, or in the case of rising ood conditions in streams for which no developed rating curves exist). The discharge can be determined by multiplying the average velocity with the cross-section area of the river channel. The Manning equation is expressed as (Subramanya, 2009) (8) v = average stream velocity [m/s] R = hydraulic radius [m] S = gradient of the stream n = Manning roughness coe cient 63
Stream Flow Measurements Determination of discharge by the velocity-area method, the dilution method, and by means of a hydraulic structure have certain limitations and are not applicable in some instances. Two relatively new methods of ow measurement in open channels are the ultrasonic method, and electromagnetic method. Subramanya (1994) gives the details of these methods. Conclusion For gauging goes, there are no xed, unchangeable rules. The main objective is to get as close as possible to the existing realities of the eld. The choice of measuring methods and the means of implementing them is a function of the conguration of site, available resources, both human and material, and the degree of accuracy expected. In di cult conditions, such as equipment breakdowns or dangerous situations, estimations of speed, photographs taken, observing oodwater marks on permanent structures, etc., are actions that can mitigate the absence of a true measurement and establish coherent limits on further evaluation of a site. The selection and adaptation of gauging sites, competence of personnel, equipment to be used for a job should all serve to advance a project toward optimum measurement precision. The most important elements for good measurements are motivated, well trained and properly equipped personnel. Not only the must the measuring equipment itself be in good condition, but also the annex equipment must be in good working order. Harmonization of practices comes with the profound motivation of men, with skilful development of collected data and a grasp of the basic notions of hydraulics. References Ackers P (1978).Weirs and Flumes for ow measurement, Wiley-Blackwell, UK. Chow V T (1964). Handbook ofApplied Hydrology, McGraw-Hill, New York, 1964 GoelNK (2003). Hydrological data collection, processing and analysis. lecture notes, IIT Roorkee, World Meteorological Organization (WMO) (2008). Guide to Hydrological Practices. Volume I: Hydrology – From Measurement to Hydrological Information, 6th Edition. Hydrology Project, training manual draft, 1999. Reddy P Jaya Rami (2011). A textbook of Hydrology. Laxmi Publications Bierkens Marc FP, Geer Frans van (2012). Stochastic Hydrology(GEO4- 44420).Department of Physical Geography, Utrecht University. Subramanya K (2009). Flow in Open Channels. Tata McGraw Hill Publishing Company, New Delhi;, 2ndEdition. Subramanya K (2013). Engineering Hydrology. Tate McGraw Hill Publishing Company Limited, New Delhi. 64
Stream Flow Measurements U.S. Bureau of Reclamation (2001). Water Measurement Manual. U.S. Government Printing O ce, Washington, DC. Bos M G (1989). Discharge measurement structures. (Publication / International Institute for Land Reclamation and Improvement; No. 20). ILRI. https://edepot.wur.nl/64285. Walkowiak DK (2006). ISCO Open Channel Flow Measurement Handbook.TeledyneIsco, Lincoln, NE. http://www.isco.com/ WMO (1994). Guide to hydrological practices-Data acquisition and processing, analysis forecasting and other applications, WHO publication no. 168, WMO, Geneva. 65
5 AN INTRODUCTION TO GROUNDWATER Soukhin Tarafdar Abstract Groundwater makes up to 99 percent of freshwater in the world. Nearly a billion people lack access to clean drinking water. Nearly sixty percent of groundwater withdrawn worldwide is used for agriculture and rest is divided in equal proportion between the domestic and industrial sectors. India is the largest groundwater user in the world, using 66.3 trillion gallons in a year.Accessibility of groundwater in areas with limited availability of surface water resources and less likelihood of being contaminated by pollutants makes groundwater a good choice for water supply as against surface water. Groundwater stored in aquifer systems sustains the river ows during the lean period as well as in periods of droughts or water scarcity. Conventionally, surface and groundwater are perceived and managed as two independent resources but there is growing recognition that the surface water bodies/wetlands and underlying aquifer act as a single connected resource, which regulates the water quantity as well as water quality. Groundwater, an essential part of water budget, can be managed and conserved through conjunctive water management and articial recharge in regions of expanding water footprints and water scarcity.Groundwater lying under the earth surface and springs and seeps act as reliable source of fresh water in mountainous basins of Indian Himalaya. The chapter is intended to give an exposure of very basics of groundwater and advance subjects can be understood by referring some of the references indicated at the end of the chapter. Keywords: Conjunctive water management; Water footprints; Water budget; Water scarcity; Articial recharge to groundwater Introduction The science of Groundwater Hydrology (Hydrogeology or Geohydrology) deals with the Soukhin Tarafdar, Ph.D. GB Pant National Institute of Himalayan Environment, Garhwal Regional Centre, Uttarakhand [email protected] Monitoring and A ssessment of Environmental Parameters Eds. V. A gnihotri, S. Rai, A . Tiwari, S. Mukherjee, K. Kumar, R. Joshi, GBPNIHE, A lmora, Uttarakhand, India ©GBPNIHE 2020 66
A n Introduction to Groundwater occurrence, movement, and quality of water beneath the surface of the Earth. It is an interdisciplinary science involving the application of physical, biological, and mathematical sciences. Groundwater hydrology is of critical importance to the welfare of humankind as well as several groundwater dependent ecosystems (Figure 5.1). Groundwater Hydrology deals with the occurrence and movement of water in an almost innitely complex subsurface environment. It is one of the most complex sciences. The purpose of this chapter is to present the basic facets of groundwater hydrology to encourage a general understanding and use. Groundwater and Hydrological Cycle The saline water present in the ocean's accounts for 96.5% (Table 5.1) whereas the water in the landmasses and atmosphere holds only 2.75% of the total. Out of this 2.75%, the water present in rivers, lakes, atmosphere, biosphere, underground and soil moisture constitutes only 25.07%, rest (74.93%) occurs as ice caps and glaciers. Groundwater represents the largest reservoir of fresh water. Therefore, its assessment, planned exploitation, conservation, and protection from pollution are important concerns. Table 5.1 : Distribution of water in Earth Reservoirs (Maidment 1993) Reservoir Percent of All Water Percent of Fresh Water Oceans 96.5 Ice and snow 1.8 69.6 Groundwater: 0.76 30.1 Fresh 0.93 Saline 0.26 Surface water: 0.007 Fresh lakes 0.006 0.03 Saline lakes 0.0008 0.006 Marshes 0.0002 0.05 Rivers 0.0012 0.04 Soil Moisture 0.001 0.003 Atmosphere 0.0001 Biosphere 67
A n Introduction to Groundwater Figure 5.1 : Global hydrological Cycle: Total global uxes in thousands of km3/year (Maidment, 1993) The residence time is the average amount of time that a water molecule resides in a reservoir before getting transferred to another reservoir. The residence time for river water could be of the order of two weeks whereas the groundwater residence time could range between two weeks to 10,000 years. The residence time Tr is calculated as the volume of a reservoir V [length3 or L3] divided by the total ux (Q) in or out of the reservoir. The Q is expressed as cubic length per time or L3/T, Tr = V/Q The atmosphere is a relatively small reservoir with a large ux moving through it, so the average residence time is short, of the order of days. The ocean is an enormous reservoir with an average residence time of the order of thousands of years. The average residence time for groundwater, including very deep and saline waters, is approximately 20,000 years. Actual residence times vary considerably. Shallow fresh groundwater would have much shorter residence times than the average, like years to hundreds of years. Vertical Distribution of Groundwater Groundwater occurrences can be categorized into two di erent zones, viz., the zone of aeration and the zone of saturation. The pore spaces in zone of aeration contain both water and air and is also referred to as the unsaturated zone. This upper most zone is underlain by a zone in which all interconnected pore spaces are lled with water. This zone is referred to as the saturated zone. Water in the saturated zone is the only underground water or groundwater that is available to supply wells and springs. The rainfall percolates through the unsaturated zone as recharge to the saturated zone. The unsaturated zone is, therefore, of great importance to groundwater hydrology. The unsaturated zone is further divided into three parts: the soil zone, the intermediate zone, and the capillary zone (Figure 5.2). The soil 68
A n Introduction to Groundwater zone extends from the land surface to a maximum depth of a meter or two and is the zone that supports plant growth. The water table or phreatic surface represents the upper surface of zone of saturation and uid pressure in the pores is at atmospheric pressure. Figure 5.2 : Cross section showing di erent water uxes and vertical distribution of unsaturated and saturated zone with sub-zones are depicted. (Fitts, 2002) Types of Aquifer The geological formations have wide range of hydraulic conductivity ranging from about 1 meter per second (m/s) to 10-13 m/s. Hydraulic conductivity is a measure of ease of groundwater ow through subsurface geological formation. Accordingly, the formations can be classied into three types, depending on their relative hydraulic conductivity which is also known as permeability. 1. Aquifer: An aquifer is a water bearing geological formation saturated with water, which has su cient hydraulic conductivity to supply reasonable quantity of water to a well or spring under ordinary hydraulic gradient. The unconsolidated sedimentary formations, like gravel and sand, form excellent aquifers. Fractured igneous and metamorphic rocks and carbonate rocks with solution cavities also form good aquifers. 2. Aquitard: An aquitard is a formation having insu cient permeability to make it a source of water supply but allows exchange of groundwater between adjacent aquifers due to vertical leakage. Therefore, aquitard serves as a layer of constrained groundwater movement and known semi-conning layer. Examples are silt, clay, shale and kankar (calcrete) etc. 69
3. Aquiclude: An aquiclude is a saturated geological formation of such a low permeability that it is incapable of transmitting signicant quantities of water under ordinary hydraulic gradients.Aquiclude includes clays, shales and metamorphic rocks. Based on hydraulic characteristics and the disposition of conning layers, aquifers are classied into unconned, conned, semi-conned or leaky aquifer and perched aquifers as shown in the gure (Figure. 5.3.). A conned aquifer is overlain and underlain by conning layer. Since the water under conned conditions is under pressure, the water will rise in a well above the upper surface of the aquifer, which is represented by potentiometric or piezometric surface. The unconned aquifers or phreatic aquifers are not overlain by conning layer and the upper surface of zone of saturation is represented by water table. Depending on the permeability of heterogeneous geological formation, leakage of water from the conning bed could render an unconned or conned aquifer into a leaky aquifer. A localised pocket of water-saturated zone with impervious layer at its base may occur within the unsaturated zone and is termed as perched aquifer. Base flow Precipitation that falls on the land surface enters the subsurface through several di erent pathways. The water that does not percolate into the subsurface and drains on the surface into a stream channel is termed as overland ow. The excess soil moisture percolates to saturated zone due to the action of gravity and forms the groundwater. Groundwater ows through the saturated zone of soil or rock until it is discharge as springs or as seepage into the streams, lake or ocean. Stream water could have its source either from the surface overland ow (quick ow) or from the groundwater. The groundwater contribution to a stream is termed as baseow (Figure 5.4). Figure 5.3 : Cross section showing unconned conned and perched aquifer systems in soft rock geological medium. (Singhal, 1999) 70
A n Introduction to Groundwater Depending on the geology, soil texture and thickness, rainfall and climatic conditions, streams could be dominated by di erent proportions of baseow. The delayed ow component is thought to represent the fraction coming from the groundwater zone. The base ow proportion of the total stream ow is termed, as base ow index (BFI). It is the Figure 5.4 : Typical stream hydrograph showing proportions of Quick ow and Baseow during a rainfall event. (Fitts, 2002) ratio of base ow volume to total ow volume. Being a relative measure, it hasno units and is indicative of the catchment ability to store and release water during the lean period. Higher BFI means the catchment ow regime is more stable as compared to a lower BFI where the stream ow regime could be ashy in nature. The BFI ranges between 0 and 1 and BFI < 0.2 results from low groundwater contribution whereas the base ow index of > 0.9 indicates high groundwater contribution (Figure 5.5). Figure 5.5 : River hydrograph of two rivers showing clear di erence of hydrograph dominated by quick ow(black line) whereas the hydrograph (blue line) showing a more stable ow regime. (Fitts, 2002) 71
A n Introduction to Groundwater The gaining or e uent streams receives groundwater discharge as the water table slopes towards the stream whereas in the inuent or losing streams, the water table is below the stream bottom depth. Inuent streams are characteristics of arid region whereas e uent streams are typical characteristics of humid region. Porosity, Specific Yield and Specific Retention The ratio of openings (voids) to the total volume of a soil or rock is referred to as its porosity. Porosity is expressed in percentage. Thus, n = (Vt-Vs) / Vt = Vv /Vt where n is porosity as a decimal fraction, Vt is the total volume of a soil or rock sample, VS is the volume of solids in the sample, and Vv is the volume of openings (voids). Porosity of unconsolidated deposits depends on grain size, sorting and on the shape of the rock particles. Fine-grained materials tend to be better sorted and, thus, tend to have the largest porosities (Figure 5.6). Figure 5.6 : Pictorial depiction of porosity. (Heath, 2004) The term primary porosity indicates the interstices or pores formed during the process of deposition. Whereas the secondary porosity refers to the interstices formed afterwards. Thus, porosities of geological formations depend on shape and arrangement of individual particle, distribution by size, degree of cementation and degree of fracturing. Porosity values ranges from zero to more than 50 percent (Figure 5.7 and Table 5.2). Specific Yield and Specific retention The specic yield (Sy) is dened as the volume of water that an unconned aquifer releases under gravity, per unit surface area of the aquifer per unit decline in water table. For 72
A n Introduction to Groundwater conned aquifer it is known as storage coe cient (S) and dened as the volume of water removed from a unit volume of a conned aquifer for unit drop in hydraulic head. It is dimensionless. Figure 5.7 : Di erent types of porosity in relation to rock texture (Meinzer, 1923). Table 5.2 : Typical values of porosity of geological material. Material Percent of Fresh Water Narrowly graded silt, sand, gravel 30-50 Widely graded silt, sand, gravel 20-35 Clay, clay-silt 35-60 Sandstone 5-30 Limestone, dolomite 0-40 Shale 0-10 Crystalline rock 0-10 The range of value depends on whether the aquifer is conned or unconned. If the aquifer is conned, the water released from storage when the head declines comes from expansion of the water and from compression of the aquifer. Relative to a conned aquifer, in unconned aquifer the expansion of a given volume of water in response to a decline in pressure is small (Figure 5.8). 73
A n Introduction to Groundwater In a conned aquifer having a porosity of 0.2 and containing water at a temperature of about 15°C, expansion of the water alone releases about 9 x 10-7 m3 of water per cubic meter of aquifer per meter of decline in head. The storage coe cient of most conned aquifers ranges from about 10-5 to 10-3 (0.00001 to 0 .001). If the aquifer is unconned, the predominant source of water is from gravity drainage of the sediments through which the decline in water table occurs. In an unconned aquifer, the volume of water derived from Figure 5.8 : The specic yield of unconned aquifer is signicantly higher compared to the storage coe cient of a conned aquifer. (Heath, 2004) in such an aquifer, the storage coe cient is virtually equal to the specic yield and ranges from about 0.1 to about 0.3. Because of the di erence in the sources of storage, the specic yield of unconned aquifers is 100 to 10,000 times the storage coe cient of conned aquifers. Specic retention (Sr) is dened as the ratio of volume of water an aquifer will hold against the pull of gravity to the volume of the aquifer. It is expressed in percent. Darcy law Henry Darcy in the year 1856 investigated the movement of water through porous medium (sand). He found that the discharge Q [L3/T] is proportional to the di erence in water level or hydraulic head (h1-h2) and cross sectional area of ow (A) and inversely proportional ow length L (Figure 5.9). Q α (h1-h2); Q α A and Q α 1/L Q = K A (h1-h2)/L or Q = -KA (dh/dl) 74
A n Introduction to Groundwater Where dh/dl is known as the hydraulic gradient and coe cient K is hydraulic conductivity has the dimension of length/time (L/T). The negative sign indicates that the ow is in the direction of decreasing hydraulic head. Figure 5.9 : Illustration of Darcy's experimental setup of ow through cylinder lled with sand (Fitts, 2002) Hydraulic conductivity or, as it is occasionally referred to in older publications, the coe cient of permeability, has dimensions of [L T−1] and is a measure of the ease of movement of water through a porous material. The hydraulic conductivity of geological materials is not only a function of the physical properties of the porous material, but also the properties of the migrating uid, including specic weight, γ (= ρg, where ρ is the density of the uid and g is the gravitational acceleration), and viscosity, μ . Springs Sparsely populated villages in rural landscape of Indian Himalaya are largely dependent of springs for their day-to-day domestic and drinking water needs. The ow in springs and seeps are primarily supported by groundwater water stored in the hillslopes as perched aquifers or shallow aquifers. Springs may be divided into gravity springs and artesian springs. Gravity springs are outow of water coming from permeable soft-rock or hard rock formation or ows from large openings in a karst rock formation, under the action of gravity. 75
A n Introduction to Groundwater Artesian springs are outow of water issuing under artesian pressure, generally through some ssure or other opening in the conning bed that overlies the aquifer. Such springs are found in the region of artesian ow, where the piezometric surface is above the land surface. Gravity springs may be divided into depression springs, contact springs, and fracture gravity springs. Mechanism of the origin of springs can be attributed to re-emergence of groundwater where relatively slow moving groundwater drainage in fracture zones above the main water table has a signicant lateral component which intersects the ground surface in a steeply sloping terrain or by appearance of groundwater at the break-in slope where water table of unconned aquifer cross-cut the ground surface which is controlled by topography (Figure 5.10). Figure 5.10 : Illustration of some of the common forms of springs in western Himalaya (Springer and Stevens, 2009). Quality of Groundwater and Groundwater Pollution Both groundwater and surface water are being utilised for drinking, domestic as well as for irrigation and industrial purposes. Although groundwater is concealed within the subsurface aquifers, making it less vulnerable to contamination as compared to surface water, but accounts of serious deterioration in groundwater quality is reported from across the country. The deterioration in groundwater quality could be caused either by geogenic processes or could be through reckless anthropogenic activities which include industrial e uents, domestic sewerage, agriculture fertilizers and pesticides, solid waste from growing urban centres and industries, mining and salt water intrusion in coastal areas. Contamination is the presence of elevated concentrations of substances in the environment above the natural background level whereas pollution results in deleterious e ects as to harm the living organisms including hazards to human health (FAO). All pollutants are contaminants, but not all contaminants are pollutants. 76
In India, Central Ground Water Board, Ministry of Jal Shakti monitors the water quality in the summer months every year as well as water table data is collected seasonally through the established network of ground water wells (i.e. dug wells, bore wells or tube wells, piezometers). The stage of groundwater development refers to the ratio of gross annual groundwater draft to net annual groundwater availability. Net annual groundwater availability is the annual groundwater potential (total annual recharge) minus the natural discharge. Depending on the stage of groundwater development at the smallest administrative units i.e. district or block level, the administrative unit is categories as safe (< 70%), semi-critical (70% - 90%), critical (90% - 100%) and more than 100% is considered overexploited. With the rising demand of groundwater for agriculture, the states like Punjab, Rajasthan, and Haryana have fallen into the category of overexploited, while states like Tamil Nadu, Gujarat and Uttar Pradesh are at Semi-critical stage of groundwater development. In addition to the water balance estimate of groundwater availability for an administrative unit, water quality also puts a constraint in terms of suitability of ground water for drinking purposes. Water quality deterioration could be due to geogenic processes resulting from rock-water interaction induced inorganic contaminants like salinity, chloride, uoride, nitrate, iron and arsenic in groundwater. Central Pollution Control Board also conducts detailed monitoring of groundwater quality in problem areas of industrial clusters and metro cities of India. Common groundwater contaminants are dissolved nitrate, trace metals, organic compounds and pathogens caused by anthropogenic activities. The sources of nitrates include sewage, fertilizers, air pollution, landlls and industries, and toxic and carcinogenic trace metals like Lead, Mercury, Cadmium, Copper, Chromium & Nickel can have its source from industrial and mine discharges and y ash from thermal power plants. Organic compounds like volatile and semi-volatile, organic compounds of petroleum derivatives, PCBs, pesticides including their sources from agricultural activities, street drainages, sewage landlls, industrial discharges, spills, vehicular emissions fall-out and pathogens like bacteria and viruses which could have sources from sewage and landlls can cause groundwater pollution. Several locations in the States of Andhra Pradesh, Gujarat, Karnataka, Madhya Pradesh, Rajasthan, Chhattisgarh, Haryana, Orissa, Punjab, Haryana, Uttar Pradesh West Bengal, Bihar, Delhi, Jharkhand, Maharashtra, and Assam where the uoride in ground water exceeds 1.5 mg/l as well as high concentration of Iron (>1.0 mg/l) in ground water. The States like West Bengal and Bihar are seriously impacted by Arsenic contamination in groundwater, where the arsenic contamination is found beyond the permissible limit of 0.01 mg/L. Also, relatively high values of Electrical Conductivity more than 3000 µS/cm are observed in many parts of the country. Districts of Rajasthan and southern Haryana where EC values of ground water is greater than 10000 μS /cm make it unsuitable for drinking. Status report on groundwater quality (CPCB, 2007) highlights serious groundwater contamination in 35 metropolitan cities and in 16 groundwater problematic areas in industrial-belt, mining areas and urban centre in the country. 77
Conclusion Groundwater is an important source of fresh water. Through the chapter, researchers and other readers can be able to understand various types of groundwater sources, it's role in hydrological cycle on earth and basic principles involved in related processes. References Fitts CR (2002). Groundwater Science. Elsevier. Heath RC (1998). Basic ground-water hydrology (Vol. 2220). US Geological Maidment DR (1993). Handbook of hydrology (Vol. 9780070, p. 397323). New York:McGraw-Hill. Meinzer OE (1923). Outline of groundwater hydrology. United State Geological Survey.Water Supply Paper, 494. Mateo-Sagasta J, Zadeh SM,Turral H and Burke J (2017). Water pollution fromagriculture: a global review. The Food andAgricultural Organization. Springer AE and Stevens LE (2009). Spheres of discharge of springs. HydrogeologyJournal, 17(1), 83-93. CPCB (2007). Status of groundwater quality in India. (www.cpcb.nic.in) Further Reading Fetter CW (2001).Applied Hydrogeology. Prentice Hall, New Jersey,4th Edition. Freeze RA and Cherry JA (1979). Groundwater. Englewood Cli s, New Jersey:Prentice- Hall.Survey,US Department of the Interior. Singhal BBS and Gupta RP (2010). Applied hydrogeology of fractured rocks. Springer Science & Business Media. Todd DK and Mays LW (2005). Groundwater Hydrology. Willy International. 78
6 INSTRUMENTATION FOR IN-SITU MONITORING OF WATER RESOURCES Shivam Sharma, Anagani Prudhvi Sagar, Shivi Saxena, Soham Adla, Deepak Arya, K Sri Harsha, Shivam Tripathi Abstract India lacks tools for enabling e ective data driven water resources management practices. There is a need for cost-e ective instrumentation network that will provide fair access of water data to all its stakeholders and can promote transparency and better advocacy for water management while pushing the boundaries of hydrology as a science. To better manage its freshwater resources, India needs to deploy a network of instrumentation on rivers, canals, ponds, lakes, tanks and farms that can monitor the water quantity and quality in real time. With the advancements in communication and sensor technologies, the cost of real-time hydrological monitoring instruments is decreasing while their reliability and robustness is increasing. Instruments like water level, owmeters, soil moisture and water quality are readily available in the market for plug and play deployments. They can transmit the data in real-time to cloud servers making it convenient for users to download and analyse the data. As the amount of data increases, we need to formulate and adopt uniform standards for in situ measurements and database management across the country. Keywords: Water monitoring, instrumentation, water quality, water level, ow measurement Introduction Access to adequate quantity and good quality of water is a fundamental requirement and right of all human beings but water management in India currently su ers from a lack of Shivam Sharma Anagani Prudhvi Sagar Shivi Saxena Soham Adla Deepak Arya K Sri Harsha, B.Tech. Kritsnam Technologies Private Limited [email protected] Shivam Tripathi Department of Civil Engineering, Indian Institute of Technology Kanpur Monitoring and A ssessment of Environmental Parameters Eds. V. A gnihotri, S. Rai, A . Tiwari, S. Mukherjee, K. Kumar, R. Joshi, GBPNIHE, A lmora, Uttarakhand, India ©GBPNIHE 2020 79
Instrumentation for In-Situ Monitoring of Water Resources accountability. Natural river channels, surface and subsurface water storages are the major sources of fresh water for the majority of Indians. Agricultural and industrial revolutions have tipped the scales towards unprecedented adulteration and disruption of the natural fresh-water systems. As a result, across the country, we are facing critical challenges of pollution and depletion of water resources. The e ects of climate change compounded by the exploitation of natural water resources are resulting in long-term negative consequences. Additionally, the lack of su cient research, technology and foresight has led to gross mismanagement of these systems. As the surface and ground water resources are simultaneously being polluted and depleted at a rapid rate, it is high time for adopting technology-driven approach to water resources monitoring and data-driven approach to water resources management. Recent advancements in sensor technology, telecommunication and embedded electronics are resulting in the development of robust, cost-e ective and energy e cient instruments for real time in situ monitoring of water resources. Similarly, improvements and simplications in high-speed cloud computing and data modeling have opened avenues for scientically managing water resources. The water resources monitoring primarily consists of measuring hydrological uxes and storages such as precipitation, inltration, streamow, sediment load, evapotranspiration, soil moisture, surface water levels and groundwater levels. It also includes monitoring water supply from rivers, reservoirs, lakes, wells and canals, and water consumption in agricultural, industrial and domestic sectors. With increase in water contamination problems, the monitoring now encompasses water quality (physical, chemical and biological) parameters as well. Presently in India, in situ water resources monitoring is mostly manual, which limits the density and accuracy of the data. The manpower for data collection comes from diverse backgrounds and carry di erent biases thus limiting the standardization of the collected data in terms of its accuracy, reliability and frequency. Automatic instruments are being used, but owing to high cost, high maintenance requirements and low on-eld support, they are sparsely installed. Moreover, lack of integrated monitoring across di erent regions, scales and variables renders the collected data to be of limited value for hydrological research. Hydrological research, which involves investigation of hydrological cycle at di erent spatial and temporal scales, is essential for understanding the functioning of our water resource systems. Such understanding is needed to derive best practices for sustainably managing water resources. The literature review suggests that access to reliable data on water quantity and quality has always been crucial for hydrological research. If we examine the progress of watershed hydrology from 1930 to present, we see how the developments in instrumentation and computational technologies have helped evolve our understanding on diverse hydrological processes responsible for life sustenance. With the advancements in monitoring techniques in each step, hydrologists around the world came up with new 80
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