H Y D R O L O G Y I R R I G A T I O N C E

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1 H Y D R O L O G Y & I R R I G A T I O N C E

2 Chapter 1: HYDROLOGICAL CYCLE AND PRECIPITATION 7 HYDROLOGIC CYCLE... 7 PRECIPITATION... 8 MEASUREMENT OF PRECIPITATION... 9 ESTIMATION OF MISSING DATA ABSTRACTIONS FROM PRECIPITATION EVAPORATION SUMMARY FORMULA TIPS PREVIOUS YEAR GATE QUESTIONS Chapter 2: INFILTRATION AND RUNOFF INFILTRATION RUNOFF RUNOFF RAINFALL RELATIONS STAGE DISCHARGE RELATIONSHIP SUMMARY TIPS PREVIOUS GATE QUESTIONS Chapter 3: HYDROGRAPHS Hydrographs Hydrograph Analysis Unit Hydrograph Career Avenues GATE Coaching by IITians

3 Synthetic Unit Hydrograph Use and Limitations of Unit Hydrograph PROBLEM SUMMARY TIPS PREVIOUS YEAR GATE QUESTIONS Chapter 4: Floods and Flood Estimation Floods Planning of Reservoirs Flood Routing PROBLEM SUMMARY FORMULA TIPS PREVIOUS YEAR GATE QUESTIONS Chapter 5: Well Hydraulics Ground Water Resources Well Irrigation Occurrence of Ground Water Well Hydraulics Equilibrium Equations Well Interference Pumping Tests (Or Aquifier Tests) SUMMARY Career Avenues GATE Coaching by IITians

4 FORMULA TIPS PREVIOUS YEAR GATE QUESTIONS Chapter 6: Introduction to Irrigation INTRODUCTION STORAGE OF SOME MAJOR DAMS (2) IMPACT OF IRRIGATION ON HUMAN ENVIRONMENT WATER RESOURCES OF INDIA NEED OF IRRIGATION IN INDIA IDEAL WEATHER FOR KHARIF AND RABI SEASONS CROPS OF KHARIF SEASON CROPS OF RABI SEASON OTHER MAJOR CROPS MULTIPLE CROPPING SUMMARY TIPS Chapter 7: Water Requirements of Crops SOIL WATER RELATIONSHIPS ROOT-ZONE SOIL WATER INFILTRATION CONSUMPTION USE (OR EVAPOTRANSPIRATION) IRRIGATION REQUIREMENT FREQUENCY OF IRRIGATION SUMMARY Career Avenues GATE Coaching by IITians

5 FORMULA TIPS PREVIOUS YEAR GATE QUESTIONS Chapter 8: Canal Irrigation ESTIMATION OF DESIGN DISCHARGE OF A CANAL SUMMARY FORMULA TIPS PREVIOUS YEAR GATE QUESTIONS Chapter 9: Canal Head Works LOCATION OF HEADWORKS ON RIVERS DIFFERENT UNITS OF HEADWORKS FISH LADDER CANAL HEAD REGULATOR DESIGN OF WEIR WEIR CREST, GLACIS, AND IMPERVIOUS FLOOR UPSTREAM AND DOWNSTREAM LOOSE PROTECTION SUMMARY FORMULA TIPS PREVIOUS YEAR GATE QUESTIONS: Chapter 10: Gravity Dams FORCES ON A GRAVITY DAM Career Avenues GATE Coaching by IITians

6 STRESS ANALYSIS OF GRAVITY DAMS ELEMENTARY PROFILE OF A GRAVITY DAM SUMMARY FORMULA TIPS PREVIOUS YEAR GATE QUESTIONS: Chapter 11: Water Logging WATERLOGGING ECONOMICS OF CANAL LINING DRAINAGE OF IRRIGATED LANDS SUMMARY FORMULA TIPS Chapter 12: Methods of Irrigation METHODS OF IRRIGATION WATER USE SUBSYSTEMS SURFACE IRRIGATION SUMMARY TIPS PREVIOUS YEAR GATE QUESTIONS Career Avenues GATE Coaching by IITians

7 Chapter 1: HYDROLOGICAL CYCLE AND PRECIPITATION The word hydrology means science of water which deals with the spatial and temporal characteristics of the earth s water in all its aspects such as occurrence, circulation, distribution, physical and chemical properties, and impact on environment and living things. HYDROLOGIC CYCLE The total water of earth, excluding deep ground water, is in constant circulation from the earth (including oceans) to atmosphere and back to the earth and oceans. This cycle of water amongst earth, oceans, and atmospheric systems is known as hydrologic cycle. Figure shown below is an enormously simplified sketch of the hydrologic cycle for which sun is the source of energy. The hydrologic cycle can be visualized to begin with the evaporation(due to solar heat) of water from the oceans, streams and lakes of the earth into the earth s atmosphere. The water vapor, under suitable conditions, get condensed to form clouds moving with wind all over the earth s surface and which, in turn, may result in precipitation (in the form of rain water, snow, hail, sleet etc.) over the oceans as well as the land surface of the earth. Part of the precipitation, even while falling, may evaporate back into the atmosphere. Another part of the precipitation may be intercepted by vegetation on the ground or other surfaces. The intercepted precipitation may either evaporate into the atmosphere or fall back on the earth s surface. The greater part of the precipitation falling on the earth s surface is retained in the upper soil from where it may return to the atmosphere through evaporation and transpiration by plants and/or find its way, over and through the soil surface as runoff, to stream (or river) channels and the runoff thus becoming stream flow. Yet another part of the precipitation may penetrate into the ground to become part of the ground water. The water of stream channels, under the influence of gravity, moves towards lower levels to ultimately meet the oceans. Water from ocean may also find its way into the adjoining aquifers. Part of the stream water also gets evaporated back into the atmosphere from the surface of the stream. The ground water too moves towards the lower levels to ultimately reach the oceans. The ground water, at times, is a source of stream flow. Hydrologic system is defined as a structure or volume in space surrounded by a boundary that receives water and other inputs, operates on them internally, and produces them as outputs. The global hydrologic cycle can be termed a hydrologic system containing three sub-systems: the atmospheric water system, the surface water system, and the subsurface water system. Another example of the hydrologic system is storm-rainfall-runoff process on a watershed. Watershed (or drainage basin or catchment) is a topographic area that drains rain water falling on it into a surface stream and discharges surface stream flow through one particular location identified as watershed outlet or watershed mouth. The term watershed used for the catchment area should be distinguished from the watershed used in the context of canal alignment 7 Career Avenues GATE Coaching by IITians

8 PRECIPITATION The atmospheric air always contains moisture. Evaporation from the oceans is the major source (about 90%) of the atmospheric moisture for precipitation. Continental evaporation contributes only about 10% of the atmospheric moisture for precipitation. The common forms of precipitation are drizzle or mist (water droplets of diameters less than 0.5 mm), rain (water drops of size between 0.5 mm and 6.0 mm), snow (ice crystals combining to form flakes with average specific gravity of about 0.1), sleet (rain water drops, falling through air at or below freezing temperatures, turned to frozen rain drops), and hail (precipitation in the form of ice balls of diameter more than about 8 mm). Most of the precipitation, generally, is in the form of rains. Therefore, the terms precipitation and rain fall are considered synonymous. Rainfall, i.e., liquid precipitation, is considered light when the rate of rainfall is upto 2.5 mm/hr, moderate when the rate of rainfall is between 2.5 mm/hr and about 7.5 mm/hr, and heavy when the rate of rainfall is higher than about 7.5 mm/hr. The temporal variation of annual rainfall at a given place is expressed in terms of the coefficient of variation, Cv defined as 8 Career Avenues GATE Coaching by IITians

9 The coefficient of variation of the annual rainfall for different places may vary between15 (for regions of high rainfall) and 70 (for regions of scanty rainfall) with an average value of about 30. MEASUREMENT OF PRECIPITATION PRECIPITATION GAUGES Precipitation (of all kinds) is measured in terms of depth of water (in millimeters) that would accumulate on a level surface if the precipitation remained where it fell. A variety of instruments have been developed for measuring precipitation (or precipitation rate) and are known as precipitation gauges or, simply, rain gauges which are classified as either recording or nonrecording rain gauges. Non-recording rain gauges only collect rain water which, when measured suitably, gives the total amount of rainfall at the rain gauge station during the measuring interval. The Indian Meteorological Department has adopted Symon s rain gauge. A glass bottle and funnel with brass rim are put in a metallic cylinder such that the top of the cylinder is 305mm above the ground level. Rain water falls into the glass bottle through the funnel. The water collected in the bottle is measured with the help of a standard measuring glass jar which is supplied with the rain gauge. The jar measures rainfall in millimeters. At each station, rainfall observations are taken twice daily at 8.30 a.m. and 5.30 p.m. Recording rain gauges automatically record the intensity of rainfall and the time of its occurrence in the form of a trace (or graph) marked on a graph paper wrapped round a revolving drum. Following three types are the most widely used recording rain gauges: Tipping bucket rain gauge, Weighing bucket rain gauge, and Siphon rain gauge. 9 Career Avenues GATE Coaching by IITians

10 Tipping bucket rain gauge: A 300 mm diameter funnel collects rain water and conducts it to one of the two small buckets which are so designed that when 0.25 mm of rainfall is collected in a bucket, it tilts and empties its water into a bigger storage tank and, simultaneously, moves the other bucket below the funnel. When any of the two buckets tilts, it actuates an electric circuit causing a pen to make a mark on a revolving drum. The recording equipment can be remotely located in a building away from the rain gauge. At a scheduled time, the rain water collected in the storage tank can be measured to yield total rainfall in the measuring duration. The rainfall intensity (and also the total rainfall) can be estimated by studying the record sheet on which each mark indicates 0.25 mm of rain in the duration elapsed between the two adjacent marks. Weighing bucket rain gauge: This gauge has a system by which the rain that falls into a bucket set on a platform is weighed by a weighing device suitably attached to the platform. The increasing weight of rain water in the bucket moves the platform. This movement is suitably transmitted to a pen which makes a trace of accumulated amount of rainfall on a suitably graduated chart wrapped round a clock driven revolving drum. The rainfall record of this gauge is in the form of a mass curve of rainfall. The slope of this curve at any given time gives the intensity of rainfall at that time. Siphon rain gauge: This gauge is also called float type rain gauge as this gauge has a chamber which contains a light and hollow float. The vertical movement of float on account of rise in the water level in the chamber (due to rain water falling in it) is transmitted by a suitable mechanism to move a pen on a clock-driven revolving chart. The record of rainfall is in the form of a mass curve of rainfall and, hence, the slope of the curve gives the intensity of rainfall. Bureau of Indian Standards has laid down the following guidelines for selecting the site for rain gauges (IS : ): The rain gauge shall be placed on a level ground, not upon a slope or a terrace and never upon a wall or roof. On no account the rain gauge shall be placed on a slope such that the ground falls away steeply in the direction of the prevailing wind. The distance of the rain gauge from any object shall not be less than twice the height of the object above the rim of the gauge. Great care shall be taken at mountain and coast stations so that the gauges are not unduly exposed to the sweep of the wind. A belt of trees or a wall on the side of the prevailing wind at a distance exceeding twice its height shall form an efficient shelter. In hills where it is difficult to find a level space, the site for the rain gauge shall be chosen where it is best shielded from high winds and where the wind does not cause eddies. The location of the gauge should not be changed without taking suitable precautions. Description of the site and surroundings should be made a matter of record. 10 Career Avenues GATE Coaching by IITians

11 AVERAGE DEPTH OF PRECIPITATION OVER AN AREA The information on the average depth of precipitation (or rainfall) over a specified area on either the storm basis or seasonal basis or annual basis is often required in several types of hydrologic problems. The depth of rainfall measured by a rain gauge is valid for that rain gauge station and in its immediate vicinity. Over a large area like watershed (or catchment) of a stream, there will be several such stations and the average depth of rainfall over the entire area can be estimated by one of the following methods: Arithmetic Mean Method This is the simplest method in which average depth of rainfall is obtained by obtaining the sum of the depths of rainfall (say P1, P2, P3, P4... Pn) measured at stations 1, 2, 3,...n and dividing the sum by the total number of stations i.e. n. Thus, This method is suitable if the rain gauge stations are uniformly distributed over the entire area and the rainfall variation in the area is not large. Theissen Polygon Method The Theissen polygon method takes into account the non-uniform distribution of the gauges by assigning a weightage factor for each rain gauge. In this method, the entire area is divided into number of triangular areas by joining adjacent rain gauge stations with straight lines, as shown in Fig. 2.7 (a and b). If a bisector is drawn on each of the lines joining adjacent rain gauge stations, there will be number of polygons and each polygon, within itself, will have only one rain gauge station. Assuming that rainfall Pi recorded at any station i is representative rainfall of the area Ai of the polygon i within which rain gauge station is located, the weighted average depth of rainfall P for the given area is given as 11 Career Avenues GATE Coaching by IITians

12 Isohyetal Method An isohyet is a contour of equal rainfall. Knowing the depths of rainfall at each rain gauge station of an area and assuming linear variation of rainfall between any two adjacent stations, one can draw a smooth curve passing through all points indicating the same value of rainfall, Fig. 2.7 (c). The area between two adjacent isohyets is measured with the help of a planimeter. The average depth of rainfall P for the entire area A is given as Since this method considers actual spatial variation of rainfall, it is considered as the best method for computing average depth of rainfall. Example 2.1 The average depth of annual precipitation as obtained at the rain gauge stations for a specified area is as shown in Fig. 2.7 (a). The values are in cms. Determine the average depth of annual precipitation using (i) the arithmetic mean method, (ii) Theissen polygon method, and (iii) isohyetal method. Solution (i) Arithmetic mean method: Using Eq. (2.2), the average depth of annual precipitation, (ii) Theissen polygons for the given problem have been shown in Fig. 2.7 (b). The computations for the average depth of annual precipitation are shown in the following Table: 12 Career Avenues GATE Coaching by IITians

13 PRECIPITATION GAUGE NETWORK The spatial variability of the precipitation, nature of the terrain and the intended uses of the precipitation data govern the density (i.e., the catchment area per rain gauge) of the precipitation gauge (or rain gauge) network. Obviously, the density should be as large as possible depending upon the economic and other considerations such as topography, accessibility etc. The World Meteorological Organization (WMO) recommends the following ideal densities (acceptable values given in brackets) of the precipitation gauge network (3): For flat regions of temperate, Mediterranean, and tropical zones, 600 to 900 sq. km ( sq. km) per station. 13 Career Avenues GATE Coaching by IITians

14 For mountainous regions of temperate, Mediterranean, and tropical zones, 100 to 250 sq. km (250 to 1000 sq. km) per station. For small mountainous islands with irregular precipitation, 25 sq. km per station. For arid and polar zones, 1500 to 10,000 sq. km per station. At least ten per cent of rain gauge stations should be equipped with self-recording gauges to know the intensities of rainfall. The Bureau of Indian Standards (4) recommends the following densities for the precipitation gauge network: In plains: 520 sq. km per station; In regions of average elevation of 1000 m: 260 to 390 sq. km per station; and In predominantly hilly areas with heavy rainfall: 130 sq. km per station. For an existing network of raingauge stations, one may need to know the adequacy of the raingauge stations and, therefore, the optimal number of raingauge stations N required for a desired accuracy (or maximum error in per cent, ) in the estimation of the mean rainfall. The optimal number of raingauge stations N is given as Example 2.2 A catchment has eight rain gauge stations. The annual rainfall recorded by these gauges in a given year is as listed in column 2 of the following Table. 14 Career Avenues GATE Coaching by IITians

15 ESTIMATION OF MISSING DATA The continuity of a record of precipitation data may have been broken with missing data due to several reasons such as damage (or fault) in a rain gauge during a certain period. The missing data is estimated using the rainfall data of the neighboring rain gauge stations. The missing annual precipitation Px at a station x is related to the annual precipitation values, P1, P2, P3...Pm and normal annual precipitation, N1, N2, N3...Nm at the neighboring stations 1, 2, 3,...M respectively. The normal precipitation (for a particular duration) is the mean value of rainfall on a particular day or in a month or year over a specified 30-year period. The 30-year normals are computed every decade. The term normal annual precipitation at any station is, therefore, the mean of annual precipitations at that station based on 30-year record. The missing annual precipitation Pxis simply given as If the normal annual precipitations at various stations are within about 10% of the normal annual precipitation at station x, i.e., Nx. Otherwise, one uses the normal ratio method which gives 15 Career Avenues GATE Coaching by IITians

16 DEPTH-AREA-DURATION (DAD) ANALYSIS Depth-area-duration (DAD) curves, Fig. 2.9, are plots of accumulated average precipitation versus area for different durations of a storm period. Depth-area-duration analysis of a storm is performed to estimate the maximum amounts of precipitation for different durations and over different areas. A storm of certain duration over a specified basin area seldom results in uniform rainfall depth over the entire specified area. The difference between the maximum rainfall depth over an area P0 and its average rainfall depth P for a given storm, i.e., P0 P increases with increase in the basin area and decreases with increase in the storm duration. The depth-areaduration curve is obtained as explained in the following example: ABSTRACTIONS FROM PRECIPITATION Prior to rain water reaching the watershed outlet as surface runoff or stream flow, it has to satisfy certain demands of the watershed such as interception, depression storage, evaporation and evapo-transpiration, and infiltration. A part of precipitation may be caught by vegetation on the ground and subsequently get evaporated. This part of precipitation is termed intercepted precipitation or interception loss (which, incidentally, is the gain for the atmospheric water) which does not include through-fall (the intercepted water that drips off the plant leaves to join the surface runoff) and stem flow(the intercepted water that runs along the leaves, branches and stem of the plants to reach the ground surface. 16 Career Avenues GATE Coaching by IITians

17 EVAPORATION Evaporation is the physical phenomenon by which a liquid is transformed to a gas. The rate of evaporation of precipitation depends on (i) the vapor pressure of water, (ii) prevailing temperature, (iii) wind speed, and (iv) atmospheric pressure. Transpiration is a phenomenon due to which water received by the plant through its root system leaves the plant and reaches the atmosphere in the form of water vapor. Evaporation and transpiration are usually considered together as evapo-transpiration (or consumptive use). SUMMARY The common forms of precipitation are drizzle or mist, rain, snow, sleet and hail. Three types are the most widely used recording rain gauges: (i) Tipping bucket rain gauge, (ii) Weighing bucket rain gauge and (iii) Siphon rain gauge. As per the World Meteorological Organization (WMO) the ideal densities (acceptable values given in brackets) of the precipitation gauge network : For flat regions of temperate, Mediterranean, and tropical zones, 600 to 900 sq. km ( sq. km) per station. For mountainous regions of temperate, Mediterranean, and tropical zones, 100 to 250 sq. km (250 to 1000 sq. km) per station. For small mountainous islands with irregular precipitation, 25 sq. km per station. For arid and polar zones, 1500 to 10,000 sq. km per station. As per the Bureau of Indian Standards the densities for the precipitation gauge network: In plains: 520 sq. km per station; In regions of average elevation of 1000 m: 260 to 390 sq. km per station; and In predominantly hilly areas with heavy rainfall: 130 sq. km per station. The rate of evaporation of precipitation depends on (i) the vapor pressure of water, (ii) prevailing temperature, (iii) wind speed, and (iv) atmospheric pressure. Evaporation and transpiration are usually considered together as evapo-transpiration (or consumptive use). 17 Career Avenues GATE Coaching by IITians

18 FORMULA Coefficient of variation, Cv defined as Arithmetic Mean Method Theissen Polygon Method Isohyetal Method The normal annual precipitation considering the mean of annual precipitations (< 10% variation) : The normal annual precipitation considering the normal ratio method (> 10% variation) : 18 Career Avenues GATE Coaching by IITians

19 TIPS This is comparatively an easy chapter to prepare for the exam and a one mark question can be expected once in two years from this chapter. An important question would be finding the missing data from the chapter. PREVIOUS YEAR GATE QUESTIONS 1. While applying the rational formula for computing the design discharge, the rainfall duration is stipulated as the time of concentration because (A) (B) (C) (D) this leads to the largest possible rainfall intensity this leads to the smallest possible rainfall intensity the time of concentration is the smallest rainfall duration for which the rational formula is applicable the time of concentration is the largest rainfall duration for which the rational formula is applicable. 2. The intensity of rainfall and time interval of typical storm are Time interval Intensity of rainfall (Min) (mm/min) The maximum intensity of rainfall for 20 min duration of the storm is (A) 1.5 mm/min (B) 1.85 mm/min (C) 2.2 mm/min (D) 3.7 mm/min 19 Career Avenues GATE Coaching by IITians

20 Answers: 1. (c) Time of concentration (t c ) is the time when entire catchment starts contributing to surface run-off at channel and this is the minimum time required. So, smallest rainfall duration for which the rational formula is applicable. 2. (b) Maximum Intensity is for time interval minutes where it is = (2.2* *10) / ( ) = 1.85 mm/min 20 Career Avenues GATE Coaching by IITians

Module 1. Lecture 2: Weather and hydrologic cycle (contd.)

Module 1. Lecture 2: Weather and hydrologic cycle (contd.) Lecture 2: Weather and hydrologic cycle (contd.) Hydrology Hydor + logos (Both are Greek words) Hydor means water and logos means study. Hydrology is a science which deals with the occurrence, circulation