COURSE CODES: AGE 411 ABE 303 COURSE TITLE: Hydrology for Agricultural Engineers

1. COURSE CODES: AGE 411 ABE 303 COURSE TITLE: Hydrology for Agricultural Engineers NUMBER OF UNITS: 3 Units COURSE DURATION: Three hours per week Course Lecturer Engr. Dr. Dada, P.O.O. (B.Eng.,M.Agric. Ph.D) Email: dadapo@unaab.edu.ng

2. Course Content Introduction, Hydrologic Cycle, Solar and Earth Radiation, Precipitation, Evapotranspiration, Infiltration, Rainfall-Runoff Over Agricultural Land, Stream Gauging, Hydrographs, Stream Flow Routing, Ground Water Hydraulics, Water –Shed Management, Flood Control. COURSE REQUIREMENTS: This is a compulsory course for all students in the University. In view of this, students are expected to participate in all the course activities and have minimum of 75% attendance to be able to write the final examination.

3. Reading List1.Hydrology and Water Resources Engineering, Santosh Kumar Garg, 6th Edition, Khanna Publisher Delhi.2. Principles of Hydrology, R.C. Ward and M. Robinson, 3rd Edition. Mc. Graw Hill Company.3. Applied Hydrology, Ven Te Chow and David Ray Medment, International Edition. Mc. Graw Hill Company.

4. Hydrology is the science of occurrence, movement and distribution of water above/below the land surface or in the atmosphere. • Surface water hydrology • Groundwater hydrology

5. Catchment Area The area of land draining into a stream or a water course at a given location is known as catchment area. Also called as drainage area or drainage basin or watershed (USA).

6. Catchment of River A at station M

8. Water Budget Equation From the continuity equation for water, i.e. Mass inflow - mass outflow = change in mass storage Water budget of a catchment for a time interval Δt is written as P – R – G – E - T = Δ S Where P = precipitation, R = surface runoff, G = net ground water flow out of the catchment, E = evaporation, T = transpiration and Δ S = change in storage. All terms in the equation may have the dimensions of volume or depth over the catchment area.

9. Example # 1 A lake had a water surface elevation of 103.2 m above datum at the beginning of certain month. In that month the lake received an average inflow of 6.0 m3/sec from surface runoff sources. In the same period the outflow from the lake had an average value of 6.5 m3/sec. Further, in that month, the lake received a rainfall of 145 mm and the evaporation from the lack surface was estimated as 6.10 cm. Write the water budget equation for the lake and calculate water surface elevation of the lake at the end of month. The average lake surface area can be taken as 5000 ha. Assume that there is no contribution to or from the ground water storage.

10. Example # 2 A small catchment of area 150 ha received a rainfall of 10.5 cm in 90 min duration due to a storm. At the outlet of the catchment, the stream draining the catchment was dry before the storm and experienced a runoff lasting for 10 hr with an average value of 2.0 m3/sec. The stream was again dry after the runoff event. What is the amount of water which was not available to runoff due to combined effect of infiltration, evaporation and transpiration? Also compute the ratio of runoff to precipitation.

11. Annual World Water Budget

12. Hydrological Cycle

13. Types of Precipitation • Cyclonic Precipitation: When air masses rise up with vapors and travel towards low pressure areas resulting into Cyclonic precipitation. i. Frontal precipitation It is due to flow of warm air mass into a cold region. ii. Non-Frontal precipitation When cold air meets with stationary warm air then Non-Frontal Precipitation occurs. 2. Convective Precipitation: It occurs due to natural rising of warm lighter air with vapors into the colder and denser region.

14. Types of Precipitation • Orthographic Precipitation When heavily moisture-laden air stops due to topographic conditions (mountains) and precipitation occurs then it is called orthographic precipitation. Just like in Himalayan regions. • Precipitation due to Turbulent Ascent. When turbulence in the velocity of clouds occur due to land surface after long travel on the ocean surface then there will be rising up of clouds into colder regions and precipitation occurs.

15. Rainfall Characteristics 1. Size and Shape • Rainfall occurs when moisture in the atmosphere condenses into drops. • Raindrops occur in any shape up to approximately 9 mm mean diameter after which they tend to break up.

16. Rainfall Characteristics 2. Intensity and Duration • Amount of water that reaches to ground surface per unit area is called intensity. • Intensity and duration are usually inversely related, i.e., high intensity storms are likely to be of short duration and low intensity storms can have a long duration.

17. Types of Rain-Gauges • Following are the main types of rain-gauges used for measurement of rainfall. • Non-automatic / Non-recording Rain-gauge • Symon’s Rain-gauge • Automatic / Recording Rain-gauge • Weighing Bucket Rain-gauge • Tipping Bucket Rain-gauge • Float type Rain-gauge

18. Non-automatic (Non-recording) Rain-gauge • These are called non-recording rain gauges because they do not record the rain but only collect the rain. • The collected rain is then measured by means of graduated cylinders so as to directly represent the rainfall volume in cm of water depth, i.e. • Depth of rain water in cm = • Note: The amount of precipitation/rainfall is expressed as the • depth in cm or inches.

19. Symon’s Rain-gauge • Most common type of non-automatic rain-gauge. • Consists of cylindrical vessel 127 mm (5”) dia with a base enlarged to 210 mm (8”) dia. • The top section is a funnel provided with circular brass rim exactly 127 mm (5”) internal dia. • The funnel shank is inserted in the neck of receiving bottle which is 75 to 100 mm (3 to 4”) dia. • Capacity of bottle is 75 to 100 mm of rainfall. • The rain-gauge is placed in concrete block 60cm×60cm×60cm (2’×2’×2’). • The rim should be 305 mm (12”) above the ground surface. • Water contained in the bottle is measured by suitably graduated measuring glass, with an accuracy up to 0.1 mm. • During a heavy rainfall, the rain should be measured 3 or 4 times in a day.

20. Measurement of rainfall by an Automatic (Recording) Rain-gauge • In general, automatic rain-gauge consists of rotating drum with a graph paper fixed around it. • There is pen in contact with graph paper, which moves up with the collected rain, and thus recording cumulative rain, with the passage of time. • These are of three types 1. Weighing Bucket Rain-gauge 2. Tipping Bucket Rain-gauge 3. Float Type Rain-gauge

21. 1.Weighing Bucket Rain-gauge • Consists of a receiver bucket supported by a spring or lever balance or any other weighing mechanism. • The movement of bucket due to its increasing weight is transmitted to a pen which traces the record on a clock driven chart.

22. 2.Tipping Bucket Rain-gauge • Consists of 30 cm dia sharp edge receiver. • At the end of receiver funnel is provided. • Under the funnel a pair of buckets are pivoted (the central point which balances) in such away that when one bucket receives 0.25 mm (0.01”) of rainfall it tips (to fall or turn over), discharging its contents into reservoir bringing other bucket under funnel. • Tipping of bucket completes an electric circuit causing the movement of pen to mark on clock driven revolving drum which carries a record sheet.

24. 3.Float Type Rain-gauge • Working is similar to weighing gauge bucket. • Funnel receives the rain water which is collected in rectangular container. • Float is provided at the bottom of container. • Float is raised as the water level rises in the container. • Movement of float is being recorded by a pen moving on recording drum actuated by clock work. • When the water level in the container rises so that float touches the top, the siphon comes into operation, and release the water; thus all the water in the box is drained out.

25. Advantages of Recording Gauge over the Non-recording Gauge. • In recording gauge rainfall is recorded automatically & therefore, there is no necessity of any attendant. • Recording rain-gauge gives the intensity of rainfall at any time while the non-recording gauge gives the total rainfall in any particular interval of time. • As no attendant is required such rain-gauge can be installed in far–off places also. • Possibility of human error is obviated.

26. Disadvantages of Recording Gauge over the Non-recording Gauge. • It is costly in comparison with non-recording gauge. • Fault may be developed in electrical or mechanical mechanism or recording the rainfall.

27. Computation of average rainfall over a basin. • In order to compute the average rainfall over a basin or catchment area, the rainfall is measured at a number of rain-gauge stations suitably located in the area. • The no. of rain-gauge stations depends upon the area and distribution of rainfall. • If a basin or catchment area contains more than one rain-gauge station, the computation of average rainfall may be done by the following methods: • Arithmetic average method. • Thiessen polygon method. • Isohytel method.

28. 1.Arithmetic average method. • Simplest method of estimating average rain fall. • Average rainfall is calculated by arithmetic average of recorded rainfall at various stations. • If P1, P2, P3…..Pn are the rainfall values measured at n gauge stations, we have • Advantages: suitable method when rainfall is uniform.

29. Example: Using Arithmetic Average Method, find average rainfall over a catchment. The rain gage data is: 12.6, 18.8, 14.8, 10.4 and 16.2 mm.

30. 2.Thiessen polygon method. • This method is a more common method of weighing the rain-gauge observation according to the area. • Also called Weighted Mean Method. • Accurate than arithmetic average method. • Average rainfall can be computed by the following expression. • Advantages: This method is based on assumption that a rain-gauge station best represents the area which is close to it.

31. Procedure: • Join the adjacent rain-gauge stations. • Construct the perpendicular bisectors of each of these lines. • The polygon formed by the perpendicular bisectors around a station encloses an area which is every where closer to that station than to any other station. • Find the area of each of these polygons, shown hatched in the figure. • Compute the average precipitation using the given formula.

32. Example: Using Thiesen Polygon Method, find average rainfall over a catchment. The data is:Rain Gauge Station A B C D EPolygon Area (km2) 40 45 38 30 43Precipitation (mm) 30.8 33.4 34.6 32.6 24.6 Solution:

33. 3.Isohytel method. • An isohyet is a line, on a rainfall map of the basin, joining places of equal rainfall readings. • An isohyetal map showing contours of equal rainfall presents a more accurate picture of the rainfall distribution over the basin. • Average rainfall can be computed by the following expression. • Advantages: The isohytel method is the most elaborate and accurate than other methods.

34. Procedure • From the rainfall values recorded at various rain-gauge stations, prepare the isohyetal map. • Measure the areas enclosed between successive isohyets with the help of planimeter. • Multiply each of these areas by the average rainfall between the isohyets. • Compute the average rainfall applying the given formula.

35. Example: Using Isohyetal Method, find average rainfall over a catchment. The data is:Isohyetes (cm) 12 13 14 15 16 17Area b/w Isohyetes (km2) 22 80 110 89 70Average Precipitation (cm) 12.5 13.5 14.5 15.5 16.5

36. Problem: Find the mean precipitation for the area sketched in figure by Thiessen’s method. The area is composed of a square plus an equilateral triangular plot of side 4 kms. Rainfall readings in cms at the various stations are also given in figure.

37. Presentation of rainfall data A few commonly used methods of presentation of rainfall data which have been found to be useful in interpretation and analysis of such data are: 1.Mass curve of Rainfall 2.Hyetograph

38. 1.Mass curve of Rainfall • If the total accumulated precipitation is plotted against time, the curve obtained is known as Mass curve of Rainfall/Storm. • The curve rises steeply in the beginning and then tends to become constant. • Mass curve of rainfall are very useful in extracting the information on the duration and the magnitude of storm. • Also, intensities at various time intervals in a storm can be obtained.

39. 2.Hyetograph. • It can be defined as a plot of intensity of rainfall (cm/hr) against the time interval, represented as a bar chart. • The area under hyetograph represents the total precipitation received in that period. • This chart is very useful in representing the characteristics of storm, and is particularly important in developing the design storms to predict extreme floods. • The time interval used depends on the purpose; in urban-drainage problems, small durations are used, while in flood flow computations in larger catchments, the intervals are of about 6 hr.

40. Precipitation Contd. Estimating Point Rainfall: In estimating point rainfall, the method of weighted averages is often used. This gives depth of rainfall that is smaller than the greatest amount and larger than the smallest amount of the area. i.e. lies between Pmax and Pmin of the area.

41. Precipitation Contd. • Estimating Point Rainfall Contd. Method: Consider that rainfall is to be calculated for point A. Establish a set of axes running through A and determine the absolute coordinates of the nearest surrounding points BCDEF. The estimated precipitation at A is determined as a weighted average of the other five points. The weights are reciprocals of the sums of the squares of X and Y, that is D2 = X2 + y2 and W = 1/D2 The estimated rainfall at the point of interest is given by: ( P x W )/ W

42. Precipitation Contd. • Example of Point Rainfall Determination: Point Rainfall X Y X2 + Y2 D2 W P x W ( mm ) (m) (m) A - - - - - - - B 160 40 20 C 180 10 16 D 150 30 20 E 200 30 30 F 170 20 20 Sums W  P x W Estimated Precipitation of A = PxW / W

43. Precipitation Contd. • Method of Estimating Point Rainfall: 1). Draw X and Y axis through the interested point 2). Mark up the distances along the X and Y axis. 3). Put them down (distances along the ordinates) 4). Calculate D2 = X2 + Y2 5). Calculate W = 1/D2 6). Have the sum of (PxW) i.e. (PxW) 7). Rainfall at the point = (PxW)/W

44. Aerial Precipitation • For most hydrologic analyses, it is important to know the aerial distribution of precipitation. Usually, the average depth for representative portions of the watershed are determined and used for this purpose. • The most direct approach is to use the arithmetic average of gauged quantities. This procedure is satisfactory if gauges are uniformly distributed and the topography is flat. • Other commonly used methods are: isohyetal and Thiessen methods.

45. Aerial Precipitation Contd. • The reliability of rainfall measured at one gauge in representing the average depth over a surrounding area is a function of: 1). the distance from the gauge to the centre of the representative area. 2). the size of the area 3). topography 4). the nature of the rainfall of concern (storm event vs. mean, monthly or annual) 5). local storm characteristics.

46. Probabilities of Handling Rainfall Data. • Rain is random (Stochastic) – element of uncertainty. • To get the probability, we use: a). Plotting position- to give an idea of the possibility. b). Carry out probability analyses. c). Frequency analyses. These give probability of P, associated with the magnitude of an event equal or exceeded in a given period of time. The probability concept gives a useful way of presenting hydrologic data. To calculate the probability of an event, one often uses the ‘Weibull’s formula in the form: P = m/n+1, where: m = rank or order number, n = number of years of record. From the probability, the return period is calculated. Tr = 1/P; or n+1/m; where: Tr is return period.

47. Example on Probability of Event • Given a Rainfall data, say for 30 years: Year Rank Annual Tr(1/P) Probability totals . . . . . . . . . . • On a log probability paper, plot annual totals against probability; one can then estimate the once in 50 years or 100 years or 75 years etc. • Note: Rank the values from max. to min. • This is useful when you want values for draught periods i.e. for irrigation requirements etc.

48. Question on Probability of Event • The following are annual rainfall totals for 33 years in mm. 108, 113, 133, 180, 115, 163, 320, 110, 230, 118, 123, 115, 118, 100, 145, 200, 238, 230, 153, 245, 160, 463, 115, 118, 100,145, 200, 238, 230, 153, 245, 160, 468. • Calculate return periods for the 33 years record. Plotting probabilities and return periods on the graph provided, estimate return periods for: 120, 180, 215 and 370 annual totals.

49. Consistency of data • This is used to determine if there is any trend in the rainfall data. • To do this, take as many number of gauging stations as possible from a Basin. • Taken seven of the gauging stations as base group. • Test each one of the stations against the base group.

50. Consistency of data Contd. • How to test? 1). With annual rainfall data, take the 7 stations and find the annual means for the number of years record available. 2). Plot cumulative annual data of station 1 against the cumulative annual data of the base group. 3). Find the slope. If it is not a straight line, then the data is inconsistent, due to certain conditions that are not known. 4). Note where the curve changes, take the slope of the curve before the change and the slope after the change. AF = Slope of change after/ slope of change before AF = average factor 5). Multiply each of the annual values for the station by AF. 6). Repeat the process for each station.