Introduction to Hydrology, Rainfall measurement

HYDROLOGY

1. Introduction to Hydrology UNIT - I Prepared by, Divakar Pamanji Assistant Professor MTIET

2. Rainfall measurement The amount of precipitation is expressed as the depth in centimetres (or inches) which falls on a level surface, and is measured by rain-gauge

3. Types of rain gauges Non-automatic rain-gauge Symons rain gauge Automatic rain gauge Weighing bucket rain-gauge Tipping bucket rain-gauge Float type rain-gauge

4. Symons rain gauge Symon's rain-gauge is most common type of non-automatic rain-gauge, and is used by Meteorological Department of Government of India. As shown in Fig, it consists of cylindrical vessel 127 mm (or 5") in diameter with a base enlarged to 210 mm (or 8") diameter. The top section is a funnel provided with circular brass rim exactly 127 mm (5") internal diameter. The funnel shank is inserted in the neck of a receiving bottle which is 75 to 100 mm (3 to 4") diameter.

5. Symons rain gauge A receiving bottle of rain-gauge has a capacity of about 75 to 100 mm of rainfall and as during a heavy rainfall this quantity is frequently exceeded, the rain should be measured 3 or 4 times in a day on day of heavy rainfall lest the receiver fill should overflow. A cylindrical graduated measuring glass is furnished with each instrument, which reads to 0.2 mm. The rainfall should be estimated to the nearest 0.1 mm. The rain-gauge is set up in a concrete block 60 cm x 60 cm x 60 cm (2'x 2'x 2'), as shown in Fig. The rim should be 305 mm (12") above the surface of the ground.

6. Symons rain gauge

7. Weighing Bucket Type Rain-gauge Self recording gauges are used to determine rates of rainfall over short periods of time. The most common type of self-recording gauge is the weighing bucket type, as shown in Fig. The weighing bucket rain-gauge essentially consists of a receiver bucket supported by a spring or lever balance or any other weighing mechanism. The movement of the bucket due to its increasing weight is transmitted to a pen which traces the record on a clock driven chart

8. Weighing Bucket Type Rain-gauge

9. Tipping Bucket Type Rain-gauge A Steven's tipping bucket type rain-gauge consists of 300 mm diameter sharp edge receiver. At the end of the receiver is provided a funnel. A pair of buckets are pivoted under the funnel in such a way that when one bucket receives 0.25 mm (0.01 inch) of precipitation it tips, discharging its contents into a container bringing the other bucket under the funnel.

10. Tipping Bucket Type Rain-gauge Tipping of the bucket completes an electric circuit causing the movement of pen to mark on clock driven revolving drum which carries a record sheet The electric pulse generated due to the tipping of the buckets is recorded at the control room far away from the rain gauge station

11. Tipping Bucket Type Rain-gauge

12. Float (or Syphon) Type Rain-gauge The working of a float or syphon type rain-gauge is similar to the weighing bucket type gauge. A funnel receives the rain water which is collected in a rectangular container. A float is provided at the bottom of the container.

13. Float (or Syphon) Type Rain-gauge The float is raised as the water level rises in the container, its movement being , recorded by apen moving on a recording drum actuated by a clock work. When the water level in the container rises so that the float touches the top, the siphon comes into operation, and releases the water; thus all the water in the box is drained out.

14. Selecting the site for a rain-gauge station The site where a rain-gauge is set up should be an open place. The distance between the rain-gauge and the nearest object should be at least twice the height of the object. In no case should it be nearer to the obstruction than 30 metres. The rain-gauge should never be situated on the side or top of a hill if a suitable site on a level ground can be found. In the hills, where it is difficult to find level space, the site for the gauge should be chosen where it is best shielded from high winds, and where the wind does not cause eddies. A fence, if erected to protect the gauge from cattle etc. should be so located that distance of the fence is not less than twice its height.

15. Advantages of recording rain gauges Following are the advantages of recording type rain-gauge over the non-recording type The rainfall is recorded automatically and therefore, there is no necessity of any attendant. The recording rain-gauge also 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.

16. Disadvantages It is costly in comparison with non-recording gauge. Fault may develop in electrical or mechanical mechanism or recording the rainfall.

17. Rain-gauge network It is absolutely essential to design a proper network of raingauges in a given catchment (water shed) to collect the necessary precipitation data. The raingauge density or network density is defined as the ratio of total area of catchment to the total number of gauges in the catchment. A question that frequently arises concerns the number and type of gauges that are necessary to ensure accurate assessment of a catchment's rainfall.

18. Rain-gauge network The World Meteorological Organisation (WMO) has laid down the following norms for minimum network density:

19. Rain-gauge network The optimum no. of raingauge stations (N) as per IS C v = coefficient of variation of the rainfall values of existing stations s x = standard deviation x = mean of rainfall values of existing stations p = desired degree of error in estimating mean rainfall

20. Presentation of rainfall data

21. Hyetograph method

22. Mass curve method Mass curves are a graphical representation of cumulative depth of rainfall over a period of time. In the case of rainfall, the vertical axis represents the cumulative rainfall for the period of interest, while the horizontal axis represents the time.

23. Point rainfall method

24. Interpretation of rainfall data Intensity of rainfall: The rate at which rainfall occurs and is expressed as cm/h or mm/h. Non-recording type rain gauges measure/record only rainfall depth in a day, or over a duration. Frequency of rainfall: The frequency of rainfall refers to how often rain occurs within a given period typically expressed as the number of rainy days over a specific timeframe, such as monthly or annually. Design of hydraulic structures such as flood control structures, soil conservation structures, waste-water systems, drains, and culverts, etc is based on the probability of extreme rainfall and runoff

25. Intensity duration analysis Intensity of rainfall decreases with increasing duration Greater intensity of rainfall has shorter duration Sherman proposed a relation between intensity and duration of rainfall:

26. Intensity duration  frequency relationship

27. Depth area relationship The depth-area relationship describes the expected relationship between rainfall depth and area The equations or curves used in the relationship are specific to a particular geographic region As rainfall depth increases, the area over which it is distributed in creases This is caused by increased runoff resulting in less infiltration

28. Depth area  duration relationship Depth-area-duration curves show the relationship between rainfall depth, area, and duration Curves show that the volume of runoff increases as rainfall depth and duration increase Relationship is due to the fact that as rainfall intensity and duration increase, the soil's ability to absorb water is exceeded DAD curves are important tools for hydrologists and engineers By understanding the relationship, engineers can estimate the volume of runoff DAD curves are used to estimate the maximum precipitation and volume of runoff from a given catchment area during a rainfall event

29. Evaporation Evaporation and evapo -transpiration are important phases of the hydrologic cycle which redistribute heat energy.  Evaporation is a process in which liquid changes to gaseous state below the boiling point with the transfer of heat energy. Solar radiation is the main source of evaporation, and in arid regions the loss due to evaporation can be up to 90% of the annual precipitation.

30. Factors affecting evaporation loss

31. Estimation or measurement of evaporation

32. Measurement using evaporation pans Most reliable method for estimation of evaporation from large bodies of water is through measurements from evaporation pans Pan is 1.22 m in diameter and 0.255 m deep. Made of 0.9 mm thick copper sheet with hexagonal wire netting of galvanized iron mesh covering it Placed over a square wooden platform of 1.29 m in width and 10 cm in height The water level in the pan is recorded by a point gauge arrangement placed inside a stilling basin Measurement is taken at least once a day by adding water to the pan in a calibrated cylindrical glass jar If there is rainfall exceeding the depth of evaporation, water is taken out of the pan in the same way by measuring jar Knowing the depth of rainfall from the rain gauge, the evaporation depth is found by subtraction

33. Use of empirical equations Meyer's formula (1915)       E = evaporation from the water body, mm/day       e s = saturation vapor pressure at the water surface        e a = actual vapor pressure of overlaying air at the sp. Height       V 9 = monthly mean wind velocity in km/h @ 9 m above the GL       K m = coefficient accounting for various other factors             = 0.36 for large deep waters             = 0.50 for small shallow waters

34. Use of empirical equations Rohwer's formula (1931)       E = evaporation from the water body, mm/day       e s = saturation vapor pressure at the water surface        e a = actual vapor pressure of overlaying air at the sp. Height       V 0.6 = mean wind velocity in km/h @ 0.6 m above the GL To find V 0.6 in the numerical problems using the following equation V h is the wind velocity at height h above the GL

35. Water budget method P = Precipitation I sf =  surface water inflow      O sf = surface water outflow I gf =  ground water inflow      O gf = ground water outflow T = Transpiration loss ΔS = change in storage

36. Energy budget method This method uses the conservation of energy by incorporating all the incoming, outgoing and stored energy of aa lake/reservoir in the following form:                               H n = H a + H e + H g + H s + H i              ------ (1) H n = net heat energy received by water surface = H c (1-r)-H b Hc (1-r) = incoming solar radiation into a surface of reflection coefficient r (approx. 0.05) Hb = back radiation from the water body Ha = sensible heat transfer from water surface to air He = heat energy used up in evaporation = 

37. Energy budget method La = latent heat of evaporation Hg = heat flux into the ground Hs = heat stored in water body Hi = net heat conducted out of the  system by water flow All the energy terms are in cal /mm 2 /day Hs and Hi can be neglected for small time periods Tw = temperature of water surface Ta = temperature of air Pa = atmospheric pressure Ha can be estimated by using Bowen's ratio(β) ------(2) Combining eq 1 & 2, we obtain the following equation for evaporation E:

38. Infiltration

39. Factors affecting infiltration

40. Measurement of infiltration Single tube infiltrometer Consists of a hollow metal cylinder of 30 cm diameter and 60 cm length, both ends open 10 cm of cylinder driven into ground, water-filled to 7 cm head above ground level Water level decreases due to infiltration, water added to maintain a constant level  Volume of water added over predetermined time interval gives infiltration rate - Observations continued till a uniform rate is obtained (3-6 hours, depending on the type of soil) - Plot of time in abscissa against the rate of water added in mm/h gives infiltration capacity curve - Drawback is infiltrated water percolates laterally at bottom of the ring, does not represent area through which infiltration takes place

41. Measurement of infiltration Double tube infiltrometer Double tube infiltrometer consists of two concentric hollow rings (or cylinders) Rings driven into soil without any tilt, to a depth of 15 cm. Diameter of rings may vary from 25 to 60 cm Water applied in both inner and outer rings to maintain a constant depth of about 5 cm Water level in inner and outer rings kept the same during observation period Measurement includes recording of volume of water added into inner compartment and corresponding elapsed time

42. Runoff Surface runoff Sub-surface runoff Base flow

43. Factors affecting runoff

44. Computation of runoff Runoff by linear or exponential regression Linear regression is a statistical method used to determine the relationship between two variables In the context of runoff prediction, linear regression is used to determine the relationship between rainfall and runoff Exponential regression is a statistical method used to determine the relationship between two variables when the data shows an exponential pattern In the context of runoff prediction, exponential regression is used when the relationship between rainfall and runoff is not linear Exponential regression can help to model and predict runoff when rainfall has a greater impact on runoff as it increases Both linear and exponential regression can be used to predict runoff, but they are typically used in different contexts R = aP+b Linear regression equation Coefficient of correlation Exponential relationship R = βP m

45. Computation of runoff By empirical equations and tables Runoff coefficient         R = kP Inglis's formula  Derived from data collected from 37 catchments in the Bombay presidency R = 0.85 P – 30.5                       (For ghat areas) R= 0.00394 P 2 – 0.0701 P       (For plain regions) Urban residents                               -0.3 - 0.5 Commercial and industrial             -0.9 Forest areas                                      -0.05 - 0.2 Parks, farm land, pasture               -0.05 - 0.3 Asphalt or concrete  pavements   -0.85

46. Computation of runoff Strange's tables and curves W.L Strange (1928) gave tables and curves for runoff resulting from rainfall in the plains of south India

47. Computation of runoff Lacey's formula  Khosla's formula                                                 R m = P m – 0.48 T m j ICAR formula                                           R = 1.511 (P 1.44 )(T m ) – 1.34 (A – 0.0613 )   Rm  = monthly runoff in cm T      = mean temperature on the catchment Pm   = monthly precipitation in cm R      = runoff in cm Tm   = mean temperature on the catchment P      = precipitation in cm S = catchment factor F = monsoon duration factor

48. Computation of runoff By infiltration method