Part A:Unit 2

1. Part A:Unit 2 LOSSES FROM PRECIPITATION

2. EVAPORATION Evaporation is the process in which a liquid changes to the gaseous state at the free surface ,below the boiling point through the transfer of heat energy.

3. Factors affecting Evaporation

4. Factors affecting Evaporation: Vapour Pressure The rate of evaporation is proportional to the difference between the saturation pressure at the water temperature,ew and the actual vapour pressure in the air ,ea. Thus EL=C(ew-ea) , Daltons law of evaporation. Where, EL = rate of evaporation(mm/day) C = a constant ew & ea are in mm of mercury Evaporation continues till ew = ea If ew > ea ,Condensation takes place

5. Temperature • The rate of evaporation increases with an increase in the water temperature. • Regarding air temperature high correlation between evaportion rate and air temperature does not exist. • Thus for the same mean monthly temperature it is possible to have evaporation to different degrees in a lake in different months.

6. Wind: • Wind aids in removing the evaporated water vapour from the zone of evaporation and consequently creates greater scope for evaporation. • However, if the wind velocity is large enough to remove all the evaporated water vapour, any further in­crease in wind velocity does not influence the evaporation. • Thus the rate of evapora­tion increases with the wind speed up to a critical speed beyond which any further increase in the wind speed has no influence on the evaporation rate. • This critical wind-speed value is a function of the size of the water surface. • For large water bodies high-speed turbulent winds are needed to cause maximum rate of evaporation.

7. ATMOSPHERIC PRESSURE: Other factors remaining same, a decrease in the baro­metric pressure, as in high altitudes, increases evaporation. • SOLUBLE SALTS : • When a solute is dissolved in water, the vapour pressure of the solution is less than that of pure water and hence causes reduction in the rate of evapo­ration. • The percent reduction in evaporation approximately corresponds to the per­centage increase in the specific gravity. • Thus, for example, under identical conditions evaporation from sea water is about 2-3% less than that from fresh water.

8. HEAT STORAGE IN WATER BODIES • Deep water bodies have more heat storage than shallow ones. • A deep lake may store radiation energy received in summer and release it in winter causing less evaporation in summer and more evaporation in winter compared to a shallow lake exposed to a similar situation. • However, the effect ofheat storage is essentially to change the seasonal evaporation rates and the annual evaporation rate is seldom affected.

9. Measurement of Evaporation: The amount of water evaporated from a water surface is estimated by the following methods: • using evaporimeter data, • Class A Evaporation Pan • ISI Standard pan (Modified class A Pan) • empirical evaporation equations, • Meyer’s Equation • Rohwer’s equation • analytical methods.

10. Class A Evaporation Pan Galvanised iron sheet (Monel metal) • Depth of water maintained between18cm and 20cm • Evaporation measurements are made by measuring the depth of water with a hook gauge in a stilling well

11. ISI Standard Pan (Modified class A Pan) IS:5973-1970:

12. • Specified by IS:5973 and known as the modified Class A Pan • A pan of diameter 1220mm and depth 255mm • The pan is made of copper sheet 0.9mm thick, tinned inside and painted white outside • The pan is placed on a square wooden platform of width 1225mm and height 100mm above ground level to allow free air circulation below the pan • A fixed point gauge indicates the level of water • Water is added to or removed from the pan to maintain the water level at a fixed mark using a calibrated cylindrical measure • The top of the pan is covered with a hexagonal wire net of GI to protect water in the pan from birds • Presence of the wire mesh makes the temperature of water more uniform during the day and night • Evaporation from this pan is about 14% lower as compared to that from an unscreened pan

13. Pan Coefficient(Cp) Evaporation pans are not exact models of large reservoirs Their major drawbacks are the following: • They differ from reservoirs in the heat storage capacity and heat transfer characteristics from the sides and the bottom. Hence evaporation from a pan depends to some extent on its size (Evaporation from a pan of about 3m dia is almost the same as that from a large lake whereas that from a pan of about 1m dia is about 20% in excess of this). • The height of the rim in an evaporation pan affects wind action over the water surface in the pan. Also it casts a shadow of varying size on the water surface. • The heat transfer characteristics of the pan material is different form that of a reservoir. • • Hence evaporation measured from a pan has to be corrected to get the evaporation from a large lake under identical climatic and exposure conditions. • • Lake Evaporation = Pan Coefficient x Pan Evaporation

14. Values of Pan Coefficient(Cp)

15. EMPIRICAL EVAPORATION EQUATION Most of the available empirical equations for estimating lake evaporation are a Dalton type equation of the general form

16. (1) Meyer’s Formula

17. (2) Rohwer’s Formula • Accounts for the effect of pressure in addition to the wind speed effect

18. Wind Velocity In the lower part of the atmosphere, up to a height of about 500m above the ground level, wind velocity follows the one-seventh power law as,

19. Methods to reduce Evaporation losses • Reduction of Surface area • Mechanical Covers • Chemical Films

20. (I) REDUCTION OF SURFACE AREA • The volume of water lost by evaporation is directly proportional to the surface area of the water body • Hence the reduction of surface area wherever feasible reduces evaporation losses. • Measures like having deep reservoirs in place of wider ones and elimination of shallow areas can be consideredunder this category.

21. (II)MECHANICAL COVERS • Permanent roofs over the reservoir, temporary roofs and floating roofs such as rafts and light-weight floating particles can be adopted wherever feasible. • Obviously these measures are limited to very small water bodies such as ponds, etc.

22. CHEMICAL FILMS • This method consists of applying a thin chemical film on the water surface to reduce evaporation. • Currently this is the only feasible method available for reduction of evaporation of reservoirs up to moderate size. • Cetyl alcohol (hexadecanol) and Stearyl alcohol (octadecanol) • These forms monomolecular layers on a water surface. • These layers act as evaporation inhibitors by preventing the water molecules to escape past them.

23. Features of thin films: • The film is strong and flexible and does not break easily due to wave action. • If punctured due to the impact of raindrops or by birds, insects, etc., the film closes back soon after. • It is pervious to oxygen and carbon dioxide; the water quality is therefore not affected by its presence. • It is colorless, odourless and nontoxic.

24. Cetyl alcohol • Most suitable chemical for use as an evaporation inhibitor. • It is a white, waxy, crystalline solid and is available as lumps, flakes or powder. • It can be applied to the water surface in the form of powder, emulsion or solution in mineral turpentine. • Roughly about 3.5 N/hectare/day of cetyl alcohol is needed for effective action. • The chemical is periodically replenished to make up the losses due to oxidation, wind sweep of the layer to the shore and its removal by birds and insects. • Evaporation reduction can be achieved to a maximum if a film pressure of 4 x 10-2 N/m is maintained.

25. Advantages: • Controlled experiments with evaporation pans have indicated an evaporation reduction of about 60% through use of cetyl alcohol. • Under field conditions, the reported values of evaporation reduction range from 20 to 50%. • It appears that a reduction of 20-30% can be achieved easily in small size lakes (< 1000 hectares) through the use of these monomolecular layers. • The adverse effect of heavy wind appears to be the only major impediment affecting the efficiency of these chemical films.

26. TRANSPIRATION Transpiration is the evaporation of water from plants. Environmental factors that affect the rate of transpiration 1. Light • Plants transpire more rapidly in the light than in the dark. This is largely because light stimulates the opening of the. • Light also speeds up transpiration by warming the leaf. 2. Temperature • Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises. At 30°C, a leaf may transpire three times as fast as it does at 20°C. 3. Humidity • When the surrounding air is dry, diffusion of water out of the leaf goes on more rapidly.

27. 4. WindWhen there is no breeze, the air surrounding a leaf becomes increasingly humid thus reducing the rate of transpiration. When a breeze is present, the humid air is carried away and replaced by drier air. 5. Soil water • A plant cannot continue to transpire rapidly if its water loss is not made up by replacement from the soil. • The volume of water lost in transpiration can be very high. It has been estimated that over the growing season, one acre of corn (maize) plants may transpire 400,000 gallons (1.5 million liters) of water. • As liquid water, this would cover the field with a lake 15 inches (38 cm) deep. An acre of forest probably does even better.

28. Evapotranspiration (consumptive use) • Evapotranspiration (ET) is a collective term for all processes through which water in liquid or solid form becomes atmospheric water vapor. • It includes evaporation from bare soil, lakes and rivers and vegetative surfaces. • It also includes transpiration, which represents evaporation from within the leaves of plants through stomatal openings.

29. Potential Evapotranspiration (PET): If sufficient moisture is always available to completely meet the needs of vegetation fully covering the area ,resulting Evapotranspiration is called Potential Evapotranspiration. • Actual Evapotranspiration (AET) : The real Evapotranspiration occurring in a specific situation is called Actual Evapotranspiration . • If water supply is adequate , soil moisture = Field capacity, then AET = PET. • If water supply is less than PET , AET/PET < 1

30. Factors affecting ET • Humidity • Temperature • Growing season of crop & cropping pattern • Monthly precipitation in the area • Irrigation depth • Wind velocity • Soil & topography • Irrigation Practices & methods of irrigation

31. Measurement of ET • Tank and Lysimeter methods • Field experimental plots • Soil moisture studies • Integration method • Inflow and outflow studies for large area

32. LYSIMETER • It is a special water tight tank containing a block of soil • Set in a field of growing plants • ET estimated in terms of the amount of water required to maintain constant moisture condition • Measured volumetrically & gravimetrically • Designed to match the field condition • It should be buried that the soil is at the same level inside & outside • Time consuming & expensive

34. Estimation of ET by Blaney criddle method: • Empirical formula based on data from arid western United states. • This formula assumes that the PET is related to hours of sunshine and temperature • The Potential Evapotranspiration in a crop growing season is given by, ET = 2.54KF & F= Σ PhTf /100 • Where, ET = PET in a crop season in cm • K=an empirical coefficient, depends on the type of the crop & stage of growth • F= sum of monthly consumptive use factors for the period • Ph = monthly percent of annual day-time hours, depends on the latitude of the place • Tf = mean monthly temperature

35. Infiltration • Zone 3: Transmission Zone • Downward motion of the moisture takes place. • Moisture content is above the field capacity but below saturation • Unsaturated flow & fairly uniform moisture content • Zone 4: Wetting Zone • Soil moisture is at or near field capacity • M.C. decreases with the depth • Boundary of wetting zone is wetting front, where large a sharp discontinuity exists between the newly wet soil & original m.c. Infiltration is the flow of water into the ground through the soil surface. • Depending upon the amount of the infiltration & physical properties of the soil Wetting front can extend from a few cm to metres.

37. Infiltration Capacity( fp ): • The maximum rate at which a given soil at a given time can absorb water is defined as the infiltration capacity ( fp ). • The actual rate of infiltration f can be expressed as : f = fp when i ≥ fp and f = i when i ≤ fp Where i= intensity of rain fall • Infiltration capacity of a soil is high at beginning of a storm & has an exponential decay as the time elapses.

38. Factors affecting infiltration • Characteristics of the soil • Condition of the soil surface • Vegetative cover • Current moisture content • Soil temperature • Precipitation • Slope of the land • Fluid Characteristics

39. Characteristics of the soil • Type of soil, i.e. sand ,silt clay, its texture,structure,permeability & under drainage are the important characteristics under this category • Loose ,permeable ,sandy soil will have a larger infiltration capacity than a tight, clayey soil. • Soil with good under drainage will have higher infiltration capacity . • Transmission capacity determines the over all infiltration rate of the layered soil. • Dry soil absorb more water.

40. Surface of entry • At the soil surface, the impact of rain drops causes the fines in the soil to be displaced and these turn can clog the pore spaces in the upper layers of the soil which affects the infiltration rate. • Thus surface covered with grass & other vegetation reduce the above said effect & pronounced influence on the infiltration rate.

41. Precipitation: • The greatest factor controlling infiltration is the amount and characteristics (intensity, duration, etc.) of precipitation that falls as rain or snow. • Precipitation that infiltrates into the ground often seeps into streambeds over an extended period of time, thus a stream will often continue to flow when it hasn't rained for a long time and where there is no direct runoff from recent precipitation.

42. Land cover: • Some land covers have a great impact on infiltration and rainfall runoff. • Vegetation can slow the movement of runoff, allowing more time for it to seep into the ground. • Impervious surfaces, such as parking lots, roads, and developments, act as a "fast lane" for rainfall - right into storm drains that drain directly into streams. • Agriculture and the tillage of land also changes the infiltration patterns of a landscape. • Water that, in natural conditions, infiltrated directly into soil now runs off into streams.

43. Slope of the land: Water falling on steeply-sloped land runs off more quickly and infiltrates less than water falling on flat land. Fluid characteristics: • More impurities present – Turbidity • The turbidity of the water ,especially clay and colloid content block the fine pores and reduce infiltration capacity. • Temperature of the water affects the viscosity by which infiltration rate also affected. Contamination of water by dissolved salts affects the soil structure & in turn affect the infiltration rate

44. Measurement of Infiltration • Using Flooding type infiltrometer. • Double ring Infiltrometer • Measurement of subsidence of free water in a large basin or pond. • Rain fall simulator • Hydrograph analysis

45. Double ring Infiltrometer Disadvantages: 1.The rain drop impact effect is not simulated 2.Driving of the tube disturbs the soil structure 3.Results depends on the their size.(Larger meters giving less rates than the smaller ones)

47. Infiltration Indices • In hydrological calculations involving floods it is convenient to use a constant value of infiltration rate for the duration of the storm. The defined average infiltration rate is called Infiltration Index. Two types of indices are in common use: • φ- index • W- index

48. φ- index This is defined as the rate of infiltration above which the rainfall volume equals runoff volume. • If the rain fall intensity is less than φ,then the infiltration rate is equal to the rain fall intensity. • If the rain fall intensity is larger than φ, the difference b/w the rain fall & infiltration in an interval of time represents the run off volume.

49. W-index This is the average infiltration rate during the time when the rainfall intensity exceeds the infiltration rate. Where , R = total storm precipitation (cm) R= Total storm run off (cm) Ia = Initial losses (cm) te = duration of the rain fall excess,i.e.the total time in which the rain fall intensity is greater than W (in hours) W = defined average rate of infiltration (cm)

50. Horton's Equation of Infiltration Horton gives infiltration capacity as a function of time as: Where: Fp = infiltration rate into soil, (mm/hr) Fc = final steady state infiltration capacity occurring at t=tc (mm/hr) Fo = initial infiltration capacity (mm/hr) t = time from beginning of storm, sec k = Horton's decay coefficient which depends upon soil characteristics & vegetation cover. (1/sec)