Evaporation Of Precipitation PDF

 EVAPORATION EVAPORATION PROCESS Evaporation is the process in which liquid changes to the gaseous state at the free surface below the boiling point through the transfer of heat energy. Rate of evaporation depends on Vapour Pressure at the water surface and air above Air and...

 EVAPORATION EVAPORATION PROCESS Evaporation is the process in which liquid changes to the gaseous state at the free surface below the boiling point through the transfer of heat energy. Rate of evaporation depends on Vapour Pressure at the water surface and air above Air and water temperature Wind speed Atmospheric pressure Quality of water Size of the water body  EVAPORIMETERS Estimation of the amount of water evaporated from a water surface Using evaporimeter data Empirical evaporation equation Analytical Methods Types of Evaporimeters Class A Evaporation Pan ISI Standard Pan (IS 5973-1970) Colorado Sunken Pan USGS Floating Pan Unpainted galvanised iron sheet Disadvantages: Difficult to detect leaks Extra care is needed to keep the surrounding area free from grass, dust etc Expensive to install Unpainted Galvanised iron sheet USGS FLOATING PAN  PAN COEFFICIENT CP Drawbacks of Evaporation Pan ØThey differ in the heat storing capacity and heat transfer from the sides and bottom. The sunken pan and floating pan aim to reduce this deficiency. As a result of this factor the evaporation from a pan depends to a certain extent on its size. While a pan of 3 m diameter is known to give a value which is about the same as from neighbouring large lake, a pan of size 1.0 m diameter indicates about 20% excess evaporation than that of the 3 m diameter pan. ØThe height of the rim in an evaporation pan affects the wind action over the surface. Also it casts a shadow of variable magnitude over the water surface. ØThe heat transfer characteristics of the pan material is different from that of the reservoir Lake evaporation = CP X Pan evaporation where CP is pan coefficient.  WMO RECOMMENDATION Arid zones: One station for every 30000 km2 Humid Temperate Climates: One station for every 50000 km2 Cold Regions: One station for every 100000 km2  EMPIRICAL EVAPORATION EQUATIONS DALTON- TYPE EQUATION E L = Kf (u )(e w - e a ) ew is saturated vapour pressure at the water surface temperature in mm of mercury and ea is actual vapour pressure of overlying air at a specified height in mm of mercury. f (u) is wind speed correction function and K = a coefficient. The term ea is measured at same height at which wind speed is measured MEYER’S FORMULA æ u9 ö E L = K M (e w - e a )ç1 + ÷ è 16 ø u9 is monthly mean wind velocity in km/hour at about 9 m above ground and KM is coefficient. KM = 0.36 for large deep waters and 0.5 for small shallow waters. ROHWER’S FORMULA E L = 0.771(1.465 - 0.000732 p a )(0.44 + 0.0733u 0 )(e w - e a ) Pa is mean barometric reading in mm of mercury. u0 is mean wind velocity in km/hour at ground level which can be taken to be velocity at 0.6 m height above ground. It is known that in the lower part of the atmosphere up to a height of about 500 m above the ground level, the wind velocity can be assumed to follow 1/7 th power law. u h = Ch 1 / 7 uh is wind velocity at a height h above the ground and C is constant.  ANALYTICAL METHODS OF EVAPORATION ESTIMATION Water budget method Energy balance method Mass transfer method Water budget method P + V IS + V IG = VOS + VOG + E L + DS + T L P = daily precipitation Vos = Daily surface outflow from the lake Vis = Daily surface inflow into the lake Vog = Daily seepage outflow Vig = Daily groundwater inflow EL = Daily Lake evaporation DS = Increase in lake storage in a day TL = Daily Transpiration loss EL = P + (VIS - VOS ) + (VIG - VOG ) - DS - TL  Energy balance method Hn = Ha + He + H g + Hs + Hi Hn = net energy received by water surface = Hc (1-r)- Hb Hc (1-r)= incoming solar radiation into a surface of reflection coefficient r Hb = Back radiation (Long wave) from water body Ha = Sensible heat transfer from water surface to air He = Heat energy used up in evaporation = rLEL where r is density of water, L is latent heat of evaporation and EL is evaporation in mm. Hg = Heat flux into the ground Hs = Heat stored in the water body Hi = Net heat conducted out of the system by water flow The sensible heat term Ha which can not be readily measured is estimated using Bowen’s ratio b Ha T w - Ta b= -4 = 6.1´ 10 ´ p a rLE L ew - ea Pa is atmospheric pressure in mm of mercury ew is saturated vapor pressure in mm of mercury ea is actual vapor pressure of air in mm of mercury Tw is temperature of water surface in 0 C and Ta is temperature of air in 0 C Hn - H g - Hs - Hi EL = rL(1 + b ) If the time periods are short, Hs and Hi can be neglected as it is very small.  RESERVOIR EVAPORATION The water volume lost due to evaporation from a reservoir in a month is VE = AEPM CP where VE is volume of water lost in evaporation in a month (m3 ) A = average reservoir area during the month (m2 ) EPM is Pan evaporation loss in meters in a month (m) CP is pan coefficient Methods to reduce Evaporation loss: Reduction of Surface Area Mechanical Covers Chemical Films Cetyl Alcohol (Evaporation reduction approx 60%) and Stearyl Alcohol are used as an evaporation inhibitor. Characteristic features of Thin film The film is strong and flexible and does not break easily due to wave action If punctured due to impact of rain drops 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 colourless, odourless and nontoxic The reduction is 20-30% can be achieved easily in small size lakes. Heavy winds appears to be only major disadvantages which affects the efficiency of films  TRANSPIRATION Transpiration is the process by which water leaves the body of a living plant and the reaches the atmosphere as a water vapour. Transpiration is confined to day light hours and rate of transpiration depends upon the growth periods of the plant. Evaporation continues through day and night Factors affecting Transpiration Atmospheric Vapour pressure Temperature Wind Light intensity Characteristics of the plant (Root and Leaf system) Potential Evapotranspiration: If sufficient Moisture is always available to completely meet the needs of vegetation fully covering the area, the resulting evapotranspiration is called Potential evapotranspiration (PET) (Depends critically on climatic factors not soil and plant factors) Actual Evapotranspiration The real evapotranspiration occurring in a specific situation is called Actual Evapotranspiration (AET)  INTERCEPTION Interception loss: It may be retained by vegetation as surface storage and returned to the atmosphere by evaporation. It is solely due to evaporation and doesnot include transpiration, throughfall and stemflow. The amount of water intercepted depends on the species composition of vegetation, its density and storm characteristics. The interception loss is 10-20% in an area during plant growing season and greater than 25% due to forest. It is large for small storm and levels off to a constant value for large storms. Throughfall: It can drip off the plant leaves to join the ground surface or the surface flow Stemflow: The rain water may run along the leaves and branches and down the stem to reach the ground surface  I i = S i + K i Et Ii is interception loss in mm. Si is interception storage whose values varies from 0.25-1.25 mm depending on nature of vegetation. Ki is ratio of vegetal surface area to its projected area. E is evaporation rate in mm/hr. t is duration of rainfall in hours. Depression Storage The volume of water trapped in depressions are known as depression storage. Depends on The type of soil The condition of surface reflecting the amount and nature of depression Slope of the catchment The antecedent precipitation. Zone 1 Thin layer of saturated zone at the top Zone 2 Beneath zone 1, there is a transition zone Zone 3 Transmission zone where downward motion of the moisture takes place. The soil moisture is above field capacity but below saturation in this zone. Characterised by unsaturated zone and fairly uniform moisture content. Zone 4 Wetting zone. The soil moisture is near field capacity and moisture content decreases with depth. The boundary of wetting zone is the wetting front where a sharp discontinuity exist between the newly wet soil and original moisture content of soil. Maximum rate it can absorb linked to infiltration capacity of soil Volume of water it can hold linked to field capacity of soil  INFILTRATION CAPACITY Infiltration capacity (fp): The maximum rate at which a given soil at a given time can absorb water. f=fp when i>=fp f=i when i F F= i when i< F The amount of rainfall in excess of the index is called rainfall excess or effective rainfall. Consider a rainfall hyetograph of event duration D hours and having N pulses of time interval Dt such that N x Dt = D Let Ii be the intensity of rainfall in ith pulse and Rd is the total direct runoff and te is the duration of rainfall excess. N Total rain fall P = å I i.Dt 1 P - f.te = Rd 1. Assume that out of given N pulses, M number of pulses have rainfall excess. Select M number of pulses in decreasing order of their intensity 2. Find the value of F that satisfies the relation M R d = å (I i - f )Dt 1 Using the value of F of step 2, find the number of pulses (Mc) which give rainfall excess. If Mc = M , then F of step 2 is correct. If not repeat step 1. W-index P - R - Ia W= te P = Total precipitation R= Total runoff Ia = Initial losses te = duration of rainfall excess. Total time in which the rainfall intensity is greater than W. W=Defined average rate of infiltration A storm with 10 cm of precipitation produced a direct runoff of 5.8 cm. The duration of the rainfall was 16 hours and its time distribution is given below. Estimate phi index of the storm Time 0 2 4 6 8 10 12 14 16 from start (h) Cumul 0 0.4 1.3 2.8 5.1 6.9 8.5 9.5 10 ative rainfall (cm)