Experiment No:1EVAPORATION AND ITS MEASUREMENT • Before rainfall reaches the outlet of a basin as runoff, certain demands of the catchment such as interception, depression storage and ini!tration have to be met • Besides these, e"aporation and transpiration processes transfer water to the atmosphere as water vapour • Evaporation from water bodies and the soil mass together with transpiration from vegetation is called e"apotranspiration #ET$ • That portion of Precipitation which is not available as surface runoff is termed as “!oss”. E"aporation Evaporation is the process in which a liquid changes to the gaseous state as the free surface, below its boiling point, through the transfer of energ Evaporation is a cooling process! the latent heat of vapourisation "#$%$ cal&g of evaporated water' must be provided b the water bod (ate of evaporation depends on ) *apour pressures at the water surface and the air above ) +ind speed ) ,ncident solar radiation ) -tmospheric pressure ) .ualit of water ) -ir and water temperatures ) /i0e of the water bod Vapo%r press%re ) (ate of evaporation is proportional to the difference between the saturation vapour pressure "/*P' at the water temperature and the actual vapour pressure in the air " ' • This equation is called 1alton2s 3aw of Evaporation • Evaporation occurs till • ,f condensation ta4es place Temperat%re ) (ate of evaporation increases with an increase in water temperature. -lthough there is an increase in the rate of evaporation with increase in air temperature, a high correlation does not e5ist between. 6or the same mean monthl temperature, evaporation from a la4e ma be different in different months. &ind ) +ind helps to remove the evaporated water vapour from the 0one of evaporation, thereb creating greater scope for evaporation. (ate of evaporation increases with increase in wind velocit up to some limit "critical wind speed' and thereafter an further increase in wind velocit does not have an effect on the evaporation rates. This critical wind speed value is a function of the si0e of the water surface "large water bodies ) high wind speeds' Atmosp'eric Press%re ) 7ther factors remaining the same, a decrease in atmospheric pressure "as in high altitude areas' increases the evaporation rate So!%(!e sa!ts ) +hen a solute is dissolved in water, the vapour pressure of the solution is less than that of pure water and hence it causes reduction in the rate of evaporation. The percentage reduction in the evaporation rate appro5imatel corresponds to the percentage increase in specific gravit 8nder identical conditions evaporation from sea water is about 9!:; less than that from fresh water )eat storage in *ater (odies ) 1eep water bodies have more heat storage capacit than shallow water bodies. - deep la4e stores radiation energ received in summer and releases it in winter resulting in less evaporation in summer and more evaporation in winter when compared to a shallow la4e e5posed to similar situations. The effect of heat storage is to change the seasonal evaporation rates and the annual evaporation remains more or less unaltered. ESTIMATION+ MEASUREMENT O, EVAPORATION This is done b the following methods • 8sing evaporimeters • 8sing empirical equations • B analtical methods T-PES O, EVAPORIMETERS E"aporimeters ) These are pans containing water which are e5posed to the atmosphere. 3oss of water b evaporation from these pans are measured at regular intervals "dail'. <eteorological data such as humidit, wind velocit, air and water temperatures, and precipitation are also measured and noted along with evaporation. #1$ US&. /!ass A E"aporation Pan • - pan of diameter =9=>mm and depth 9$$mm • 1epth of water is maintained between =% and 9>cm • The pan is made of unpainted ?, sheet • The pan is placed on a wooden platform of height =$cm above ground level to allow free air circulation below the pan • Evaporation is measured b measuring the depth of water in a stilling well with a hoo4 gauge 6igure@ 8/?/ Alass - Evaporation Pan #0$ ISI Standard Pan • /pecified b ,/@$BC: and 4nown as the modified Alass - Pan • - pan of diameter =99>mm and depth 9$$mm • The pan is made of copper sheet >.Bmm thic4, tinned inside and painted white outside • The pan is placed on a square wooden platform of width =99$mm and height =>>mm above ground level to allow free air circulation below the pan • - fi5ed point gauge indicates the level of water • +ater is added to or removed from the pan to maintain the water level at a fi5ed mar4 using a calibrated clindrical measure • The top of the pan is covered with a he5agonal wire net of ?, to protect water in the pan from birds • Presence of the wire mesh ma4es the temperature of water more uniform during the da and night • Evaporation from this pan is about =D; lower as compared to that from an unscreened pan 6igure@ ,/, Evaporation Pan #1$ /o!orado S%n2en Pan • B9>mm square pan made of unpainted ?, sheet, DE>mm deep, and buried into the ground within =>>mm of the top • <ain advantage of this pan ) its aerodnamic and radiation characteristics are similar to that of a la4e • 1isadvantages ) difficult to detect lea4s, e5pensive to install, e5tra care is needed to 4eep the surrounding area free from tall grass, dust etc 6igure@ Aolorado /un4en Pan #3$ US4S ,!oating Pan • - square pan of B>>mm sides and D$>mm deep • /upported b drum floats in the middle of a raft of si0e D.9$m 5 D.%Cm, it is set afloat in a la4e with a view to simulate the characteristics of a large bod of water • +ater level in the pan is maintained at the same level as that in the la4e, leaving a rim of C$mm • 1iagonal baffles are provided in the pan to reduce surging in the pan due to wave action • 1isadvantages ) Figh cost of installation and maintenance, difficult in ma4ing measurements Pan /oeicient • Evaporation pans are not e5act models of large reservoirs • Their maGor drawbac4s are the following@ ) The differ from reservoirs in the heat storage capacit and heat transfer characteristics from the sides and the bottom "sun4en and floating pans aim to minimise this problem'. Fence evaporation from a pan depends to some e5tent on its si0e "Evaporation from a pan of about :m dia is almost the same as that from a large la4e whereas that from a pan of about =m dia is about 9>; in e5cess of this'. ) The height of the rim in an evaporation pan affects wind action over the water surface in the pan. -lso it casts a shadow of varing si0e on the water surface. ) The heat transfer characteristics of the pan material is different form that of a reservoir. • Fence evaporation measured from a pan has to be corrected to get the evaporation from a large la4e under identical climatic and e5posure conditions. • 3a4e Evaporation H Pan Aoefficient 5 Pan Evaporation Ta(!e: Va!%es o Pan /oeicients Sl. No. Types of Pan Average Value Range 1 Class A Land Pan 0.70 0.60 – 0.80 2 ISI Pan (Modified Class A) 0.80 0.65 – 1.10 3 Colorado Sunken Pan 0.78 0.75 – 0.86 4 USS !loa"in# Pan 0.80 0.70 – 0.8$ • Evaporation pans are normall located at stations where other hdro!meteorological data are collected E"aporation Stations +<7 recommends the following values of minimum densit of evaporimeters • -rid Iones ) = station for ever :>,>>> sq.4m • Fumid Temperate Iones ) = station for ever $>,>>> sq.4m • Aold regions ) = station for ever =,>>,>>> sq.4m A t5pica! '5dro6meteoro!ogica! station 'as t'e o!!o*ing: • (ecording raingauge and non!recording raingauge • /tevenson bo5 with ma5imum, minimum, wet, and dr bulb thermometers • +ind anemometer and wind vane • Pan evaporimeter • /unshine (ecorder etc EMPIRI/A7 E8UATIONS <ost of the available empirical equations for estimating la4e evaporation are a 1alton tpe equation of the general form #1$ Me5er9s ,orm%!a #0$ Ro'*er9s ,orm%!a -ccounts for the effect of pressure in addition to the wind speed effect &ind Ve!ocit5 ,n the lower part of the atmosphere, up to a height of about $>>m above the ground level, wind velocit follows the one!seventh power law as ANA7-TI/A7 MET)ODS O, EVAPORATION ESTIMATION • +ater Budget <ethod • Energ Budget <ethod • <ass Transfer <ethod #1$ &ater .%dget Met'od can onl be measured. can onl be estimated. ,f the unit of time is 4ept ver large, estimates of evaporation will be more accurate. ,t is the simplest of all the methods, but the least reliable. #0$ Energ5 .%dget Met'od • ,t involves application of the law of conservation of energ • Energ available for evaporation is determined b considering the incoming energ, outgoing energ, and the energ stored in the water bod over a 4nown time interval • Estimation of evaporation from a la4e b this method has been found to give satisfactor results, with errors of the order of $;, when applied to periods less than a wee4 6igure@ Energ Balance in a water bod This is the energ balance in a period of = da. -ll energ terms are in calories& sq.mm&da. • ,f time periods are short, can be neglected as the are negligibl small • -ll terms e5cept can either be measured or evaluated indirectl • is estimated using Bowen2s ratio /OMPARISON O, MET)ODS • -naltical methods can provide good results. Fowever, the involve parameters that are difficult to assess. • Empirical equations can at best give appro5imate values of the correct order of magnitude. • ,n view of the above, pan measurements find wide acceptance in practice.