4 PRINCIPLE : Installed in the field Filled with a known quantity of water (the surface area of the pan is known and the water depth is measured)allowed to evaporate during a certain period of time (usually 24 hours).After 24 hours, the remaining quantity of water (i.e. water depth) is measuredThe amount of evaporation per time unit (the difference between the two measured water depths) is calculated; this is the pan evaporation: E pan (in mm/24 hours)The E pan is multiplied by a crop coefficient, Kc, to obtain the ETo.
5 FORMULA :ETo = KC × E pan ETo : reference crop evapotranspiration KC : crop coefficient E pan : pan evaporation
6 KUMULUR PAN-EVAPORIMETER DATA 1998 Month'sMean Monthly Pan Evaporation(in mm/day)January2.8258February3.5821March4.5968April5.1933May5.3613June6.74JulyAugust4.0129September4.2733October2.7581November1.7133December1.8097
8 Calculation Table: Month Mean Monthly Pan Evaporation (in mm/day) Epan KcMean Monthly Crop EvaporationEcEvapotranspiration for month( in mm)January2.82580.6052.56February3.58210.550.15March4.59681.15163.86April5.1933May5.36130.699.72June6.740.755.055151.65July0.80122.64August4.01290.85105.74September4.2733108.97October2.75810.9076.95November1.71330.9548.83December1.80971.0051.10∑ =