Potential evaporation is average annual Penman Potential Evapotranspiration for 1960-2006. It is interpolated from the potential evaporation data calculated at climate stations.
Actual evaporation is an estimated average annual value for 1960-2006. The values are calculated using AE = PE .* ( 1 + P/PE - (1+ (P/PE)^w)^(1./w) )whereAE = average annual actual evaporationPE = average annual potential evaporation (item 2 above)P = average annual precipitation (item 1 above)W = 4.35 (empirical constant value)