Evaluation of the temporal variability of the evaporative fraction in a tropical watershed

Abstract

Evaporation exhibits diurnal variation in response to the changes in the available energy at the land surface. This requires continuous measurements of evaporation to determine daily total evaporation. This is not feasible without sophisticated field equipment, which at the end, only provides field scale evaporation rates. Remote sensing methods are a good alternative but these give snapshot measurements. If the partitioning of available energy into the different surface fluxes can be assumed to be diurnally constant, then instantaneous remotely sensed measurements could be used to derive daily total evaporation. In situ evaporation measurements were obtained for about a year at a grassland and woodland site in the Lake Naivasha basin, Kenya. These measurements were used to test the validity of the diurnal constancy of the partitioning of the available energy, expressed as evaporative fraction, and the extrapolation of evaporation from instantaneous to daily totals. A good relationship between midday and average day evaporative fraction was obtained at the two sites. Estimated daily evaporation from midday evaporative fraction was within 10% of measured evaporation for both sites. The deviation reduced if evaporation is further integrated in time. The seasonal progression of evaporative fraction is gradual at both sites although grassland evaporative fraction responds faster to changes in rainfall and moisture availability. The results provide a basis for the determination of regional evaporation across a season in tropical watersheds if evaporative fraction is determined instantaneously at intermittent intervals of 5–10 days.