Evaluation of actual Evapotranspiration from Agricultural catchments, Kenya
Actual evapotranspiration was evaluated using three models viz water balance, Morton and Grindley. The catchments used in the study were Oruba, Tugenon, Ndarugu and Kimakia each having an area less than 100 sq. km. All the catchments lie in the humid regions of Kenya. The first three catchments are chiefly vegetated with pasture, annual and perennial crops whereas Kimakia is largely under forest. The water balance method was implemented using daily rainfall data and runoff hydrographs in a manner that allowed the estimation of actual evapotranspiration as the difference between rainfall and runoff over a period of some days. The period in days was determined from the receding limbs of the daily runoff hydrograph assuming the linearity of runoff processes. The Morton model was implemented as documented in the journal of hydrology (Netherlands) by Morton (1983) and all the calculations were done on a monthly basis. Grindley model was implemented as documented in the text "Hydrology in practice" by Shaw (1984). This model in essence is the extension of Penman root constant concept and therefore proceeds with firstly computing the Penman potential evapotranspiration values. The potential values are reduced to actual values by involving soil moisture deficits and root constants for specified vegetations. Computation of the above mentioned quantities in the Grindley model were done on a ten-day time interval as prescribed by Grindley and then converted to monthly values. The estimates of actual evapotranspiration by the three methods were compared basing the water balance estimates as the standard ones. The annual evapotranspiration by the water balance method ranged from 958.1 mm to 1352.1 mm while the coefficients of variation varied between 0.05 and 0.08 in all the catchments. The results indicated that the Grindley model tended to overestimate the actual evapotranspiration such that the estimates were either equal or close to Penman potential evapotranspiration values in all the catchments. Morton model performed better and actual evapotranspiration estimates by the method, though marginally higher, were closer to the water balance based estimates. The study, therefore, recommends that Morton model may be used in the evaluation of the evapotranspiration component of the water balance. The additional merit of the method lies in its ability to provide the estimates of actual evapotranspiration solely on meteorological data, which are readily available in Kenya.