INFLUENCE OF COMPONENTS OF THE ADVECTION-ARIDITY APPROACH ON EVAPOTRANSPIRATION ESTIMATION

Several methods have been proposed to estimate areal evapotranspiratio n from common meteorological data. The advection-aridity approach is o ne such method which requires no site-calibrated parameters, and uses measurements at a single elevation which are commonly available from a meteorological station. The method is based on a complementary relati onship between actual and potential evapotranspiration which postulate s that a decrease in actual evapotranspiration will result in a comple mentary or symmetrical increase in potential evapotranspiration for a given energy input. In this paper, the advection-aridity equations wer e rewritten to isolate terms which account for available energy, advec tion or drying power of the air, and sensible heat flux. The advection -aridity method was tested on a daily basis with data for 43 days coll ected from a Bat, semi-arid grassland over a 4 month period. The influ ence of each of the terms was examined to determine its relative influ ence in generating the complementary or symmetrical structure between actual and potential evapotranspiration. Advection and sensible heat f lux were found to be very significant, whereas available energy was no t. A comparison between reference values of evapotranspiration E-en ob tained from the energy budget with measurements of net radiation, grou nd heat flux, and sensible heat fluxes obtained by the eddy correlatio n method, and evapotranspiration estimates from the advection-aridity method, E-aa, produced a small root mean square error of 13.1 W m(-2). However, significant bias was present in that E-aa overestimated E-en for large values of E-en, but underestimated E-en for small values. M easurement error and external energy sources not accounted for in the complementary relationship or the advection-aridity approach are discu ssed as potential causes of this bias.