Evaluation of daily evapotranspiration estimates from instantaneous measurements

Abstract Evapotranspiration on a regional scale is useful information for agricultural and hydrological practice and can be determined by combining remote sensing data with ground-based measurements. However, estimates of evapotranspiration using remote sensing data are usually instantaneous values and to obtain daily evapotranspiration at a regional scale, it is necessary to convert these values into daily totals. By using the data obtained during the HAPEX-MOBILHY experiment in southwestern France, two different methods of converting instantaneous evapotranspiration into daily totals were evaluated. The first method is based on the assumption that the diurnal course of evapotranspiration is similar to that of solar irradiance and can be approximated by a sine function. As a result, daily evapotranspiration can be obtained from one instantaneous measurement. The second method assumes that the evaporative fraction, defined as the ratio of the latent heat flux and the available energy flux, is constant during the daytime period. Hence daily evapotranspiration can be determined from the evaporative fraction and daily total available energy. Comparisons of estimated and measured daily evapotranspiration indicated that both the methods are accurate for cloud-free days. However, the first method is preferable for the purpose of estimating regional evapotranspiration using remote sensing data.