Estimating Evapotranspiration from a Small Area on a Grass-Covered Slope Using the BBH Model of Soil Hydrology

Abstract

Evapotranspiration (ET) from a small area on a grass-covered slope with an inclination of 8.7 degrees was estimated using the bucket-with-a-bottom-hole (BBH) model of soil hydrology. The BBH model has seven parameters, two of which were determined by experiment. The other five parameters were identified by trial and error, based on the decision criterion that the root mean square (RMS) of the difference between the predicted and measured values of soil moisture contained in the active soil surface layer (ASSL) takes a minimum value. Estimates of ET made using the BBH model were compared to the measurements taken using the Bowen ratio method. If the measurements can be regarded as the true values, the results are summarized as follows: - The estimates of annual ET made using the BBH model are reliable, although they may be somewhat smaller than the true values. - The estimates of monthly ET made using the BBH model can be used in analyses of water balance if an accuracy of better than ±10% is not required. - The accuracy of estimating daily ET using the BBH model was about ±60%. The main characteristics of water balance on the grass-covered slope obtained using the BBH model were summarized as follows: - Only less than one quarter of precipitation was allotted to ET on the slope. - Spatial variation in ET within an area several meters in size on the grass-covered slope was shown to be rather large, especially in the summer months. During the winter months, however, it was insignificant. The variation in annual ET among the three observation points within the area was about 20% as great as their mean.