Exploring ground-based methods for the validation of remotely sensed evapotranspiration

Abstract

Accurate estimates of evapotranspiration (ET) are essential when calculating catchment-scale water balances. An open top chamber (OTC), a ground-based method for measuring actual ET, was used to validate modeled ET from earth observation MODIS and agrometeorological data. We studied and compared the modeled ET (ET_MODIS of two adjacent, quaternary catchments from the Bushman's River primary catchment, namely P10A and Q91C, representing light and heavy stocking density, respectively. Within each catchment, a relatively homogenous 1 km² pixel of grassland was selected for intensive study. Mean annual ET_MODIS results from 2009 to 2011 showed that P10A (light) was higher by 14 mm than Q91C (heavy). No significant difference was observed between hourly ET_MODIS and hourly ET_OTC. A good relationship was observed between daily ET_MODIS and ET_OTC (r² = 0.7065, p value < 0.01, df = 8) for each 1 km² pixel during the 2010/11 growing season. This confirmed that the OTC is a useful instrument to determine actual ET over grassland and short shrubland, and may be used in model validation.