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 (ETMODIS 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 ETMODIS results from 2009 to 2011 showed that P10A (light) was higher by 14 mm than Q91C (heavy). No significant difference was observed between hourly ETMODIS and hourly ETOTC. A good relationship was observed between daily ETMODIS and ETOTC (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.

Keywords: evapotranspiration, MODIS LAI, open top chamber