Evaluation of the use of remotely sensed evapotranspiration estimates into AnnAGNPS pollution model

The utilization of evapotranspiration (ET) estimates, derived from satellite remote sensing, into the Annualized Agricultural Non-Point Source (AnnAGNPS) pollution model was investigated. Modifications within AnnAGNPS were performed to allow the internal calculations of ET based on climate parameters, collected from ground-based measurements, to be replaced by ET estimates produced with the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor. Direct comparison of these sources of ET indicated that AnnAGNPS calculated ET estimates were 35% higher than the MODIS-calculated ET estimates resulting in a 10% increase in runoff at the outlet of the Yalobusha River Watershed for 2004. Simulated monthly runoff values based on MODIS and AnnAGNPS ET estimates for the year 2004 were compared with observed values from stream gages indicating slightly better agreement from the simulation using MODIS-derived ET. Analysis of the spatial distribution of the percent difference in runoff between AnnAGNPS-based ET and MODIS-based ET simulations showed higher differences at the most downstream and upstream of the watershed. These differences could be attributed to different soil properties or to the effect of distant field/weather station assigned to these locations. Although these findings should not be generalized to watersheds located in different ecosystems, the results indicate that the utilization of remotely sensed ET in hydrological modelling is feasible and that use of MODIS to estimate ET provided a more comprehensive spatial coverage than the often available through ground-based measurements. Copyright (C) 2010 John Wiley & Sons, Ltd.