Defining watershed-scale evaporation using a normalized difference vegetation index

Monthly composites of the Normalized Difference Vegetation Indices (NDVI), derived from the National Oceanic and Atmospheric Administration's (NOAA) A dvanced Very High Resolution Radiometer (AVHRR), were transformed linearly into monthly evaporation rates and compared with detailed hydrologic-model simulation results for five watersheds across the United States. Model-simu lated monthly evaporation values showed high correlations (mean R-2 =.77) w ith NDVI-derived evaporation estimates. These latter estimates, used in a c lassical water balance model, resulted in equally accurate simulations of m onthly runoff than when the model was run to estimate monthly evaporation v ia soil moisture accounting. Comparison of NDVI-derived evaporation estimat es with pan data showed promise for transforming NDVI values into evaporati on estimates under both wet and water-limiting conditions without resorting to the application of any kind of calibrated hydrologic models.