Hydrologic models are useful management tools for assessing water resources solutions and estimating the potential impact of climate variation scenarios.  A comprehensive understanding of the water budget components and especially the evapotranspiration (ET) is critical and often overlooked for adequate model performance, not only within the calibration and validation periods, but also when predicting future climate responses.  This study was conducted on the South Fork watershed of the Iowa River in central Iowa, U.S.A.  The watershed covers about 780 km² (193,000 ac) and is one of the Benchmark Watersheds of the USDA Conservation Effects Assessment Project (CEAP).  Daily measurements of ET values on corn and soybean fields through three growing seasons were used along with MODIS NDVI product to spatially estimate ET throughout the study area for the Soil and Water Assessment Tool (SWAT) model calibration.  This research provides insight of the value of remote sensing and field observations for the application of a physically-based watershed models.