Application and Validation of a MODIS-based Vegetation Transpiration Model Over the Southern Great Plains Using IHOP 2002 Data

The International H2O Project 2002 (IHOP 2002), which was conducted during May and June 2002, sought to better understand spatial and temporal variations in the water vapor field in the Southern Great Plains (SGP) of the United States. One fundamental influence on this water vapor field is transpiration. Data collected at six IHOP 2002 surface sites, which are representative of the crop and grassland environments typical of the SPG as a whole, were used to validate a remote sensing based vegetation transpiration model (VTM). The model uses several vegetation indices derived from remotely sensed data in conjunction with surface observation data, for example air temperature and incident solar radiation, to estimate transpiration as a function of gross primary production and water use efficiency. While VTM has been tested over forest environments, this is the first evaluation of the model over crop and grassland environments. Since these land use types represent a significant proportion earth's terrestrial surface including nearly 40 percent of the land cover of the contiguous United States, this research marks an important step toward modeling transpiration over a region that plays a critical role in numerous biogeochemical cycles on both regional and global scales. By comparing model output with observations, it was found that the VTM represented temporal trends reasonable. It was found that the modeled values for transpiration were consistently less than the total observed moisture flux. This is to be expected since the VTM model currently considers only transpiration neglecting the moisture stream due to evaporation. Research is ongoing to develop an evaporation component for the VTM model so that it is able to accurately describe all streams of moisture transfer to the atmosphere.