Diurnal evapotranspiration estimates in the Walnut River Watershed

摘要

Evapotranspiration is an essential component of the surface hydrological balance, but obtaining accurate estimates of the water vapor flux over large terrestrial areas can be difficult because of the substantial temporal and spatial variability in surface moisture conditions that can occur. This variability is often very large in the Great Plains and other portions of the Mississippi River Basin. Nevertheless, variations in soil moisture content, groundwater levels, and runoff in streams and rivers cannot be fully assessed without some knowledge of evapotranspiration rates. Here, observations made at the Walnut River Watershed (WRW), which is near Wichita, Kansas, and has an area of approximately 5000 km(sup 2), are used to improve and test a modeling system that estimates long-term evapotranspiration with use of satellite remote sensing data with limited surface measurements. The techniques may be applied to much larger areas. As is shown in Fig. 1, the WRW is located in the Red River Basin and is enclosed by the southern Great Plains Clouds and Radiation Testbed (CART) of the US Department of Energys Atmospheric Radiation Measurement (ARM) program. The functional relationships involving the satellite data, surface parameters, and associated subgrid-scale fluxes are modeled in this study by the parameterization of subgrid-scale surface (PASS) fluxes scheme (Gao, 1995; Gao et al., 1998), which is used in a modified and improved form (PASS2). The advantage of this modeling system is that it can make effective use of satellite remote sensing data and can be run for large areas for which flux data do not exist and surface meteorological data are available from only a limited number of ground stations. In this study, the normalized difference vegetation index (NDVI) or simple ratio (SR) and surface brightness temperature at each pixel for the WRW were derived from advanced very high resolution radiometers data collected by a ground station at Argonne National Laboratory from the National Oceanic and Atmospheric Administrations NOAA-12 and NOAA-14 satellites. The satellite data were subjected to atmospheric corrections for three intensive observation days of the 1997 Cooperative Atmosphere-Surface Exchange Study (CASES-97) experiment, which was conducted in cooperation with the Argonne Boundary Layer Experiments (ABLE) effort and the ARM Program.