Estimating watershed evapotranspiration with PASS. Part I: Inferring root-zone moisture conditions using satellite data

A model framework for parameterized subgrid-scale surface fluxes (PASS) has been modified and applied as PASS1 to use satellite data, models, and limi ted surface observations to infer root-zone available moisture (RAM) conten t with high spatial resolution over large terrestrial areas. Data collected during the 1997 Cooperative Atmosphere-Surface Exchange Study field campai gn at the Atmospheric Boundary Layer Experiments site in the Walnut River w atershed in Kansas were used to evaluate applications of the PASS1 approach to infer soil moisture content at times of satellite overpasses during clo udless conditions. Data from Advanced Very High Resolution Radiometers on t he NOAA-14 satellite were collected and then adjusted for atmospheric effec ts by using LOWTRAN7 and local atmospheric profile data from radiosondes. T he input variables for PASS1 consisted of normalized difference vegetation index and surface radiant temperature, together with representative observa tions of downwelling solar irradiance, air temperature, relative humidity, and wind speed. Surface parameters, including roughness length, albedo, sur face conductance for water vapor, and the ratio of soil heat flux to net ra diation, were estimated with parameterizations suitable for the area using satellite data and land-use information; pixel-specific near-surface meteor ological conditions such as air temperature, vapor pressure, and wind speed were adjusted according to local surface forcing; and RAM content was esti mated using surface energy balance and aerodynamic methods. Comparisons wit h radar cumulative precipitation observations and in situ soil moisture est imates indicated that the spatial and temporal variations of RAM at the tim es of satellite overpasses were simulated reasonably well by PASS1.