Estimating watershed evapotranspiration with PASS. Part II: Moisture budgets during drydown periods

The second part of the parameterization of subgrid-scale surface fluxes mod el (PASS2) has been developed to estimate long-term evapotranspiration rate s over extended areas at a high spatial resolution by using satellite remot e sensing data and limited, but continuous, surface meteorological measurem ents. Other required inputs include data on initial root-zone available moi sture (RAM) content computed by PASS1 for each pixel at the time of clear-s ky satellite overpasses, normalized difference vegetation index (NDVI) from the overpasses, and databases on available water capacity and land-use cla sses. Site-specific PASS2 parameterizations evaluate surface albedo, roughn ess length, and ground heat flux for each pixel, and special functions dist ribute areally representative observations of wind speed, temperature, and water vapor pressure to individual pixels. The surface temperature for each pixel and each time increment is computed with an approximation involving the surface energy budget, and the evapotranspiration rates are computed vi a a bulk aerodynamic formulation. Results from PASS2 were compared with obs ervations made during the 1997 Cooperative Atmosphere-Surface Exchange Stud y field campaign in Kansas. The modeled diurnal variation of RAM content, l atent heat flux, and daily evapotranspiration rate were realistic in compar ison to measurements at eight surface sites. With the limited resolution of the NDVI data, however, model results deviated from the observations at lo cations where the measurement sites were in fields with surface vegetative conditions notably different than surrounding fields. Comparisons with airc raft-based flux measurements suggested that the evapotranspiration rates ov er distances of tens of kilometers were modeled without significant bias.