VARIABILITY IN SURFACE-ENERGY FLUX PARTITIONING DURING WASHITA-92 - RESULTING EFFECTS ON PENMAN-MONTEITH AND PRIESTLEY-TAYLOR PARAMETERS

During the Washita '92 field experiment, the local surface energy bala nce was evaluated at four locations in the USDA-ARS Little Washita Riv er Watershed near Chickasha, OK, using the Bowen ratio-energy balance (BREB) approach, For any given day, differences in the partitioning of the available energy appeared to be mostly a function of the type of vegetation at the site, while the actual magnitude of the fluxes was m ostly affected by cloud cover, The soil surface was initially wet, and gradually dried during the field experiment, However, there was not a corresponding decrease in the evaporative fraction, which would have indicated a decreasing contribution of soil evaporation to the total l atent heat flux. Ground weather data indicated a large shift in the di rection and magnitude of the surface winds, and a significant increase in air temperature and vapor pressure deficit. During this period, th e evaporative fraction actually increased at two of the four sites. Th e response of the different sites to the changing near-surface atmosph eric conditions was studied in more detail by evaluating the canopy re sistance (r(c)) to evaporation using the Penman-Monteith equation and the Priestley-Taylor parameter (alpha), Midday averages of r(c) and (a lpha) tended to decrease (increase) with increasing vapor pressure def icit for two of the sites while such a trend was not evident for the o ther two sites, Estimates of stomatal resistances indicated that signi ficant plant physiological differences existed between the sites conta ining weedy vegetation versus the grasses at the pasture/rangeland sit es. Even though soil moisture conditions were relatively wet, alpha wa s less than 1 at all sites and there was no trend in alpha as a functi on of surface soil moisture conditions, These findings suggest that ve getation types in mixed agricultural/rangeland ecosystems can have sig nificantly different responses to similar atmospheric forcing conditio ns.