LOCAL ENERGY FLUX ESTIMATES FOR UNSTABLE CONDITIONS USING VARIANCE DATA IN SEMIARID RANGELANDS

A network of meteorological stations was installed during the Monsoon '90 field campaign in the Walnut Gulch experimental watershed. The stu dy area has a fairly complex surface. The vegetation cover is heteroge neous and sparse, and the terrain is mildly hilly, but dissected by ep hemeral channels. Besides measurement of some of the standard weather data such as wind speed, air temperature, and solar radiation, these s ites also contained instruments for estimating the local surface energ y balance. The approach utilized measurements of net radiation (R(n)), soil heat flux (G) and Monin-Obukhov similarity theory applied to fir st- and second-order turbulent statistics of wind speed and temperatur e for determining the sensible heat flux (H). The latent heat flux (LE ) was solved as a residual in the surface energy balance equation, nam ely, LE = -(R(n) + G + H). This procedure (VAR-RESID) for estimating t he energy fluxes satisfied monetary constraints and the requirement fo r low maintenance and continued operation through the harsh environmen tal conditions experienced in semiarid regions. Comparison of energy f luxes using this approach with more traditional eddy correlation techn iques showed differences were within 20% under unstable conditions. Si milar variability in flux estimates over the study area was present in the eddy correlation data. Hence, estimates of H and LE using the VAR -RESID approach under unstable conditions were considered satisfactory . Also, with second-order statistics of vertical velocity collected at several sites, the local momentum roughness length was estimated. Thi s is an important parameter used in, modeling the turbulent transfer o f momentum and sensible heat fluxes across the surface-atmosphere inte rface.