RELATIONSHIPS BETWEEN EVAPORATIVE FRACTION AND REMOTELY-SENSED VEGETATION INDEX AND MICROWAVE BRIGHTNESS TEMPERATURE FOR SEMIARID RANGELANDS

Measurements of the microwave brightness temperature (TB) with the Pus hbroom Microwave Radiometer (PBMR) over the Walnut Gulch Experimental Watershed were made on selected days during the MONSOON 90 field campa ign. The PBMR is an L-band instrument (21-cm wavelength) that can prov ide estimates of near-surface soil moisture over a variety of surfaces . Aircraft observations in the visible and near-infrared wavelengths c ollected on selected days also were used to compute a vegetation index . Continuous micrometeorological measurements and daily soil moisture samples were obtained al eight locations during the experimental perio d. Two sites were instrumented with time domain reflectometry probes t o monitor the soil moisture profile. The fraction of available energy used for evapotranspiration was computed by taking the ratio of latent heat flux (LE) to the sum of net radiation (Rn) and soil heat flux (G ). This ratio is commonly called the evaporative fraction (EF) and nor mally varies between 0 and 1 under daytime convective conditions with minimal advection. (-)A wide range of environmental conditions existed during the field campaign, resulting in average EF values for the stu dy area varying from 0.4 to 0.8 and values of TB ranging from 220 to 2 80 K. Comparison between measured TB and EF for the eight locations sh owed an inverse relationship with a significant correlation (r(2) = 0. 69). Other days were included in the analysis by estimating TB with th e soil moisture data. Because transpiration from the vegetation is mor e strongly coupled to root zone soil moisture, significant scatter in this relationship existed at high values of TB or dry near-surface soi l moisture conditions. It caused a substantial reduction in the correl ation with r(2) = 0.40 or only 40% of the variation in EF being explai ned by TB. The variation in EF under dry near-surface soil moisture co nditions was correlated to the amount of vegetation cover estimated wi th a remotely sensed vegetation index. These findings indicate that in formation obtained from optical and microwave data can be used for qua ntifying the energy balance of semiarid areas. The microwave data can indicate when soil evaporation is significantly contributing to EF, wh ile the optical data is helpful for quantifying the spatial variation in EF due to the distribution of vegetation cover.