DAYTIME NET-RADIATION ESTIMATED FOR A SEMIARID RANGELAND BASIN FROM REMOTELY-SENSED DATA

The relationships between daytime averages of net radiation (R(n,dt)) and solar radiation (R(s,dt)) were investigated with data collected from the MONSOON 90 experiment conducted in a semiarid rangeland basin. Just as in many previous studies, a regression between R(n,dt) and R(s,dt) was performed. In an attempt to account for more of the variability in R(n,dt), soil moisture was included as an additional independent variable. The behavior of the ratio R(n,dt)/R(s,dt) was also investigated. It was found that both temporal and spatial variations in this ratio were mainly correlated with surface shortwave albedo and soil moisture. Measurements of microwave brightness temperatures from an L-band (21 cm wavelength) radiometer, sensitive to near-surface soil moisture, also showed a high correlation with R(n,dt)/R(s,dt). Estimates of R(s,dt) from the GOES-7 satellite were used with the soil moisture data to compute R(n,dt). The results indicate that improved estimates of basin-scale R(n,dt) for semiarid rangelands can be obtained by including soil moisture measurements. This may lead to more accurate regional-scale estimates of daily ET with operational methods using satellite-based measurements. Comparisons between R(n,dt) and R(s,dt) during the dry and wet seasons suggested that the variation in surface albedo and temperature are required in order to fully account for observed differences in equations predicting R(n,dt) and the ratio R(n,dt)/R(s,dt).