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 fr om 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 variabili ty 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 investig ated. It was found that both temporal and spatial variations in this r atio were mainly correlated with surface shortwave albedo and soil moi sture. Measurements of microwave brightness temperatures from an L-ban d (21 cm wavelength) radiometer, sensitive to near-surface soil moistu re, 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 dat a to compute R(n,dt). The results indicate that improved estimates of basin-scale R(n,dt) for semiarid rangelands can be obtained by includi ng 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 an d temperature are required in order to fully account for observed diff erences in equations predicting R(n,dt) and the ratio R(n,dt)/R(s,dt).