PUSH BROOM MICROWAVE RADIOMETER OBSERVATIONS OF SURFACE SOIL-MOISTUREIN MONSOON 90

The push broom microwave radiometer (PBMR) was flown on six flights of the NASA C-130 to map the surface soil moisture over the U.S. Departm ent of Agriculture's Agricultural Research Service Walnut Gulch experi mental watershed in southeastern Arizona. The PBMR operates at a wavel ength of 21 cm and has four horizontally polarized beams which cover a swath of 1.2 times the aircraft altitude. By flying a series of paral lel flight lines it was possible to map the microwave brightness tempe rature (T(B)), and thus the soil moisture, over a large area. In this case the area was approximately 8 by 20 km. The moisture conditions ra nged from very dry, <2% by volume, to quite wet, >15%, after a heavy r ain. The rain amounts ranged from less than 10 mm to more than 50 mm o ver the area mapped with the PBMR. With the PBMR we were able to obser ve the spatial variations of the rain amounts and the temporal variati on as the soil dried. The T(B) values were registered to a Universal T ransverse Mercator grid so that they could be compared to the rain gag e readings and to the ground measurements of soil moisture in the 0- t o 5-cm layer. The decreases in T(B) were well correlated with the rain fall amounts, R2 = 0.9, and the comparison of T(B) with soil moisture was also good with an R2 of about 0.8. For the latter, there was some dependence of the relation on location, which may be due to soil or ve getation variations over the area mapped. The application of these dat a to runoff forecasts and flux estimates will be discussed.