RUNOFF SIMULATION SENSITIVITY TO REMOTELY-SENSED INITIAL SOIL-WATER CONTENT

A variety of aircraft remotely sensed and conventional ground-based me asurements of volumetric soil water content (SW) were made over two su bwatersheds (4.4 and 631 ha) of the U.S. Department of Agriculture's A gricultural Research Service Walnut Gulch experimental watershed durin g the 1990 monsoon season. Spatially distributed soil water contents e stimated remotely from the NASA push broom microwave radiometer (PBMR) , an Institute of Radioengineering and Electronics (IRE) multifrequenc y radiometer, and three ground-based point methods were used to define prestorm initial SW for a distributed rainfall-runoff model (KINEROS; Woolhiser et al., 1990) at a small catchment scale (4.4 ha). At a med ium catchment scale (631 ha or 6.31 km2) spatially distributed PBMR SW data were aggregated via stream order reduction. The impacts of the v arious spatial averages of SW on runoff simulations are discussed and are compared to runoff simulations using SW estimates derived from a s imple daily water balance model. It was found that at the small catchm ent scale the SW data obtained from any of the measurement methods cou ld be used to obtain reasonable runoff predictions. At the medium catc hment scale, a basin-wide remotely sensed average of initial water con tent was sufficient for runoff simulations. This has important implica tions for the possible use of satellite-based microwave soil moisture data to define prestorm SW because the low spatial resolutions of such sensors may not seriously impact runoff simulations under the conditi ons examined. However, at both the small and medium basin scale, adequ ate resources must be devoted to proper definition of the input rainfa ll to achieve reasonable runoff simulations.