Runoff Calculation on Semiarid Watersheds Using Infiltration Curves Determined from the Rotadisk Rainulator

Abstract

A method of determining runoff from ungaged semi-arid unit watersheds is presented. The method consists of using the realistic rotating disk rainfall simulator ( Rotadisk Rainulator) to measure the infiltration rate combined with data from recording raingages. The infiltration rate is expressed mathematically as a function of rainfall volume by using a non-linear correlation fit method. A runoff hydrograph for the unit watershed is then calculated using a computer program which essentially subtracts the infiltration from the rainfall volume as measured by existing recording rainfall records. This hydrograph determination is repeated for increasing increments of surface storage and detention. The final selection of storage and detention is based in part on direct measurement using the Rotadisk Rainulator, and using the resulting hydrographs combined with past experience of the hydrologist with gaged watersheds. The method was tested using four one-acre gaged watersheds on the Atterbury Experimental Watershed on the valley floor east of Tucson, Arizona. In general, the correlation between measured and calculated runoff was better than 0.9 over a three-year period for the four unit-watersheds tested.