EFFECT OF ANTECEDENT RAINFALL ON RUNOFF DURING LOW-INTENSITY RAINFALL

The objective of this study was to determine the relative significance of several rainfall characteristics of the occurrence and magnitude of runoff from small ( less than 300 ha) watersheds during very low-intensity rainstorms (maximum rainfall intensities less than 15 mm/hr). This was done by making hourly measurements of rainfall and runoff at 1-hr intervals for four years on five watersheds in western Oregon, U. S. A. The amount of runoff was computed for each rainfall event. Discriminant analysis indicated that the magnitude of rainfall during the 12 and 120 hrs preceding the events was more significant in determining the occurrence of runoff than either rainfall magnitude, average intensity or maximum intensity. The majority of rainfall events on all of the watersheds resulted in less than 25% runoff. The results suggest that in climates characterized by low-intensity rainfall, antecedent rainfall is more important in controlling the occurrence and amount of runoff than rainfall magnitude or intensity.