This paper establishes a novel approach to estimate monthly and annual direct runoff by combining the curve number method of the Natural Resources Conservation Service with an exponential distribution of rainfall depths. The approach was tested against observed rainfall and runoff for 544 watersheds throughout the contiguous United States. For more than half of the watersheds, the performance of the new approach is indistinguishable from the application of the method to daily rainfall when curve numbers are determined via calibration. For all watersheds, the uncertainty introduced by the approximation of the distribution of rainfall depths is far less than the uncertainty associated with the use of tabulated curve numbers based on soil and land-cover characteristics. The new approach does not appreciably increase the overall uncertainty associated with the application of the curve number method in ungauged watersheds. The approach provides reasonable estimates of monthly and annual direct runoff that can inform land-management decisions when daily rainfall records are unavailable
