Predicting runoff producing areas and corresponding risks is important for protecting water quality from nonpoint source pollution. However, the currently proposed engineering methods to do this do not account for antecedent soil wetness status, which may substantially impact risk estimates, especially where variable source area hydrology is a dominate storm runoff process. In this study, I developed a bivariate approach to estimate spatially-distributed risks of runoff production by incorporating both rainfall and antecedent soil moisture conditions into a method based on the Natural Resource Conservation Service-Curve Number equation. I used base flow immediately preceding storm events as an index of antecedent soil wetness status. Using the data from a study hillslope near Ithaca, NY, I demonstrated that my estimates agreed with independent field-observations. I further applied the proposed approach to the Upper Susquehanna River Basin and mapped predicted saturated areas with a Geographic Information System using a Soil Topographic Index.