Depressional areas (e.g., sinks, wetlands, potholes) are naturally-occurring features in many landscapes. During runoff events the runoff volume reaching a depressional area is not contributed downstream until the runoff volume exceeds the depressional area’s volume. If the runoff volume does not exceed the depressional area volume, the area is categorized as “non-contributing”. This determination is dependent on the size of the runoff event analyzed. The 10-year, 24-hour precipitation event is often used to evaluate non-contributing areas for water quality purposes. The non-contributing determination is performed using a series of iterative GIS processes in which the available storage of a depressional area was compared to the runoff volume generated from the contributing watershed of the depressional area. This is an iterative “fill and spill” process in which the excess runoff of contributing areas is routed through subsequent downstream depressional areas until no excess runoff is produced. This process results in a hydrologically conditioned DEM that accounts for non-contributing areas. All depressional areas determined to be contributing are “filled” by adjusting their elevation values to equal the surface spill out elevation to create a continuous flow path that traverses the depressional area. Flow paths terminate at the minimum elevation cell within each non-contributing depressional area.