The runoff model states even though an area has high infiltration some runoff still exists. Additionally, the runoff model allows for small amounts of infiltration to occur despite steep slopes. The accepted runoff model requires field-testing based on slope in order to permit construction of BMP’s. No construction in areas over 20 percent slope is permitted. Whereas the infiltration model dictates no infiltration occurs on slopes over 25 percent.
Secondly, the infiltration model does not take initial abstractions into account. This includes surface storage by wetlands and water retention on leaves and buildings. Neither model accounts for evapotranspiration of rainfall. These conditions vary based on region, season, weather, and time of day. The Potential Infiltration Model is not designed to account for these temporal variables.
A possible solution to better map potential surface water infiltration is a compromise between the two models. Since most error in the infiltration model is derived from slope the inverse of the runoff model can determine where infiltration does or does not occur. Two separate intersections of steep slopes and impervious surfaces extracts restrictions in infiltration on the Runoff Model to indicate levels of potential infiltration.