Computing infiltration using the Green and Ampt method includes computing the time to ponding. The time to ponding is the length of time required for the precipitation to saturate the soil so that surface runoff begins. In some circumstances, the precipitation rate may decrease after the ponding condition is satisfied. Once this happens, there may not be enough precipitation to maintain the ponding condition. When the precipitation rate increases again the calculations must be adjusted to resatisfy the ponding condition. HEC-1 did not perform this adjustment correctly and would convert all precipitation to excess during any time step when the adjustment was required. HEC-HMS correctly switches between ponded and unponded conditions.