Clark Unit Hydrograph

The Clark method produces a unit hydrograph that theoretically has an infinite number of ordinates, because each ordinate on the tail of the unit hydrograph is computed by multiplying the preceding ordinate by a constant fraction. In both HEC-HMS and HEC-1, the tail of the unit hydrograph is truncated when the volume represented by the unit hydrograph exceeds 99.5% of the unit depth. In HEC-HMS the ordinates are then adjusted using a flow weighting scheme to produce a volume of exactly 100%. No adjustment was made in HEC-1.