A Design Oriented Continuous Synthesis Model

In the field of hydrology the "unit hydrograph" method of transforming an impulse of rain or snowmelt into a time distribution of runoff is widely used in the designing of hydraulic structures. While there are some features of the method that are subject to question, designers are usually sufficiently familiar with the procedure and its limitations to apply it as the prime hydrologic tool in designing large structures. Applications of the unit hydrograph procedure to a design problem usually involves the analysis of past storms and the resulting flood to arrive at a unit hydrograph. Usually a discrete storm is selected as opposed to complex storms consisting of numerous rain impulses. This would provide a unit hydrograph, but perhaps not an adequate evaluation of the soil moisture condition at the start of or during the storm or of its effect on the volume of storm or direct runoff. Thus it may be desirable to analyze moisture inputs and runoff from the basin or watershed under study for a selected period in advance of the storm of interest and so provide a quantitative measure of the soil moisture condition at the start of and during the storm. An antecedent precipitation index would be one possible measure of this variable. Obviously, the index or other method of analyzing the soil moisture condition should increase (or decrease) with precipitation or snowmelt and move in the opposite direction for dry periods.