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.
Hydrologic Engineering Center, U.S. Army Corps of Engineers
Pabst, Arthur; Kim, Kwonshik.
A Design Oriented Continuous Synthesis Model.
St. Anthony Falls Hydraulic Laboratory.
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