The use of a dimensionless unit hydrograph to derive unit hydrographs for some Pacific Northwest basins

A simple trial-and-error method of developing unit hydrographs from long-period storms by making use of a general dimensionless unit hydrograph is presented. The development of a general dimensionless unit hydrograph is based on approximately 20 unit hydrographs from the Willamette Valley in Oregon. A method of using the general dimensionless unit hydrograph to derive unit hydrographs from long-period storms and the drainage area of the basin is also presented. The method consists in first converting the dimensionless unit hydrograph into a unit hydrograph by using the drainage area of the basin and assuming a time base. This unit hydrograph is then applied to a long-period storm to produce a storm hydrograph. The synthetic storm hydrograph is then compared with the actual storm hydrograph. If they compare favorably, the unit hydrograph used is the unit hydrograph for the basin. If they do not compare favorably, another unit hydrograph is used, assuming a new time base. This process is repeated until a favorable comparison is obtained. The method is adaptable to the electronic computer and has been successfully tested on the IBM 650 computer.