Synthesizing instantaneous unit hydrographs in rural, ungaged watersheds with readily available information

A statistically based procedure was developed to estimate instantaneous unit hydrographs (IUHS) for rural, ungaged watersheds using data from Alabama, South Carolina, and Tennessee. This synthesized IUH may be used to construct a design storm hydrograph for a precipitation event of selected magnitude and duration.

One hundred and twenty-two watersheds were used in this research. The physical and hydrologic characteristics of these watersheds are diverse, with watershed areas ranging from 0.17 to 518 square miles and main stream channel slopes from 3 to 237 feet per mile. Each watershed was characterized by fourteen parameters including: watershed area, main stream channel length and slope, elevation, mean annual precipitation, forest cover, storage, and others. Multiple linear regression was employed to develop nine equations to predict points on the IUH of these watersheds. These nine points include the time to peak flow, peak flow, the time to base flow, and the times to 25- 50- and 75-percent of peak flow on the rising and falling limbs. This analysis concludes that each of the nine points may be determined with simple multiplicative predictive equations containing two independent parameters: watershed area, and main stream channel slope.

Using the nine points these equations generate, one may readily construct the remaining portion of the IUH for an ungauged watershed. These predictive equations describe over 75 percent of the variation in an IUH, with standard errors ranging from 6.2 50.6 percent. Because the parameters needed for this technique are readily available either from topographic maps or other sources, this technique may be applied to ungaged watersheds in a relatively non-resource intensive manner.