Equations were developed to estimate the magnitude of floods for various occurrence frequencies at ungaged sites on ephemeral streams that drain small, relatively undeveloped basins in the semiarid part of eastern Washington. The equations were developed from regression analyses that used the logarithms of the longitude indexes of gaged sites, forest cover, and drainage areas of the upstream basins as independent variables and the logarithms of the discharges for selected exceedance-probabilities at the gaged sites were used as dependent variables. These equations may be used to estimate floodflows with probabilities of being exceeded in any year of 50, 20, 10, 4, 2, and 1 percent in natural-flow ephemeral streams that drain relatively undeveloped basins with areas and forest covers less than about 40 square miles and 30 percent, respectively. The standard errors of estimate for the equations range from 57 to 100 percent. Equations that use longitude index as a surrogate for a precipitation index more accurately estimate floods than do equations that use mean annual precipitation, which, in turn, are better than the equations that use the 24-hour precipitation with an exceedance probability of 50 percent. (Kosco-USGS)
