Dependence of mean and peak streamflow on basin area in the conterminous United States

Basin area is the primary control on both mean and peak streamflows, but relationships vary with regional meteorological conditions. Rich historical data sets permit deduction of these relationships for most areas of the United States, revealing both expected and unexpected correlations. On log-log plots, mean flows for most basins with > 75 cm/y rainfall have unit slopes, with the y-intercept approximating the mean annual runoff. Lower slopes characterize regions where runoff is greatest at high topographic elevations, or where significant withdrawals for irrigation occur at lower elevations. Peak flows also correlate strongly with basin area, but the regressions for most regions have slopes ranging from 0.4 to 0.9, and y-intercepts that increase with increasing flood recurrence interval. The slopes on these log-log plots for peak flows are highest in cool regions with low sunshine and low evapotranspiration, and lowest where sunshine is abundant, evapotranspiration is high, and small convective storm cells are common. Effects of relief are small and inconsistent. Peak flows of small watersheds are huge compared to their mean flows, commonly being several thousand times greater, particularly in the USA midcontinent where the slopes for peak flows are low.