Hydrology of the North Cascades Region, Washington: 1. Runoff, precipitation, and storage characteristics

Abstract

The time and space distributions of measured precipitation and measured runoff and of spring storage, which is approximately equal to the subsequent summer runoff of snowmelt and stored groundwater, have been analyzed for the North Cascades region of Washington. Neither precipitation nor runoff shows a consistent relationship with altitude, chiefly because of precipitation‐shadowing effects in this region of high relief. The relationship between mean annual precipitation and altitude is improved considerably when a topographic mean altitude is used instead of the actual altitude of the weather station. However, the improvement is not sufficient to explain a number of discrepancies that still occur. Some of the scatter in a precipitation‐altitude plot appears to be due to a deficient precipitation catch by high‐altitude gages. When the dependence of runoff on precipitation was examined on an annual, seasonal, and monthly basis, it was found that only a few low‐altitude gages correlated well with basin runoff. In several instances, annual (water year) runoff could be better predicted from winter precipitation than from annual precipitation. Precipitation and runoff were tested for changes with time and both appeared to have been increasing slightly but not at a statistically significant level. When the summer (May–September) fraction of the total annual runoff is plotted against mean basin altitude, a consistent, physically reasonable, mildly curvilinear relationship is revealed. On the basis of this relationship, by using several drainages with diverse area‐altitude profiles the spring storage versus altitude distribution can be estimated for each basin. The area‐altitude distribution is used instead of the mean altitude in this calculation because of the apparent nonlinear relationship between runoff and altitude.