Variability of streamflow and precipitation in Washington

Patterns of variation in monthly records of 55 streamflow stations and 38 precipitation stations were evaluated using chronologies of cumulative departure of monthly values from mean-monthly values. The cumulative departures for each station were rescaled (standardized) by dividng them by the standard deviation of the annual values for that station. The degree of similarity between the chronologies for each pair of either streamflow or precipitation stations was measured by the coefficient of correlation between them. Matrices of these coefficients of correlation were evaluated using cluster analysis to identify regions of similarity among the chronologies. The cluster analysis of the streamflow-station matrix defined geographic regions of similarity in south- western, northwestern, and northeastern Washington. The cluster analysis of the precipitation-station matrix defined two regions of similarity--western and eastern Washington-- and indicated that there is a tendency for persistent periods of above-average precipitation in western Washington to correspond with persistent periods of below-average precipitation in eastern Washington, and conversely. A water- resource availability index was developed that provides a quantitative interpretation of present conditions in the context of historical records of streamflow and precipitation. The index value for a given month and year is calculated as the summation of time-weighted, rescaled, monthly departures during the preceding 3-year period. Methods were described for the use of chronologies of cumulative departure (1) in estimating monthly streamflow or precipitation values for ungaged sites from monthly values for gaged sites, and (2) in examining the adequacy of the sizes of existing or proposed reservoirs.