Relations between topography and annual precipitation in western Oregon and Washington

Average annual precipitation in western Oregon and Washington varies from less than 20 inches to over 150 inches, in obvious general relation to the topographic influences of the Coast and Cascade mountain ranges. If these influences could be isolated and measured, it would result in better definition of the precipitation resource in the mountainous areas of sparse measurements. Testing of the parameters and methods used by investigators in other regions led to the development of simple indexes to terrain elevation and barrier elevation. Together with a latitude index, they explain most of the variation in precipitation among the more than 280 stations. The key to the method is in the empirical definition of the contours of ‘effective’ barrier elevation. The pattern of residual errors from the graphical correlation shows no need for the usual zone parameter.