Net Precipitation under a Douglas-Fir Forest

Under dense stands of old-growth Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco) and associated species typical of Douglas-fir forests of western Oregon and Washington, throughfall averaged 76 percent of gross summer precipitation. Throughfall varied with storm size from near 0 percentin storms under 0.05 inch to about 82 percent in storms over 3 inches. Density of old-growth stands, which ranged from 75 percent to 92 percent, had some influence on interception. However, since estimates of density are not generally available, a relationship based on storm size was determinedto be more useful. A linear relation, which fits the data best, explained 96 percent of the variation in throughfall in summer months. Throughfall in winter months increased to an average of about 86.3 percent. A precise relationship with storm size was not determined, but in storms producing8 inches or more gross precipitation, throughfall was estimated to approach 96 percent. Stemflow was relatively unimportant for nearly all species. Weighted average stemflow measured in the 1959-60 water year was only slightly more than 0.27 percent of the total precipitation.