Rates and Implications of Rainfall Interception in a Coastal Redwood Forest

Throughfall was measured for a year at five-min intervals in 11 collectors randomly located on two plots in a second-growth redwood forest at the Caspar Creek Experimental Watersheds. Monitoring at one plot continued two more years, during which stemflow from 24 trees was also measured. Comparison of throughfall and stemflow to rainfall measured in adjacent clearings indicates throughfall and stemflow accounted for 75.1 and 2.5 percent, respectively, of annual rainfall, while 22.4 percent was intercepted and evaporated by the forest canopy. Average interception loss remains above 20 percent even for the largest storms monitored. Models that predict pre-logging peakflows from below-canopy rainfall suggest that altered interception and transpiration could account for the 54 to 70 percent average increases in peakflow observed in five gauged watersheds for two years after clearcutting. Results such as these can be used to estimate the influence of interception loss on landslide frequency at sites for which relationships between landslide frequency and storm rainfall have been defined.