Do forests reduce flood risk
Flow has been monitored in subwatersheds at North Fork Caspar Creek since 1985, providing a record of peakflows before and after clearcut logging of second-growth coast redwood in 5 of 8 gaged subwatersheds. Comparison of peakflow magnitudes between the logged and forested subwatersheds showed an average increase of about 50%, a result that indicated that existing theory was inadequate to explain the mechanisms by which a forest affects storm flows. We hypothesized that rainfall interception by the forest canopy may be influencing peakflows, and began to measure rainfall both in clearings and under adjacent forest stands. Results showed that about 21% of even the largest rainstorms was trapped and evaporated by the forest canopy. The rainfall interception measurements, in combination with estimates of transpiration, then allowed calibration of rainfall-runoff models for pretreatment conditions. When the same models were applied to clearcut conditions, they closely predicted the observed response (see figure 1), and indicated that interception was responsible for about 2/3 of the effect and transpiration for about 1/3. Results of early phases of the study are already being used by the California Department of Forestry and Fire Protection to aid planning of timber harvest operations upstream of sites susceptible to flooding.
Forest Service Partners