Simulated Flood Frequency Response to Forest Cover Removal for an Upstream Watershed in Central Taiwan

Abstract

The effect of forest harvesting on flooding is a controversial subject. Determining such effects is constrained by the ability to directly measure how forest harvesting affects stormflow responses to extreme events. Stormflow caused by extreme rainfall or snowmelt events must be subjected to a frequency analysis to adequately describe them. In this study, the modified Peatland Hydrologic Impact Model (PHIM) was used to simulate forested and clearcut conditions in an upland firstorder watershed in central Taiwan. The results of simulations using 47 yr of precipitation records indicated that the overall daily average and maximum discharges were affected by clearing of upland forests in such watersheds; however, only the daily average discharge was significantly increased. In addition, the annual water yield increased by 11.2%, a difference that was statistically significant. Frequency analysis using the log-Pearson type Ⅲ distribution showed that quantiles of discharge for events of a small recurrence interval of approximately 5 and 2 yr after removal of the forest cover appeared to significantly differ from that of the original forested conditions for the daily average and maximum discharges, respectively. Since the frequency analysis showed only small differences in quantiles for before and after forest removal for events with a large recurrence interval, the effects of forest cover on large floods in central Taiwan are considered negligible.