Influences of forests on water flows from headwater watersheds in Taiwan

Abstract

Forests cover 59% of Taiwan island where disastrous floods, landslides and debris flows occur often due to heavy typhoon-season rainstorms, steep terrain, fragile geologic formations and frequent earthquakes. This study evaluated the hydrologic influences of forests on Taiwan’s headwaters watersheds and supports the century-old policy of designating protection forests for streamflow regulation and soil conservation. Despite rainfall intensity that often exceeds 100 mm/h overland flow rarely occurs on Taiwan’s permeable forest soils. High evapotranspiration totaling 800–1200 mm annually contributes to reduced streamflows. In Taiwan forests reduce stream sedimentation from landslides by enhancing slope stability with roots and protect water quality by minimizing stream temperature fluctuation, regulating nutrient concentration and filtering contaminants. Floods in Taiwan are mainly caused by heavy rainstorms exceeding 250 mm and are not significantly affected by the currently low level of annual forest removal. Rapid urbanization of some forested watersheds may cause increased peak flows and decreased low flows due to significantly reduced soil infiltration capacities. Forests’ influences are minimal on landslides, debris flows or floods caused by extreme natural events such as the 7.3 Richter-scale earthquake in September 1999 or the rainstorms exceeding 1000 mm during Typhoon Herb in August 1996.