Modelling the potential role of forest thinning in maintaining water supplies under a changing climate across the conterminous United States

The goal of this study was to test the sensitivity of water yield to forest thinning and other forest management/disturbances and climate across the conterminous United States (CONUS). Leaf area index (LAI) was selected as a key parameter linking changes in forest ecosystem structure and functions. We used the Water Supply Stress Index model to examine water yield response under 18 scenarios that combine hypothetical LAI changes (+10%, ±20%, 50%, and 80%), uniform increases in temperature (+1 °C and +2 °C) and precipitation change (±10%), and four climate change scenarios projected by general circulation models (GCMs) for the year 2050. Approximately 2100 large basins produced approximately 2003 billion cubic metres of water annually from 2002 to 2007. Forest lands covered 23% of the land surface area, but contributed 43% of the total water yield for the CONUS. As a whole, water yield increased by 3%, 8%, and 13% when LAI was reduced 20%, 50%, and 80%, respectively, while water yield decreased by 3% when LAI increased by 20%. Temperature increases of 2 °C alone could decrease water yield by 11%. A reduction of precipitation by 10% and 20% could result in a decrease of water yield by 20% and 39%, respectively. The direction and magnitude of water yield response to the combinations of LAI (+10%), climate warming (+1 °C), and precipitation change (±10%) were dominated by the change in precipitation. Climate change projected by the four GCMs (CSIROMK2 B2, CSIROMK3.5 A1B, HADCM3 B2, and MIROC32 A1B) resulted in a large change in water yield (+18% to 64%) by 2045–2055 when compared with the baseline. A 50% reduction in forest LAI under the four GCMs scenarios could greatly mitigate or exacerbate future climate change impacts on water yield in forest-dominated watersheds with high precipitation. This study provides the first quantitative estimate of the effects of forest thinning options on water yield under future climate across the CONUS. Effective forest water management for climate mitigation should focus on those watersheds identified. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.