Can reduced forest density increase resilience to severe drought in municipal water supply watersheds across a gradient of urbanization?

Droughts are expected to become increasingly frequent and severe due to increasingly extreme precipitation patterns. In the Southeastern US, trends indicate a shift toward large precipitation events and longer, drier, interstorm periods. Due to the importance of forest land use to regional water supply, forest management might stabilize or increase forest water yield under; however, urban development might constrain the effectiveness. We investigated to role of forest management (20% reduction in biomass) in mitigating extreme drought across forested and developed water supply watersheds to determine whether there are thresholds at which forested land use no longer effects water yield. Experimental droughts included a. removing all precipitation from June-August or b. reducing precipitation totals by 20% year-round. Land use in the forested, mountain watershed is expected to remain stable (>96% forested), while urban developed land use in the developed watershed is expected to approach 50% by 2060. Using 2060 land use and climate projections, we found that a 20% reduction in precipitation reduced annual water yield 27-47% in the forested watershed and 38-40% in the developed watershed over five simulation years. When thinning treatments were applied, the reductions were 9-21% in the forested and 23-28% in the developed watershed. Removing summer precipitation decreased water yield 12-53% in the forested and 35-63% in the developed watershed. With thinning, these reductions were 2-13% in the forested and 9-47% in the developed watershed. Our study suggests that maintaining low-density forests might increase municipal water supply resilience to drought.