Effects of Urbanization on Watershed Water Yield and Gross Primary Productivity in the Conterminous United States

Urbanization represents a permanent land-use change that has great implications for watershed ecosystem functions and services. We applied an ecohydrological model (WaSSI) to evaluate the likely impacts of projected land-use change (EPA ICLUS scenarios) on water and carbon balances across the lower 48 states in the U.S. for the periods of 2000, 2010, 2050, and 2100 at a 12-digit Hydrologic Unit Code (HUC) watershed scale. We found that although the simulated impact of future urbanization on mean change in water yield (ΔQ) was small at the national level, significant changes (ΔQ >50 mm) were found in 1,046 and 3,747 watersheds by 2050 and 2100, respectively. The total CONUS Q increased from 2.03×10⁶ million m³ yr^-1 in 2000, to 2.04×10⁶ million m³ yr^-1 in 2010, to 2.06×10⁶ million m³ yr^-1 in 2050, and 2.09 ×10⁶ million m³ yr^-1 in 2100. Total CONUS GPP declined from 8.68 Pg C yr^-1 in 2000, to 8.54 Pg C yr^-1 in 2010, to 8.36 Pg C yr^-1 in 2050, and to 8.13 Pg C yr^-1 in 2100. Although total ΔGPP was less than 0.55 Pg yr^-1, or <8%, large changes (ΔGPP >300 gC^-1 m^-2 yr^-1) were found in 245, 1,984, and 5,655 of the 81,900 watersheds by 2010, 2050 and 2100, respectively. Overall, the impact of evapotranspiration, water yield, and GPP in the CONUS were influenced by background climate, previous land cover characteristics, and the magnitude and extent of land-use change. This study provides an update on the effects of urbanization on water and carbon balances in natural ecosystems. Climate change that occurs alone with projected urbanization could alter these urbanization impacts. Effective national scale watershed management strategies must consider novel combinations of local climatic and land cover conditions to minimize negative hydrologic impacts of urbanization.