Modeling the impacts of urbanization on watershed-scale gross primary productivity and tradeoffs with water yield across the conterminous United States
The objective of this study was to examine the impacts of urbanization on gross primary productivity (GPP) and the interactions between carbon and water fluxes, including precipitation, evapotranspiration (ET), and water yield (Q). A water-centric ecosystem model, Water Supply Stress Index model (WaSSI) that operates at the 12- digit (81,900 watersheds) Hydrologic Unit Code (HUC) scale for the conterminous United States (CONUS) during 2000–2010, 2000–2050, and 2000–2100 was used. Linear regression and causal-based models were then applied to identify key factors controlling urbanization impact on GPP. Simulations of GPP patterns compared favorably with a global, 0.05-degree product of solar-induced chlorophyll fluorescence (SIF). We found that 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. Total GPP decreased from 6.81 Pg C yr−1 to 6.26 Pg C yr−1 for those watersheds affected by urbanization (~55,000). 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, while Q increased from 1.68 × 106 million m3 yr−1 to 1.74 × 106 million m3 yr−1 for urbanized watersheds alone (~55,000). Although total CONUS ΔGPP was less than 0.55 Pg C yr−1, or<8%, large changes (ΔGPP>300 g C m−2 yr−1) were found in 245, 1984, and 5655 of the 81,900 watersheds by 2010, 2050 and 2100, respectively. Overall, the impacts of urbanization on GPP in the CONUS were influenced by background climate, previous land cover characteristics, and the magnitudes of land use change. Effective integrated watershed management that attempts to minimize the negative ecological and environmental impacts of urbanization must consider regional hydrologic differences and fit local climatic and watershed conditions.