Land use and land cover changes (LULCC) associated with urbanization affect watershed functions and services through fundamental alteration of biogeochemical cycles. Quantifying the potential changes in water, carbon, and energy due to urbanization helps sustainable city planning and integrated watershed management. We hypothesize that ‘impacts of urbanization do not create equal’. We conducted two case studies in the US and China at watershed to national scales to show how urbanization affect watershed hydrology, meteorology, and Gross Primary Productivity (GPP). We used both empirical data and ecohydrological models including WaSSI, SWAT, and MIKE SHE. We used stepwise regression and geographically weighted regression models to assess the variable impacts of urbanization on watershed water and carbon balances across a large disturbance and climatic gradient. We show that LULCC may overwhelm the impacts of climate warming on hydrology and urban microclimate (Urban Heat Island and Urban Dry Island) in the humid southern China. We found that the impacts of urbanization on both water yield and GPP are more pronounced in the area with high precipitation and forest covers. The magnitude of changes in ecosystem functions were influenced by many factors such as the background climate (high precipitation vs. low precipitation), previous land use and cover types and land use and cover changes, and the magnitude of urbanization (change in % impervious surface). We conclude that effective environmental management measures and strategies such as maintaining forest vegetation and wetlands to mitigate the negative effects must be designed to fit local watershed conditions.

Quantifying species-specific tree transpiration across watershed zones is important for estimating watershed evapotranspiration (ET) and predicting drought effects on vegetation. The objectives of this study are to 1) assess sap flux density (Js) and tree-level transpiration (Ts) across three contrasting zones (riparian buffer, mid-hillslope, and upland-hillslope), 2) determine how species-specific Js responds to vapor pressure deficit (VPD), and 3) compare watershed-level transpiration (Tw) derived from each zone. We measured Js and Ts in eight tree species in the three zones in a 12-ha forested watershed. In the dry year of 2015, loblolly pine (Pinus taeda), Virginia pine (Pinus virginiana), and sweetgum (Liquidambar styraciflua) Js rates were significantly higher in the buffer when compared to the other two zones. In contrast, Js in tulip poplar (Liriodendron tulipifera) and red maple (Acer rubrum) were significantly lower in the buffer than in the mid-hillslope. Daily Ts varied by zone and ranged from 10 to 93 liters in the dry year and 9 to 122 liters in the wet year. Js responded nonlinearly to VPD in all trees and zones. Annual Tw based on scaled-Js data was 447 mm, 377 mm, and 340 mm for the buffer, mid-hillslope, and upland-hillslope, respectively. We conclude that large spatial variability in Js and scaled Tw were driven by differences in soil moisture at each zone and forest composition. Consequently, spatial heterogeneity of vegetation and soil moisture must be considered when accurately quantifying watershed level ET.