Nonlinear groundwater influence on biophysical indicators of ecosystem services

Groundwater is a fundamental control on biophysical processes underpinning essential ecosystem services (ES). However, interactions and feedbacks among groundwater, climate and multiple ES remain less well understood. We investigated groundwater effects on a portfolio of food, water and biogeochemical ES indicators in an urbanizing agricultural watershed. Our results show that food production, water quality and quantity, and flood control are most sensitive to groundwater, with the strongest responses under wet and dry climate extremes. Climate mediates groundwater effects, such that several ES have synergies during dry climate, but trade-offs (groundwater increased some ES but declined others) under wet climate. There is substantial spatial heterogeneity in groundwater effects on ES, which is driven primarily by water table depth (WTD) and is also sensitive to soil texture and land cover. Most ES indicators respond nonlinearly to WTD when groundwater is within a critical depth (approximately 2.5 m) of land surface, indicating that small WTD changes can have disproportionately large effects on ES in shallow groundwater areas. Within this critical WTD, increasingly shallow groundwater leads to nonlinear increases in surface flood risk, sediment erosion and phosphorus yield; nonlinear decreases in drainage to the deep vadose zone and thus groundwater recharge; and bidirectional responses of crop and grass production, carbon storage and nitrate leaching. Our study illustrates the complex role of groundwater in affecting multiple ES and highlights that strategically managing groundwater may enhance ES resilience to climate extremes in shallow groundwater settings.