Future Orchard Expansion May Decrease Groundwater Recharge and Increase Nitrate Contamination in An Intensively Cultivated Loess Critical Zone

Extensive cropland-to-orchard transition alters water flow and nitrate transport in the vadose zone (VZ) of the Earth’s Critical Zone (CZ), which may impact groundwater recharge and threaten future water quality from intensive nitrogen fertilizer application. Understanding the regional unsaturated water and nitrate fluxes and travel times in the deep VZ is crucial for the sustainable management of the groundwater system. Here, a regional-scale model was developed to estimate the recharge and nitrate transport in the cultivated loess CZ of China’s Guanzhong Plain (CGP), where cropland-to-orchard transition has been extensively promoted in the past few decades. Besides, uncertainties and sensitivities in estimated fluxes of water and nitrate induced by variations in soil hydraulic parameters (SHPs) were evaluated. A comparison between model simulations and observations at 12 sites exhibits good simulation performance. Comparing the measured SHPs, SHPs from Rosetta and Global SHPs products introduced 86.28% and 48.94% uncertainties in the simulation of nitrate leaching fluxes from cropland and orchard, respectively, as well as 44.76% uncertainties in the simulation of groundwater recharge fluxes from the orchard. Application over the CGP based on measured SHPs indicates that the central and eastern CGP were the hotspots for groundwater nitrate contamination. By comparing traditional cropland and orchard scenarios, simulations reveal that cropland-to-orchard transition results in a 39.3-fold increase in nitrate leaching fluxes and a 9.8% decrease in groundwater recharge fluxes. Modeled nitrate travel times through the deep VZ range between decades and centuries under both land use scenarios; however, the cropland-to-orchard transition would extend the time (~22.4 years) it takes for nitrate to reach the aquifer. Although cropland-to-orchard transition delays nitrate transport to the aquifer, the increased nitrate leaching flux will increase the risk of nitrate groundwater pollution, especially in areas with shallow VZs and coarse soil texture. This study provides valuable information for assessing the future vulnerability of groundwater resources under agricultural land use and management changes in the cultivated loess CZ.