Long-term groundwater dynamics affected by intense agricultural activities in oasis areas of arid inland river basins, Northwest China

Oasis areas of arid inland river basins in northwest China have been facing intensified water use conflicts between agricultural sector and eco-environmental systems since 1990s. The reduction of river water allocation to oasis has resulted in the undesirable declines of groundwater levels (GWLs) with the increase in irrigated area and groundwater pumping. Improving water management and restoring GWLs become a great concern for those areas. In this study, the middle oasis of Ideihe River basin (HRB) was selected as the representative case for such an endeavor. A three-dimensional groundwater flow model was established for the Zhangye basin, a sub-basin of HRB to obtain a better understanding of groundwater dynamics in middle oasis, particularly for investigating the effects of agricultural water use. A major advantage of this model is that the spatial and temporal recharge from irrigation has been described in details with considering the result obtained by an ago-hydrological model (SWAP-EPIC) simulation. The model was well calibrated and validated over the period of 1991-2010. Simulation of GWis matched well with the observed 20-year GWLs in the 50 wells. Then, spatiotemporal groundwater dynamics and groundwater budget were quantitatively analyzed for the Zhangye basin during 1991-2010. In particular, the modeling results revealed three different changing trends of GWLs based on the analysis of groundwater dynamics and budget for four representative zones. Results indicated that negative balance of groundwater was mainly caused by over exploitation of groundwater for irrigation, resulting in a GWL decline of 9 cm a(-1) in average and even 2 m decline in some years at local areas. The area with critical groundwater depth (e.g. < 5 m) has reduced about 30% in 2010 as compared to that in 1991. Finally, recommendations on how to restore GWLs were proposed with emphasis on irrigation water and land use adjustment and groundwater pumping control. Our results are expected to provide implications for recovering the groundwater status in oasis areas of inland river basins in arid northwest China.