An Integrated Hydrological Model for the Restoration of Ecosystems in Arid Regions: Application in Zhangye Basin of the Middle Heihe River Basin, Northwest China

River basins in or across arid regions have been facing intensified water scarcity and ecological problems, mainly due to the intense irrigated agriculture. Integrating the multiprocesses in hydrological cycle is quite necessary to make reasonable management strategies. In this paper, an integrated multiprocess hydrological model was proposed by coupling river water flow, groundwater flow, canal conveyance, and vadose water flow processes. It was applied to the Zhangye basin of middle Heihe River basin for searching management strategies to restore the ecosystems (i.e., ensure surface runoff into downstream and also recover local groundwater levels). The integrated model was calibrated and validated during 2005-2007 and 2008-2010, respectively. Simulation of groundwater levels (GWLs, 32 wells) and surface runoff both matched well with the observed values, with Nash and Sutcliffe model efficiency > 0.38 and R-2 > 0.57. Then various scenarios were designed with considering five alternatives of different farmland area decrease and three alternatives of groundwater exploitation. Responses of surface runoff and GWLs were predicted for 20 years. Surface runoff change was compared with the water diversion curve, and GWL recovery was also discussed. Results revealed that ecosystems could not be restored with current agricultural area, even shutting down groundwater abstraction for irrigation. A decrease of about 30% of farmland area and using surface river water instead of pumping groundwater for irrigation could satisfy water diversion demand with only a slight GWL decline. Furthermore, the extra irrigation with diverted surface water during nongrowing season could further lead to the recovery of GWLs while without causing negative effects on surface runoff.