Basin-scale multi-objective simulation-optimization modeling for conjunctive use of surface water and groundwater in northwest China

Abstract. In the arid inland basin of China, the long-term unregulated agriculture irrigation from surface water diversion and groundwater abstraction has caused unsustainability of water resources and degradation of ecosystems. This requires integrated management and conjunctive use of surface water (SW) and groundwater (GW) at basin scale to achieve scientific decision supports for water resources allocation in China. This study developed a novel multi-objective simulation-optimization (S-O) framework for sustainably conjunctive use of SW and GW in Yanqi Basin (YB), a typical arid region with intensive agricultural irrigation in northwest China. The S-O model integrates the new epsilon multi-objective memetic algorithm (e-MOMA) with the MODFLOW-NWT based simulation model for examining the hydraulic interactions between SW and GW. Four conjunctive management objectives, involving maximizations of total water supply rate, groundwater storage change and surface runoff inflow to Bosten Lake, and minimization of total water delivery cost, were considered to explore the tradeoffs between socioeconomic development and environmental demands. The combined multi-objective SW and GW management model can achieve the tradeoffs in high-order objective spaces by considering groundwater abstraction in the irrigation districts and surface water diversion from the river, so as to avoid the prevalence of decision bias caused by the low-dimensional optimization formulation. Decision-makers are then able to identify their desired water use schemes with preferred objectives in the post-optimization and achieve maximal socioeconomic and ecological benefits. Furthermore, three representative runoff scenarios under changing climatic conditions were specified to quantify the influence of decreasing runoff in Kaidu River on the YB water management. Results show that runoff reduction would be of great negative impact on the total water supply, surface runoff inflow to the lake and regional groundwater storage in the aquifer. Therefore, the integrated SW and GW management is of critical importance for the protection of the fragile hydro-ecosystem under changing climatic conditions.