Modeling irrigation strategy impacts on conjunctive surface–groundwater use in the semi-arid Sarab Basin, Iran

Water scarcity in the semi-arid Sarab Basin, exacerbated by population growth and inefficient irrigation, demands integrated water management. This study aims to apply a novel WEAP-HYDRUS-MODFLOW-MT3DMS framework linking all hydrological compartments to assess the quantity and quality impacts of irrigation modernization and management strategies in Urmia Lake Basin, Iran. Simulations for 2009–2018 evaluated sprinkler, 60% (S60) and 70% (S70) efficiency, and drip systems, 80% (D80) and 90% (D90) efficiency under two strategies: water conservation via demand management strategy (DMS) and agricultural development strategy (ADS) using saved water. Results show irrigation modernization (S60 to D90) under DMS increases streamflow from 84.3 mcm/yr to 112–131 mcm/yr, while groundwater levels rise by 1.5–2.4 m with TDS increasing by 1.3–2 mg/L. Conversely, ADS reduces streamflow to 74.5–79.8 mcm/yr, lowers groundwater levels by 0.6–1.2 m, and raises TDS by 0.9–1.9 mg/L. Surface water quality, with groundwater return flow considered, improves by 94–136 mg/L (DMS) and 25–100 mg/L (ADS). The study underscores irrigation modernization advantages under DMS while warning against reallocation in ADS. Findings emphasize policymakers should prioritize water constraints over land expansion for sustainable development in semi-arid regions like Iran, as irrigation modernization alone cannot restore Urmia Lake.