Shahre Rey Plain, as one of the agricultural hubs of Tehran Province, is facing a crisis of declining groundwater levels, land subsidence, and low water productivity. This study aims to evaluate the effects of a water market on cropping patterns, economic water productivity, and farmers' incomes in this plain. To this end, farmers' behavior was simulated using a Positive Mathematical Programming (PMP) model calibrated with the Shannon entropy method. Data on cultivated area, yield, product prices, and production costs for the 2022–2023 cropping year were collected from the Tehran Province Agricultural Jihad Organization, and the model was implemented in the GAMS environment under three scenarios: (1) baseline (existing conditions), (2) water pricing at 8,000 IRR/m³ (based on full-cost pricing), and (3) a water market with endogenous equilibrium prices. The results showed that under baseline conditions, water consumption is 200 million cubic meters, gross profit is 1,000 billion IRR, and water productivity is 5,000 IRR/m³. The pricing scenario reduced water consumption by 15 Percent and profit by 11 Percent, while increasing productivity to 5,235 IRR/m³. In the water market scenario, the equilibrium price was estimated at 10,800 IRR/m³, with a 5% reduction in water consumption, a 5.8 Percent increase in gross profit, and a rise in water productivity to 5,621 IRR/m³ (a 12.4 Percent improvement). The findings indicate that, compared to a mere pricing policy, a water market creates a more efficient balance between economic and environmental objectives and can serve as an effective tool for sustainable water resource management in semi-arid plains of the country.