Water demand and surface water supply dynamics in the changing climate of semi-arid basins

Climate change is expected to increase vulnerability in the agricultural sector by increasing crop demand for water, limiting water supply, and adding pressure on freshwater resources. In developing countries, limited hydrological data and research make it difficult to quantify changes in surface water flow (Q) and Potential Evapotranspiration ( PET ), making effective water resource management challenging. This paper supports stakeholders in agricultural water management by examining short- and long-term effects of climate change, in the Beqaa Valley of Lebanon, an ungauged, semi-arid sub-basin (9 km 2 ). The paper projects the impact of climate change on PET (linked to crop water requirements) and Q (linked to water supply), under Shared Socioeconomic Pathways (SSP) SSP1 “Sustainability Road” and SSP3 “Regional Rivalry Road” during 2020–2100. PET was calculated using the Hargreaves method, and Q was calculated using the Modèle du Génie Rural à 2 paramètres Mensuel (GR2M) lumped model. Short term results show insignificant change of PET by 2041; however, Q is expected to drop by 38-52% by 2041. Similar patterns were observed in the long term, thus while the increase in water demand is nominal, the decline in water supply poses a serious concern. To address this challenge, the paper uses sensitivity analysis to provide quantitative hydrological projections aimed at supporting evidence-based decision-making, and identify physical parameters and anthropogenic drivers that impact surface-water flow. These insights will help farmers, policymakers, and water user associations foster inclusive dialogue, develop data-informed policies, and implement sustainable watershed management strategies.