Balancing ecosystem sustainability and irrigated smallholder agriculture: A modeling approach for water resource management

• Balancing e-flows and small-scale farmers’ needs ensures river ecosystem sustainability. • Holistic modeling blends insights, revealing connections and forecasting outcomes. • Assessing impact on smallholders involves evaluating risks under varying flow conditions. • Model simulations reveal conflicts between environmental flows and smallholder needs. • Evaluating e-flows should include impacts on smallholder farming communities. Implementing environmental flows (e-flows) is crucial for protecting river ecosystems but often competes with water needs for small-scale agriculture. Understanding these trade-offs is essential for effective water management. We employ holistic modeling to explore the socioecological impacts of e-flows on agriculture along the lower Great Letaba River. Our approach uses conceptual impact pathways and quantitative models to account for smallholder water use, forecasting outcomes under varying river flow conditions. Our findings reveal that the irrigation water demand of small-scale agriculture amounts to only one-tenth of the required e-flow volume. However, farmers face increasing crop water gaps—shortfalls in available water for irrigation—especially during the dry season, which jeopardizes agricultural sustainability as e-flow needs persist. Addressing this challenge may require upstream water flow supplementation, though it would necessitate restricting current upstream water use. We also introduce a robust decision-making model, integrating expert knowledge and Monte Carlo simulations, to assess e-flow management options under uncertainty. By extending the e-flow concept to include the needs of vulnerable farming communities, our model provides decision-makers with an accessible tool for balancing ecological and agricultural demands, even when precise data are unavailable.