Exploring the potential of minimum flows to mitigate the impacts of small farm reservoirs on low-flows

Small farm reservoirs are human-made storage systems (from few hundreds m³ to 1Mm³) used all around the world to store water for agricultural uses (crop irrigation, livestock watering) . They are usually built directly across the stream (small dams) or in local depressions to store surface runoff water (hill reservoirs). In the absence of regulation from authorities, small dams can intercept and store upstream water throughout the year without any restrictions. This may have downstream impacts on streamflow and aquatic ecosystem, particularly during the dry season. Many countries, such as Spain and South-Africa, have incorporated the concept of environmental flow in their regulations to protect downstream water uses.

In France, since 2014, all dams irrespective of their size or construction date are required to maintain a downstream flow whenever upstream flow occurs. Reservoirs can only be filled if the upstream flow exceeds a minimum flow, which must be transmitted downstream. This minimum flow is set at 10 % of the inter-annual mean discharge at the location of the reservoir.

This study aims to evaluate the effect of the minimum flow on streamflow, especially low-flow, and water availability in small reservoirs. From a water management perspective, it addresses the question of the mitigation of the hydrological impacts of small reservoirs. We used the spatially-distributed agro-hydrological model MHYDAS-small-reservoirs (Lebon et al., 2022¹) to test four minimum flow values: 0%, 5%, 10%, and 20% of inter-annual mean discharge. The study site was the Gélon catchment (20 km²) located in South-Western France, characterized by hilly terrain, clay loam soils, and a groundwater dominated hydrology. The four values of minimum flow were tested with three different hypothetical 14-reservoirs networks to consider the effect of reservoir distribution (Lechevallier et al., 2025²), and with two values for total reservoir capacity. This results in a total of 24 simulations, in addition to the reference simulation without reservoirs or irrigation. The agronomic context involved intensive reservoir use to irrigate maize and soybeans crops. Simulations were run on 20 years at a hourly time step.

The impact on low-flows was assessed spatially as the change in the number in low-flow days compared to the reference situation without reservoirs. Our on-going research reveals that the greatest impact occurs at 0% of minimum flow, and the effect diminishes with increasing minimum flow, reaching a no-impact situation at 20% of minimum flow. Additionally, withdrawals and water availability are little impacted by the implementation of minimum flows, except for upstream reservoirs. These findings demonstrate that minimum flows effectively mitigate the impacts of small farm reservoirs on low-flows. Future analysis will explore the effects of minimum flows on crop yields and simulate alternative management strategies with restrictions on reservoir refill during the dry season.

 

¹ https://doi.org/10.1016/j.envsoft.2022.105409

² https://doi.org/10.5194/egusphere-egu25-6876