Reservoir classification based on hydrological efficiency to improve water governance in the drylands

Water management in dry regions is challenging due to high climatic variability, frequent droughts, and projected climate change, which cause water scarcity. In Ceará, Brazil, water security has historically relied on a dense network of reservoirs of varying sizes and playing distinct roles in the system, but management has focused on a few large reservoirs supplying strategic demands, while thousands of smaller reservoirs remain unmonitored and underused. These small reservoirs are particularly vulnerable to rapid water loss through evaporation, limiting their operation under conventional, risk-avoiding management strategies. Recent work has demonstrated that larger and hydrologically more efficient reservoirs are most appropriate to supply strategic human consumption that requires high reliability of water supply, whereas small reservoirs can be managed under more intense water withdrawal strategies for agricultural use. To support this water management approach, we propose a classification method based on hydrological efficiency, defined as a reservoir’s capacity to convert inflow into reliable withdrawal. The method uses the Triangular Regulation Diagram (TRD) to partition inflows into withdrawals, evaporation, and spillage, integrating reservoir characteristics (capacity, geometry, potential evaporation) and streamflow variability (coefficient of variation). Preliminary results indicate that, adopting a withdrawal threshold of 5% of the inflow, about 14,000 reservoirs (approximately 60% of the total network, which collectively represent only 3% of the total storage capacity of the system), can be allocated for agriculture without compromising human water supply. This study shows that non-conventional, efficiency-based management of small reservoirs can enhance resilience to water scarcity through a clear scientific grounded definition of reservoir classification and their respective roles, improving the use of the existing water infrastructure.