Developing water supply reservoir operating rules for large-scale hydrological modelling

Reservoirs are key components of many water supply systems, providing functional capability to manage, and often mitigate, hydrological variability across space and time. The presence and operation of a reservoir controls the downstream flow regime, such that in many locations understanding reservoir operations is crucial to understanding the hydrological functioning of a catchment. Although substantial progress has been made in modelling reservoir operations, several key challenges remain, particularly for large-scale applications including hundreds of reservoirs. In these cases, generic and uncalibrated reservoir operating rules are often applied. However, these rules were developed from global reservoir databases that consist mostly of large irrigation reservoirs and thus are not transferable to smaller reservoirs or those fulfilling other purposes, such as water supply. An alternative option is to use a calibrated, data-driven approach but such techniques require reservoir inflows, outflows and storage data which are rarely available across hundreds of reservoirs. To overcome these problems, here we design a set of simple reservoir operating rules (with only two calibrated parameters) focused on simulating small water supply reservoirs across large scales with various types of open access data (general catchment attributes such as surface area or reservoir capacity, and flows at downstream gauges). Using Great Britain as a case study, we integrate our rules into a national-scale hydrological model and compare hydrological simulations from two modelling scenarios, with and without the new reservoir component. Our simple reservoir operating rules significantly increase model performance in reservoir-impacted catchments, particularly when the rules are calibrated individually at each downstream gauge. We also test the feasibility of using transfer functions (which transform reservoir and catchment attributes into operating rule parameters) to identify a nationally-consistent parameterisation. This works well in ~50 % of the catchments, while nuances in individual reservoir operations limit performance in others. We suggest that our approach should provide a lower benchmark for simulations in catchments containing water supply reservoirs, and that more complex methods should only be considered where they outperform our simple approach.