Forecast-based operation of repurposed small reservoirs for floods, farms, and (low) flows

Abstract. The increased frequency and intensity of hydrological extremes, including drought, due to anthropogenic climate change will drive the need for enhanced water supply resilience, even in water-rich countries. Previous studies have shown that small reservoirs have considerable potential for expanding water supply for various purposes, including when repurposed from flood-only reservoirs for both flood and drought protection. However, whether these repurposed reservoirs retain the same flood protection ability when operating under forecasts is still unclear, as reservoir operation under forecasts has primarily been researched in the context of large reservoirs. In this study, we investigated potential operating rules under forecasts for 30 small-to-midsize flood reservoirs to a) determine if the uncertainty introduced by forecasts degrades the performance of repurposed reservoirs so significantly as to render the concept unusable, b) identify patterns in the relationship between forecast accuracy and optimal reservoir performance, and c) identify patterns in optimal reservoir operation rules, under the constraint that flood protection should not be compromised. Performance is determined by the modelled ability to either supplement streamflow to avoid low flows or to provide water for irrigation purposes in the area of the reservoir. 1000 combinations of three operation parameters—the warning threshold at which flood pre-release begins, the rate at which water is released from the reservoir for flood pre-release, and the inflow at which the reservoir begins storing water—were tested for maintenance of flood protection (viability) and benefit for the reservoir’s additional uses. While some reservoirs indeed were no longer beneficial when optimized to operate under forecasts, many still maintained benefits above 40 %, with a couple even surpassing their performance under perfect knowledge. Comparing changes in benefit from the perfect-knowledge operation to forecast accuracy indicated that high rates of hits, false alarms and misses, and misses (HFM) could explain the largest decreases in performance, while other forecast accuracy metrics were less impactful. However, even if HFM were low but nonzero, a poorly-timed false alarm could drain a reservoir’s storage before a spike in demand, causing a noticeable loss in performance. Investigation of reservoirs’ potential benefits under forecasts should therefore be done via simulation rather than approximated via characterizing indices. Optimal operation rules tended to be those that most closely mimicked the perfect knowledge operation, i.e. aggressive storage thresholds and a tendency to hold onto the water storage for as long as is safe, but more conservative operating rules were also able to provide benefits as well. The models for forecast operation and optimization produced for this study can be used by water managers to assess if existing small flood reservoirs can feasibly be used to increase water supply resilience in a changing world.