Impact of emergency drawdown in off-stream brackish reservoirs – The case of La Loteta dam in Spain

Abstract

• Off-stream reservoirs produce changes in water regime to diversion catchments. • Off-stream reservoirs in saline soils enrich the ion concentration in dammed waters. • Emergency drawdown at off-stream brackish reservoirs can have meaningful impacts. • Extension and duration of the flood and physicochemical changes are meaning impacts. • Conductivity exceedance probability curves allow comparison and impact assessment. Reservoirs are the main component of regulation of water resources in basins. Off-stream reservoirs are a particular solution where water is stored in a secondary catchment other than the basin that produces the water resource. This under certain conditions can modify the water regime of the downstream secondary catchment. Although uncommon, one of the reservoir management operations is emergency emptying in the event of possible incidents that could put proper reservoir management at risk. One of the environmental requirements of the drawdown operation of reservoirs is the study of environmental impacts in the event of a complete discharging event from a full reservoir state, given the huge amount of water to be emptied and the duration of the operation, which can last for days. Logically, the impact will be greater when it comes to diversion basins. It is imperative to evaluate the physical impact of prolonged flooding in downstream areas, especially in areas of special ecosystem sensitivity. Additionally, on certain occasions, a physicochemical impact can be produced on flooded areas when the chemical composition of the discharged water differs from the usual concentrations. Exceedance probability curves for conductivity allow identification and assessment of potential impacts of off-stream reservoirs. This article presents a practical case assessment of the impact of an off-stream reservoir located in evaporitic soils with a high content of salts increasing the chemical concentration of stored water that is distributed in different compositional strata. The emptying is carried out on a wetland with important ecological value, and the impact of extension and duration of the flood on the wetland are studied, as well as exposure to the presence of water with different chemical concentrations over the time the incident lasts.