This paper discusses the role and limitations of using WEI+ as a water resource management tool for highly regulated river basins, with a conjunctive use of surface and groundwater resources. By considering flow regulation by reservoirs and aquifer systems, seasonality of water availability and demand, returns from water uses and environmental flow requirements, WEI+ constitutes an improvement to existing quantitative water scarcity indexes. However, the computation of WEI+ in complex river basins systems requires detailed data on water availability and water allocation to various uses, which are hard to obtain from monitoring records. The paper describes how the combined use of hydrological and water allocation models can help to overcome data gaps in water accounting and contribute to an improved analysis of water scarcity in heterogeneous and intricate river basins. It also examines the information provided by WEI+ and by other widely used water scarcity indexes, such as the Water Stress Index and the Criticality Ratio, as well as discusses the ability of WEI+ to measure the performance of hydraulic systems, usually evaluated by parameters such as reliability, vulnerability, and resilience. The Tagus River transboundary basin was selected as case study due to massive flow regulation by multi-purpose reservoirs and significant seasonality of water availability and demand. Results show the benefits of using WEI+ to define levels of water scarcity, over other indexes. Within the Tagus River systems, high values of WEI+ are reached during the summer months in regions with intensive agriculture, denoting severe water stress conditions in most sub-basins. The analysis also reveals the strong dependence of Portugal, the downstream country, on flows from Spain, the upstream country.
