Abstract
Abstract. Numerous indices exist for the description of hydrological drought. The EURO FRIEND-Water Low flow and Drought Group has repeatedly discussed changing paradigms in the perception and use of existing and emerging new indices for hydrological drought identification and characterization. Group members have also tested the communication of different indices to stakeholders in several national and international transdisciplinary research projects. This contribution presents the experience gained with regard to the purpose and applicability of different classes of drought indices. A recent paradigm shift is the use of anomalies, traditionally from climatology, in hydrology. For instance, anomaly-based indices, such as the Standardized Streamflow Index (SSI) and the variable threshold level method to define streamflow deficiencies, are used increasingly for real-time monitoring. How these indices relate to low flows and their impacts may have become less clear as a result. Assessments of the severity of a particular drought may also differ depending on whether return periods based on traditional low flow or drought frequency analyses or whether SSI time series index values are used. These experiences call for a systematic comparison, classification and evaluation of different low flow and drought indices and their usages.
Highlights
Numerous indices exist for the definition and description of hydrological drought, which here refers primarily to streamflow drought
Members of the EURO FRIEND-Water Low Flow and Drought Group have over time experienced changing paradigms in low flow and drought indices, including and beyond the use of anomalies
The approach is robust as large-scale models tend not to represent absolute streamflow values very accurately (e.g. Gudmundsson et al, 2014). It allows to communicate future relative changes. It presents a change from the traditional viewpoint of hydrological drought as extreme low flow with immediate impacts, e.g. on river habitats and water use restrictions, to a relative lower seasonal flow, which may not have simultaneously impacts
Summary
Numerous indices exist for the definition and description of hydrological drought, which here refers primarily to streamflow drought. Hydrological drought indices have been developed and used for a number of purposes These include, but are not limited to, planning of water resources/flow allocations, analysis of temporal trends and the influence of climate change, analysis of spatial patterns and regionalization of drought characteristics (estimation at the ungauged site), large-scale comparative monitoring (and forecasting) of river flows and drought conditions, and drought vulnerability and risk analysis. The purposes these indices serve vary and may have changed over time. It aims to spawn further discussion and to provide a starting point for a more comprehensive classification of the multitude of indices and their recommended usage
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