Abstract
Abstract. Streamflow droughts, characterized by low runoff as consequence of a drought event, affect numerous aspects of life. Economic sectors that are impacted by low streamflow are, e.g., power production, agriculture, tourism, water quality management and shipping. Those sectors could potentially benefit from forecasts of streamflow drought events, even of short events on the monthly time scales or below. Numerical hydrometeorological models have increasingly been used to forecast low streamflow and have become the focus of recent research. Here, we consider daily ensemble runoff forecasts for the river Thur, which has its source in the Swiss Alps. We focus on the evaluation of low streamflow and of the derived indices as duration, severity and magnitude, characterizing streamflow droughts up to a lead time of one month. The ECMWF VarEPS 5-member ensemble reforecast, which covers 18 yr, is used as forcing for the hydrological model PREVAH. A thorough verification reveals that, compared to probabilistic peak-flow forecasts, which show skill up to a lead time of two weeks, forecasts of streamflow droughts are skilful over the entire forecast range of one month. For forecasts at the lower end of the runoff regime, the quality of the initial state seems to be crucial to achieve a good forecast quality in the longer range. It is shown that the states used in this study to initialize forecasts satisfy this requirement. The produced forecasts of streamflow drought indices, derived from the ensemble forecasts, could be beneficially included in a decision-making process. This is valid for probabilistic forecasts of streamflow drought events falling below a daily varying threshold, based on a quantile derived from a runoff climatology. Although the forecasts have a tendency to overpredict streamflow droughts, it is shown that the relative economic value of the ensemble forecasts reaches up to 60%, in case a forecast user is able to take preventive action based on the forecast.
Highlights
Droughts, which can occur on a wide spatial and temporal range, can be defined in various ways
We focus on the evaluation of low streamflow and of the derived indices as duration, severity and magnitude, characterizing streamflow droughts up to a lead time of one month
Streamflow drought forecasts on time scales up to one month are potentially useful for hydro-power generation, agriculture, conventional power production, water quality, navigation and tourism, in general, all sectors that can use the information about upcoming streamflow drought events to take preventive action
Summary
Droughts, which can occur on a wide spatial and temporal range, can be defined in various ways. Typical and possibly interrelated processes are a temporally and spatially extended lack of precipitation (meteorological droughts), reduced soil moisture (agricultural droughts) and low levels of runoff or groundwater (hydrological droughts) (Heim Jr., 2002) This variety in the way droughts are defined is a direct consequence of the range of socioeconomic impacts they have on different interest groups. Streamflow drought forecasts on time scales up to one month are potentially useful for hydro-power generation, agriculture (irrigation), conventional power production (supply of heat exchange water), water quality, navigation (shipping) and tourism (e.g., water supply for snow cannons), in general, all sectors that can use the information about upcoming streamflow drought events to take preventive action. To estimate the value for a variety of users, the resulting forecasts are evaluated in terms of their economic value
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