Abstract
Abstract. Switzerland has faced extended periods of low river flows in recent years (2003, 2011, 2015 and 2018), with major economic and environmental consequences. Understanding the origins of events like these is important for water resources management. In this work, we provide data illustrating the individual and joint contributions of precipitation and evapotranspiration to low flows in both typical and dry years. To quantify how weather drives low flows, we explore how deviations from mean seasonal climate conditions (i.e., climate anomalies) of precipitation and potential evapotranspiration correlate with the occurrence and magnitude of annual 7 d lowest flows (Qmin) during the warm season (May through November) across 380 Swiss catchments from 2000 through 2018. Most warm-season low flows followed periods of below-average precipitation and above-average potential evapotranspiration, and the lowest low flows resulted from both of these drivers acting together. Low-flow timing was spatially variable across Switzerland in all years, including the driest (2003, 2011, 2015 and 2018). Low flows in these driest years were associated with much longer-lasting climate anomalies than the ≤2 month anomalies which preceded typical warm-season low flows in other years. We found that snow water equivalent and winter precipitation totals only slightly influenced the magnitude and timing of warm-season low flows in low-elevation catchments across Switzerland. Our results provide insight into how precipitation and potential evapotranspiration jointly shape warm-season low flows across Switzerland and potentially aid in assessing low-flow risks in similar mountain regions using seasonal weather forecasts.
Highlights
In recent decades, Europe has experienced several severe droughts (Van Lanen et al, 2016)
Annual low flows did not occur simultaneously across Switzerland but instead occurred primarily during winter in the Alpine regions and in summer and autumn across the Swiss Plateau (Fig. 1). Within these two subregions, the timing of low flows was still spatially variable, indicating that annual low flows may be surprisingly asynchronous across Switzerland, even in unusually dry years
Distinct site-to-site differences exist; at elevations below approximately 1500 m above sea level (a.s.l.), almost all annual low flows occur after periods of anomalously high potential evapotranspiration and anomalously low precipitation (Fig. 2a and b)
Summary
Europe has experienced several severe droughts (Van Lanen et al, 2016). Severe low flows in the years 2003, 2011, 2015 and 2018 led to substantial economic losses by limiting water availability for households, industry, irrigation and hydropower as well as impacting river transportation (Stahl et al, 2016; Munich Re, 2019). Such effects are expected to become more severe and frequent as water demand rises and as droughts are anticipated to increase in frequency and intensity in the future (e.g., De Stefano et al, 2012; Wada et al, 2013), leading to calls for improved understanding and management of droughts and their effects on low flows across Europe (e.g., Seneviratne et al, 2012a; Van Lanen et al, 2016; WMO, 2008). Switzerland has two low-flow seasons in which the distinction between warmseason low flows and winter low flows is strongly connected to elevation
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