Changing Low Flow and Streamflow Drought Seasonality in Central European Headwaters

Vojtech Vlach,Milada Matouskova,Ondrej Ledvinka

doi:10.3390/w12123575

Abstract

In the context of the ongoing climate warming in Europe, the seasonality and magnitudes of low flows and streamflow droughts are expected to change in the future. Increasing temperature and evaporation rates, stagnating precipitation amounts and decreasing snow cover will probably further intensify the summer streamflow deficits. This study analyzed the long-term variability and seasonality of low flows and streamflow droughts in fifteen headwater catchments of three regions within Central Europe. To quantify the changes in the low flow regime of selected catchments during the 1968–2019 period, we applied the R package lfstat for computing the seasonality ratio (SR), the seasonality index (SI), mean annual minima, as well as for the detection of streamflow drought events along with deficit volumes. Trend analysis of summer minimum discharges was performed using the Mann–Kendall test. Our results showed a substantial increase in the proportion of summer low flows during the analyzed period, accompanied with an apparent shift in the average date of low flow occurrence towards the start of the year. The most pronounced seasonality shifts were found predominantly in catchments with the mean altitude 800–1000 m.a.s.l. in all study regions. In contrast, the regime of low flows in catchments with terrain above 1000 m.a.s.l. remained nearly stable throughout the 1968–2019 period. Moreover, the analysis of mean summer minimum discharges indicated a much-diversified pattern in behavior of long-term trends than it might have been expected. The findings of this study may help identify the potentially most vulnerable near-natural headwater catchments facing worsening summer water scarcity.

Highlights

Regional and local hydrological studies are of high importance because they present both spatial and temporal variability of runoff characteristics, on a relatively small scale [1,2].The assessment of the streamflow extremes on a local scale should begin with the identification of typical seasonal variations occurring over time [3]
0.45 to 1.2 and 0.49 to 1.14, respectively. This indicates that the low-flow and drought regime in most of the catchments is closer to the summer type, but the seasonality is generally weak with values close to 1
We provided a comparison of different low-flow characteristics and seasonality indicators computed on the basis of daily streamflow data series in the 1968–2019 period in a set of fifteen near-natural headwater catchments located along the Czech–German and the Czech–Polish borders

Summary

Introduction

Regional and local hydrological studies are of high importance because they present both spatial and temporal variability of runoff characteristics, on a relatively small scale [1,2]. The assessment of the streamflow extremes on a local scale should begin with the identification of typical seasonal variations occurring over time [3]. Due to the changing climate, these patterns may be either aggravating or weakening throughout a longer period of time [4,5]. In the context of increasing severity of droughts in Europe during the last decade [9,10,11], the effects of climate change on runoff regime will very likely affect the runoff seasonality, variability of discharge and the risk of extreme streamflow drought occurrence [12,13]. Under conditions of rising air temperature, increasing evaporation rates, decreasing soil-moisture [14,15,16] and decreasing snow cover in the Central

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