Comment on hess-2020-667

doi:10.5194/hess-2020-667-rc2

Abstract

Abstract. Future changes in river runoff will impact many sectors such as agriculture, energy production, or ecosystems. Here, we study changes in the seasonality, frequency, and magnitude of moderate low and high flows and their time of emergence. The time of emergence indicates the timing of significant changes in the flow magnitudes. Daily runoff is simulated for 93Â Swiss catchments for the periodÂ 1981â2099 under Representative Concentration PathwayÂ 8.5 with 20Â climate model chains from the most recent transient Swiss Climate Change Scenarios. In the present climate, annual low flows typically occur in the summer half-year in lower-lying catchments (<1500âmâa.s.l.) and in the winter half-year in Alpine catchments (>1500âmâa.s.l.). By the end of the 21stÂ century, annual low flows are projected to occur in late summer and early autumn in most catchments. This indicates that decreasing precipitation and increasing evapotranspiration in summer and autumn exceed the water contributions from other processes such as snowmelt and glacier melt. In lower-lying catchments, the frequency of annual low flows increases, but their magnitude decreases and becomes more severe. In Alpine catchments, annual low flows occur less often and their magnitude increases. The magnitude of seasonal low flows is projected to decrease in the summer half-year in most catchments and to increase in the winter half-year in Alpine catchments. Early time of emergence is found for annual low flows in Alpine catchments in the 21stÂ century due to early changes in low flows in the winter half-year. In lower-lying catchments, significant changes in low flows emerge later in the century. Annual high flows occur today in lower-lying catchments in the winter half-year and in Alpine catchments in the summer half-year. Climate change will change this seasonality mainly in Alpine catchments with a shift towards earlier seasonality in summer due to the reduced contribution of snowmelt and glacier melt in summer. Annual high flows tend to occur more frequent, and their magnitude increases in most catchments except some Alpine catchments. The magnitude of seasonal high flows in most catchments is projected to increase in the winter half-year and to decrease in the summer half-year. However, the climate model agreement on the sign of change in moderate high flows is weak.

Highlights

Assessments of climate change impacts on hydrology are crucial for future water management and adaptation planning
In most Alpine catchments, annual low flows occur in late winter or early spring in the reference period
Our results show that projections of moderate high flows are less robust among climate models than are those of moderate low flows

Summary

Introduction

Assessments of climate change impacts on hydrology are crucial for future water management and adaptation planning. Annual and seasonal moderate low and high flows are relevant for several reasons. Even moderate extremes are important for water management planning. Very extreme high flows and very extreme low flows are difficult to simulate, because many processes are not fully understood or not yet resolved in hydrological models. Hydrological models are calibrated on observed flow conditions and may miss plausible extreme events that have not been experienced. Climate change projections incorporate large uncertainties regarding small-scale extreme events, for extremes in precipitation, which are potential flood triggers. We focus on moderate extremes: events that occur on average once every year or sea-

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