A new approach to transfer the discharge recession behavior from gauged to ungauged catchments

Many streams in the Swiss Plateau experienced in 2003, 2011, 2015 and 2018 extremely low streamflow conditions. In 2018, on several stream and groundwater gauging stations the lowest values since the start of observations were registered. Climate and hydrological impact models project that by the end of the century in the future summer half such dry periods with similar duration and intensity might occur more frequently. According to the models, extreme dry periods will last even longer as the drought in summer 2018.

To be prepared on that, the public authorities want to know and to get an overview on how streams react in case of such scenarios predicting dryer conditions. A requirement for that is to understand the relevant factors controlling the available storages for low-flow generation and the drainage behavior of small to mesoscale catchments during low flow under recent climate conditions.

Analysis of flow duration curves of over 200 gauged catchments in Swiss Plateau with information of mean annual precipitation and evapotranspiration, geological maps and some topographic properties allowed to identify factors reducing and increasing the low flow q95 percentile and controlling low flow behavior. With analysis of the resulting spatial pattern of specific discharges (l s^-1 km^-2) of more than 200 additional singular discharge measurements on selected sites during low flow periods, the effects of the relevant factors could be isolated more precisely. The relevant factors essentially consist of:

• Lithological construction of bedrock
• Infiltration and exfiltration of stream water in and from sediments of river bed
• Extend and permeability of quaternary deposits like morains, rubbly deposits, talus or sagging.
• Water withdrawal for water supply, irrigation and power plants.

The spatial low flow pattern all over the Swiss Plateau will be presented with maps and the effects of the mentioned factors will be shown on expressive examples.

A new automatic method is developped to calculate so-called master recession curves from the 200 available long-term discharge data series. With the intention to transfer the recession behavior from gauged to ungauged catchments, further analysis are planned in gauged catchments to identify the correlation between the recession curves and the effect of the above mentioned factors.