‘Teflon Basin’ or Not? A High-Elevation Catchment Transit Time Modeling Approach

Jan Schmieder,Markus Weiler,Stefan Seeger,Ulrich Strasser

doi:10.3390/hydrology6040092

Abstract

We determined the streamflow transit time and the subsurface water storage volume in the glacierized high-elevation catchment of the Rofenache (Oetztal Alps, Austria) with the lumped parameter transit time model TRANSEP. Therefore we enhanced the surface energy-balance model ESCIMO to simulate the ice melt, snowmelt and rain input to the catchment and associated δ18O values for 100 m elevation bands. We then optimized TRANSEP with streamflow volume and δ18O for a four-year period with input data from the modified version of ESCIMO at a daily resolution. The median of the 100 best TRANSEP runs revealed a catchment mean transit time of 9.5 years and a mobile storage of 13,846 mm. The interquartile ranges of the best 100 runs were large for both, the mean transit time (8.2–10.5 years) and the mobile storage (11,975–15,382 mm). The young water fraction estimated with the sinusoidal amplitude ratio of input and output δ18O values and delayed input of snow and ice melt was 47%. Our results indicate that streamflow is dominated by the release of water younger than 56 days. However, tracers also revealed a large water volume in the subsurface with a long transit time resulting to a strongly delayed exchange with streamflow and hence also to a certain portion of relatively old water: The median of the best 100 TRANSEP runs for streamflow fraction older than five years is 28%.

Highlights

High-elevation catchments were often seen as ’Teflon’-like basins [1], meaning that water input to the catchment immediately runs off as overland flow or fast subsurface flow with very limited subsurface interaction [2] and minimal infiltration, due to the permeability of bedrock typically assumed to be negligible [3]
We aim to identify the role of subsurface water in a glacierized high-elevation catchment with the use of δ18 O
Being the first study carried out in such a simulation setup and environment, we found large storages and a relatively high Fow, but contrarily a high Fyw. This led to the conclusion that the basin behaves to some degree like a ‘Teflon basin’, especially because of the large contribution of fast transmitted ice melt, and to some degree like a huge sponge with a very much delayed release of water, especially due to the large potential subsurface water storage volume

Summary

Introduction

High-elevation catchments were often seen as ’Teflon’-like basins [1], meaning that water input to the catchment immediately runs off as overland flow or fast subsurface flow with very limited subsurface interaction [2] and minimal infiltration, due to the permeability of bedrock typically assumed to be negligible [3]. Very recent studies mentioned the formerly unrecognized subsurface storage potential of mountain catchments [4,8,9], i.e., water is temporarily stored as snow and ice, and in the soil, fractured bedrock, moraines, talus, alluvium, alluvial fans, permafrost, rock glaciers and rock slides [4,10,11,12,13] This water contributes to streamflow by shallow to deep flow paths [3,14], both in periods with rain, snowmelt and ice melt input and in periods without water input to the catchment (i.e., when the catchment is in a frozen state [8]), and is an important contributor to streamflow. The water volume and flow (the quotient is the transit time) thereby affect both, downstream water supply and quality [4,23]

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Discussion

Conclusion