Reconstructing five decades of sediment export from two glaciated high-alpine catchments in Tyrol, Austria, using nonparametric regression

Abstract. To date, knowledge on the effects of decadal-scale changes in climatic forcing on sediment export from glaciated high alpine areas is still limited. This is primarily due to the extreme scarcity of sufficiently long records of suspended sediment concentrations (SSC), which precludes robust explorations of longer-term developments. Aggravatingly, insights are not necessarily transferable from one catchment to another, as sediment export can heavily depend on local preconditions (such as geology or connectivity). However, gaining a better understanding of past sediment export is an essential step towards estimating future changes, which will affect downstream hydropower production, flood hazard, water quality and aquatic habitats. Here we test the feasibility of reconstructing decadal-scale sediment export from short-term records of SSC and long time series of the most important hydro-climatic predictors, discharge, precipitation and air temperature (QPT). Specifically, we test Quantile Regression Forest (QRF), a non-parametric, multivariate approach based on Random Forests. We train independent models for the two nested and partially glaciated catchments Vent (98 km2) and Vernagt (11.4 km2) in the Upper Ötztal in Tyrol, Austria (1891 to 3772 m a.s.l.), to gain a comprehensive overview of sediment dynamics. In Vent, daily QPT records are available since 1967, alongside 15 years of SSC measurements. At gauge Vernagt, QPT records started in 1975 in hourly resolution, which allows comparing model performances in hourly and daily resolution (Validation A). Challengingly, only four years of SSC measurements exits at gauge Vernagt, yet consisting of two 2-year datasets, that are almost 20 years apart, which provides an excellent opportunity for validating the model’s ability to reconstruct past sediment dynamics (Validation B). As a second objective, we aim to assess changes in sediment export by analyzing the reconstructed time series for trends (using Mann-Kendall test and Sen’s slope estimator) and step-like changes (using two complementary change point detection methods, the widely used Pettitt’s test and a Bayesian approach implemented in the R package ‘mcp’). Our findings demonstrate that QRF performs well in reconstructing past daily sediment export (Nash-Sutcliffe efficiency of 0.73) as well as the derived annual sediment yields (Validation B), despite the small training dataset. Further, our analyses indicate that the loss of model skill in daily as compared to hourly resolution is small (Validation A). We find significant positive trends in the reconstructed annual suspended sediment yields at both gauges, with distinct step-like increases around 1981. This coincides with a crucial point in glacier melt dynamics: we find co-occurring change points in annual and summer mass balances of the two largest glaciers in the Vent catchment. This is also reflected in a coinciding step-like increase in discharge at both gauges as well as a considerable increase in the accumulation area ratio of the Vernagtferner glacier. We identify exceptionally high July temperatures in 1982 and 1983 as a likely cause, as July is the most crucial month with respect to firn and ice melt. In contrast, we did not find coinciding change points in precipitation. This study demonstrates that the presented QRF approach is a promising tool with the ability to deepen our understanding of the response of high-alpine areas to decadal climate change. This in turn will aid estimating future changes and preparing management or adaptation strategies.