Christophe Hissler

Bedrock geology controls on catchment storage, mixing and release: a comparative analysis of 16 nested catchments

The bedrock controls on catchment mixing, storage, and release have been actively studied in recent years. However, it has been difficult to find neighboring catchments with sufficiently different and clean expressions of geology to do comparative analysis. Here, we present new data for 16 nested catchments (0.45 to 410 km2) in the Alzette River basin (Luxembourg) that span a range of clean and mixed expressions of schists, phyllites, sandstones and quartzites to quantify the relationships between bedrock permeability and metrics of water storage and release. We examined 9 years’ worth of precipitation and discharge data, and 6 years of fortnightly stable isotope data in streamflow, to explore how bedrock permeability controls (1) streamflow regime metrics, (2) catchment storage and (3) isotope response and catchment mean transit time (MTT). We used annual and winter precipitation-runoff ratios, as well as average summer and winter precipitation-runoff ratios to characterize the streamflow regime in our 16 study catchments. Catchment storage was then used as a metric for catchment comparison. Water mixing potential of 12 catchments was quantified via the standard deviation in streamflow δD (σδD) and the amplitude ratio (AS/AP) of annual cycles of δ18O in streamflow and precipitation. Catchment MTT values were estimated via both stable isotope signature damping and hydraulic turnover calculations. In our 16 nested catchments, the variance in ratios of summer vs. winter average runoff was best explained by bedrock permeability. While active storage (defined here as a measure of the observed maximum inter-annual variability in catchment storage) ranged from 107 to 373 mm, total catchment storage (defined as the maximum catchment storage connected to the stream network) extended up to ~1700 mm [+/- 200 mm]. Catchment bedrock permeability was strongly correlated with mixing proxies of σδD in streamflow and δ18O AS/AP ratios. Catchment mean transit time (MTT) values ranged from 0.5 to 2 years, based on stable isotope signature damping, and from 0.5 to 10 years, based on hydraulic turnover.