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What is bedrock? Bedrock is a deposit of solid rock that is typically buried beneath soil, and other broken or unconsolidated material. Bedrock is made up of igneous, sedimentary, or metamorphic rock, and it often serves as the parent material. The processes of weathering and erosion affect bedrock. Outcrops exposed to wind and water are often decomposed, or weathered, over time into regolith or smaller particles. In environments characterized by humid conditions that extend for many thousands to millions of years, water may penetrate deep into bedrock to form saprolite, a rock made from the consolidation of clay minerals that remain from the chemical weathering process. Although some bedrock deposits may be strong enough to resist the passage of glaciers and ice sheets over their exposed surfaces, others may be scratched or deeply striated. Erosion may also shape the bedrock of some mountains that serve as the source of glaciers into semicircular basins called cirques.
The bedrock controls on catchment mixing, storage, and release have been actively studied in recent years. However, it has been difficult to find neighbouring catchments with sufficiently different and clean expressions of geology to do comparative analysis. Here, we present new data for 16 nested catchments in the Alzette River basin 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 streamflow regime metrics, catchment storage, and isotope response and catchment mean transit time. We used annual and winter precipitation–runoff ratios, as well as average summer and winter precipitation–runoff ratios to characterise the streamflow regime in our 16 study catchments. Catchment storage was then used as a metric for catchment comparison. Water mixing potential of 11 catchments was quantified via the standard deviation in streamflow 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 versus winter average runoff was best explained by bedrock permeability. Whereas active storage (defined here as a measure of the observed maximum interannual 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 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.
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