Abstract
Headwaters make up a large part of the global stream length. They are also especially sensitive to droughts, which affect the stream's water balance, chemistry, and ecology. Climate change scenarios predict an increasing frequency of extreme weather events. For streams, rivers, and their catchments, this implies a higher intensity and frequency of severer droughts and floods. It is likely that during drought streams depend to a significant extent on groundwater to maintain flow. This study contributes to ongoing research on the effects of drought on headwater catchments and the role of groundwater in the water balance of these systems. Monthly Radon (222Rn) measurements combined with mass balance calculations were used to quantify the spatial and temporal variability of groundwater influx to the Mähringsbach, a headwater catchment in northern Bavaria, Germany. Sampling was conducted in 2019 and 2020, a multi-year drought period, with 2019 being the seventh driest year since the start of records. Thus measurements covered a broad range of flow regimes (0.04 m3 s−1 to ~ 3 m3 s−1). 222Rn activities ranged between ~500 Bq m−3 and ~8,500 Bq m−3 in the headwater, while further downstream, the activities and variability in activities were lower (~500 Bq m−3 to ~2,000 Bq m−3). Results from the 222Rn mass balance showed that in the headwater reaches, the proportion of groundwater varied between 10 and 70 %, while further downstream, it ranged between only 0 and 30%. There was a clear negative correlation between river discharge and the proportion of groundwater inflow to the stream. Less than 10% of the total discharge was derived from groundwater during high flow conditions, while under low flow in the headwater reaches, it increased to 70%. We conclude that aquatic ecosystems in headwaters become increasingly dependent on groundwater during drought periods as a source of water. This dependency will increase in the summer months given current climate predictions. This knowledge should be used to develop, refine, and apply management strategies for streams and the important habitats located in stream sediments (hyporheic zone) under a changing climate.
Highlights
Headwater streams make up nearly 90% of the global stream system length (Downing, 2012; Allen et al, 2018; Ward et al, 2020)
In the headwater, we measured radon activities ranging from 560 to 7,970 Bq m−3 for group 1, 430 to 8,670 Bq m−3 for group 2, and 880 to 5,830 Bq m−3 for group 3
Radon has been applied as a tracer for groundwater inflow in several studies from various continents, climate zones, and environments ranging from creeks to oceans and from arid to Humid climates (Burnett and Dulaiova, 2006; Cartwright et al, 2011; Bourke et al, 2014; Cranswick et al, 2014; Frei and Gilfedder, 2015; Frei et al, 2019)
Summary
Headwater streams make up nearly 90% of the global stream system length (Downing, 2012; Allen et al, 2018; Ward et al, 2020) They are crucial for water quantity and quality downstream (Alexander et al, 2007). Headwaters are essential sources of high-quality water, diluting nutrient and other contaminant and sediment inputs, influencing habitat quality for aquatic organisms (Gomi et al, 2002). As such, they are a vital feature of aquatic ecosystems. Small fluvial aquatic ecosystems without water storage reserves (e.g., snowpack or glaciers) are especially threatened by effects of climate change (e.g., shifted precipitation patterns, the distribution between rain and snow, and more frequent and severe droughts) (Bennett et al, 2012)
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