Abstract
Determining the mean transit times (MTTs) and water sources in catchments at different flow conditions helps better understand river functioning, manage riverine system health and water resources, and discern the responses to climate change and global water stress. Despite being common in a range of environments, understanding of MTTs and variable water sources in intermittent streams remain incomplete compared to perennial streams. Major ion geochemistry, stable isotopes, 14C, 3H and were used in this study to identify water sources and MTTs of a periodically-intermittent river from southeast Australia at different flow conditions, including zero-flow periods. The disconnected pool waters during the zero-flow period in the summer months of 2019 had 3H activities of 0.64 to 3.29 TU. These and the variations in total dissolved solids and stable isotopes imply that these pools contained a mixture of groundwater and younger evaporated stream water. 3H activities during the high-flow period in July 2019 were 1.85 to 3.00 TU, yielding MTTs of up to 17 years. The 3H activities at moderate and low-flow conditions in September and November 2019 ranged from 2.26 to 2.88 TU, implying MTTs of 1.6 to 7.8 years. Regional groundwater near the Wimmera River has 3H activities of < 0.02 to 0.45 TU and 14C activities of 57 to 103 pMC and is not recharged by the river at high flows. The Wimmera River and other intermittent streams in southeast Australia are sustained by smaller volumes of younger catchment waters than comparable perennials streams, indicating that near-river stores have significant impacts on maintaining streamflow during low-flow periods than older deeper regional groundwater. These smaller reservoirs result in the intermittent streams being more susceptible to changes of climate and streamflow and necessitate protection of near-river corridors to maintain the health of the riverine systems.
Highlights
Despite being common in a range of environments, understanding of mean transit times (MTTs) and variable water sources in intermittent streams remain incomplete compared to perennial streams
25 stable isotopes, 14C, and 3H were used in this study to identify water sources and MTTs of a periodically-intermittent river from southeast Australia at different flow conditions, including zero-flow periods
Streamflow of up to 50 m3 day-1 was recorded at Glynwylln during the summer months (January to March), the river largely consisted of disconnected pools with only minor flowing sections at this time and all samples were collected from these pools
Summary
Understanding the timescale of water flow through catchments to rivers at different hydrological conditions is vital for effective water resources management, protecting riverine system, and 45 predicting the changes in river functioning due to climate variability, changes in land use and water utilization (Sophocleous, 2002; Cook., 2013; Van Dijk et al, 2013; Gleeson et al, 2016; Segura et al, 2019). While transit time 50 distributions provide better information on catchment processes than MTTs (McDonnell et al, 2010), MTTs are important for understanding how the water stores that discharge to streams vary at different flow conditions (McGuire and McDonnell, 2006; Blavoux et al, 2013; Duvert et al, 2016; Howcroft et al, 2018). Streams with long MTTs may be sustained by larger volumes of water from within the catchments (e.g., Morgenstern et al, 2010; Gusyev et al, 2016; Howcroft 55 et al, 2018) and be less sensitive to short-term climate variability (e.g., drought lasting years to decades). Numerous studies have focused on perennial streams and have revealed the presence of long-lived water stores contributing to streamflow especially during low-flow periods (Rice and Hornberger, 1998; Soulsby et al, 2006; Hrachowitz et al, 2009; Cartwright and Morgenstern, 2015; Gusyev et al., 70 2016; Howcroft et al, 2018; Cartwright et al, 2020). The connection between intermittent streams and regional groundwater may be less important than for perennial streams, especially during the periodic cease-to-flow times when the water table falls
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