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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/hess-2017-219
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
09 May 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).
Mean Transit Times in Headwater Catchments: Insights from the Otway Ranges, Australia
William Howcroft1, Ian Cartwright1,2, and Uwe Morgenstern3 1School of Earth, Atmosphere and Environment (SEAE), 9 Rainforest Walk (Building 28), Monash University, Clayton Campus, Clayton, VIC, 3800, Australia
2National Centre for Groundwater Research and Training, GPO Box 2100, Flinders University, Adelaide, SA, 5001, Australia
3GNS Science, 1 Fairway Drive, Avalon, PO Box 368, Lower Hutt 5040, New Zealand
Abstract. Understanding the timescales of water flow through catchments and the origins of stream water at different flow conditions is critical for understanding catchment behaviour and managing water resources. Here, tritium (3H) activities, major ion geochemistry and discharge data were used in conjunction with Lumped Parameter Models (LPMs) to investigate mean transit times (MTTs) and the stores of water in six headwater catchments of the Otway Ranges in southeast Australia. 3H activities of stream water ranged from 0.20 to 2.14 TU, which are far lower than those of modern local rainfall (2.4 to 3.2 TU). The 3H activities of the stream water are lowest during the low summer flows and increase with stream discharge. Calculated MTTs vary from approximately 7 to 234 years which, in many cases, exceed those reported for river systems globally. The MTT estimates, however, are subject to a number of uncertainties, including, uncertainties in the most appropriate LPM to use, aggregation errors, and uncertainty in the modern and bomb-pulse 3H activity of rainfall. These uncertainties locally result in uncertainties in MTTs of several years; however, they do not change the overall conclusions that the water in these streams has MTTs of several years to decades. There is discharge threshold of approximately 104 m3 day−1 in all catchments above which 3H activities do not increase appreciably above ~ 2.0 TU. The MTT of this 3H activity is approximately ten years, which implies that changes within the catchments, including drought, deforestation, land use and/or bush fire, would not be realised within the streams for at least a decade. A positive correlation exists between 3H activities and nitrate and sulphate concentrations within several of the catchments, which suggests that anthropogenic activities have increasingly impacted water quality at these locations over time.

Citation: Howcroft, W., Cartwright, I., and Morgenstern, U.: Mean Transit Times in Headwater Catchments: Insights from the Otway Ranges, Australia, Hydrol. Earth Syst. Sci. Discuss., https://doi.org/10.5194/hess-2017-219, in review, 2017.
William Howcroft et al.
William Howcroft et al.
William Howcroft et al.

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Short summary
Documenting mean transit times is critical for understanding and managing catchments. Mean transit times in six headwater catchments of the Otway Ranges, Australia, determined using tritium, range from 7 to 234 years. These long mean transit times implies that short-term rainfall variability may not affect these catchments. However, they may respond to longer term climatic or land-use changes. A new method of estimating aquifer recharge based on tritium activities is developed.
Documenting mean transit times is critical for understanding and managing catchments. Mean...
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