Mean transit times in headwater catchments: insights from the Otway Ranges, Australia

Hydrol. Earth Syst. Sci., 22, 635–653, 2018 https://doi.org/10.5194/hess-22-635-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 3.0 License. Mean transit times in headwater catchments: insights from the Otway Ranges, Australia William Howcroft1 , Ian Cartwright1,2 , and Uwe Morgenstern3 1 School of Earth, Atmosphere and Environment, 9 Rainforest Walk, Monash University, Clayton, VIC 3800, Australia Centre for Groundwater Research and Training, G.P.O. Box 2100, Flinders University, Adelaide, SA 5001, Australia 3 GNS Science, 1 Fairway Drive, Avalon, P.O. Box 368, Lower Hutt 5040, New Zealand 2 National Correspondence: William Howcroft () Received: 12 April 2017 – Discussion started: 9 May 2017 Revised: 19 December 2017 – Accepted: 19 December 2017 – Published: 25 January 2018 Abstract. Understanding the timescales of water flow through catchments and the sources of stream water at different flow conditions is critical for understanding catchment behaviour and managing water resources. Here, tritium (3 H) activities, major ion geochemistry and streamflow 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 in the Otway Ranges of southeastern Australia. 3 H activities of stream water ranged from 0.20 to 2.14 TU, which are significantly lower than the annual average 3 H activity of modern local rainfall, which is between 2.4 and 3.2 TU. The 3 H activities of the stream water are lowest during low summer flows and increase with increasing streamflow. The concentrations of most major ions vary little with streamflow, which together with the low 3 H activities imply that there is no significant direct input of recent rainfall at the streamflows sampled in this study. Instead, shallow younger water stores in the soils and regolith are most likely mobilised during the wetter months. MTTs vary from approximately 7 to 230 years. Despite uncertainties of several years in the MTTs that arise from having to assume an appropriate LPM, macroscopic mixing, and uncertainties in the 3 H activities of rainfall, the conclusion that they range from years to decades is robust. Additionally, the relative differences in MTTs at different streamflows in the same catchment are estimated with more certainty. The MTTs in these and similar headwater catchments in southeastern Australia are longer than in many catchments globally. These differences may reflect the relatively low rainfall and high evapotranspiration rates in southeastern Australia compared with headwater catchments elsewhere. The long MTTs imply that there is a long-lived store of water in these catchments that can sustain the streams over drought periods lasting several years. However, the catchments are likely to be vulnerable to decadal changes in land use or climate. Additionally, there may be considerable delay in contaminants reaching the stream. An increase in nitrate and sulfate concentrations in several catchments at high streamflows may represent the input of contaminants through the shallow groundwater that contributes to streamflow during the wetter months. Poor correlations between 3 H activities and catchment area, drainage density, land use, and average slope imply that the MTTs are not controlled by a single parameter but a variety of factors, including catchment geomorphology and the hydraulic properties of the soils and aquifers. 1 Introduction Determining the timescales over which precipitation is transmitted from a recharge area through a catchment to where it discharges into rivers or streams (the transit time) is important for understanding catchment behaviour and is of inherent interest to resource managers. Streams with long MTTs are connected to relatively large stores of water in the underlying aquifers (Maloszewski and Zuber, 1982; Morgenstern et al., 2010) that may sustain streamflow during droughts that last up to a few years. However, longer-term changes, such Published by Copernicus Publications on behalf of the European Geosciences Union. 636 W. Howcroft et al.: Mean transit times in headwater catchments as deforestation, agricultural development, climate change, and/or landscape change following bushfires, are likely to affect both the quality and the quantity of river flows. Headwater streams are important as they commonly support diverse ecosystems, provide recreational opportunities and in many catchments contribute a significant proportion of the total river flow (Freeman et al., 2007). Headwater streams also differ from lowland rivers in terms of their potential water inputs. Unlike lowland rivers, which typically receive groundwater inflows from regional aquifers or nearriver floodplain sediments, the sources of water in headwater streams are far less well understood. Headwater streams are commonly developed at elevations well above those of the regional water tables and/or occur on relatively impermeable bedrock. Yet such streams continue to flow even during prolonged dry periods. There are several potential water stores that could contribute to stream flow, including the soil zone, weathered or fractured basement rocks, and/or perched aquifers at the soil–bedrock interface (e.g. Sklash and Farvolden, 1979; Kennedy et al., 1986; Swistock et al., 1989; Bazemore et al., 1994; Fenicia et al., 2006; Jensco and McGlynn, 2011). Estimates of MTTs in headwater catchments range from a few months to several decades (e.g. Soulsby et al., 2000; McGuire and McDonnell, 2006; Hrachowitz et al., 2009; McDonnell et al., 2010; Stewart and Fahey, 2010; Stewart et al., 2010; Mueller et al., 2013; Stockinger et al., 2014; Atkinson, 2014; Cartwright and Morgenstern, 2015, 2016a, b; Duvert et al., 2016). However, in many regions globally the range of MTTs in headwater catchments is not well known. Additionally, it is not always clear why MTTs vary between different areas. This lack of knowledge limits our abilities to protect and manage headwater catchments. 1.1 Estimating mean transit times (MTTs) Groundwater follows a myriad of flow paths between the recharge areas to where it discharges into streams or rivers. Consequently, groundwater discharge does not have a discrete age but rather has a distribution of transit times. MTTs are commonly estimated using lumped parameter models (LPMs) that describe the distribution of water with different ages or tracer concentrations in simplified aquifer geometries (Maloszewski and Zuber, 1982, 1996; Maloszewski et al., 1983; Cook and Bohlke, 2000; Maloszewski, 2000; Zuber et al., 2005). LPMs represent a viable and commonly used alternative to estimating MTTs using numerical groundwater models that rely upon hydraulic parameters that are seldom known with certainty and which vary spatially. However, the LPMs are only approximations of actual flow systems and the MTTs may be broad estimates rather than specific values. Th (...truncated)