Wetlands and low‐gradient topography are associated with longer hydrologic transit times in Precambrian Shield headwater catchments

Intermittent headwater catchments constitute a significant proportion of many stream networks. In semi-arid climates, intermittent headwater streams flow only following periods of sustained rainfall. There is commonly a rapid response of streamflow to rainfall; however, whether this is the input of recent rainfall or displacement of water stored in the catchments for several years is not well known. Understanding the sources and transit times of water that contribute to streamflow is important for the maintenance of stream health and predicting the response of land-use changes.The study focuses on two intermittent streams from two contrasting land-use (pasture and forest) in southeast Australia. The native eucalyptus forests in this region were originally cleared for grazing following European settlement ~180 years ago and then partially replaced by plantation in the last ~15 years. Stream water and groundwater from the riparian zone adjacent to the streams were sampled between May and October 2018.The stream water has 3H activities of 1.30 to 3.17 TU in the pasture and 1.84 to 3.99 TU in the forest, with higher activities recorded during the higher winter flows. Groundwater from the riparian zone has 3H activities of 0.16 to 0.79 TU in the pasture and 2.01 to 4.10 TU in the forest. Aside from one riparian zone groundwater sample, all 3H activities of groundwater in the riparian zone are lower than those of recent local rainfall (~2.79 TU). The single high 3H activity in riparian zone possibly reflects recharge by winter rainfall with higher 3H activities.The mean transit times (MTTs) of water were estimated using a range of tracer lumped parameter models. The riparian zone groundwater has greater MTTs of hundreds of years in the pasture and up to 9 years in the forest. At high streamflow, the stream water has MTTs of <6 years in the pasture and the forest. The MTTs of stream water at low streamflow vary from 15 to 42 years in the pasture and from 3 to 16 years in the forest. The long MTTs of water from streams indicate that the source water is not just recent rainfall, rather water stored in the riparian zone is mobilised at the commencement of flow and recent rainfall makes a larger contribution at higher flows. The observation is consistent with the major ion geochemistry of the stream water, which most closely represents that of the riparian zone groundwater. The differences in MTTs of stream water between two contrasting land-use imply that the streamflow has been being most likely impacted by land-use changes. Thus, it is necessary to improve the strategies for catchment management to protect stream health from land-use practices.

Download Full-text

Transit times from rainfall to baseflow in headwater catchments estimated using tritium: the Ovens River, Australia

Hydrology and Earth System Sciences ◽

10.5194/hess-19-3771-2015 ◽

2015 ◽

Vol 19 (9) ◽

pp. 3771-3785 ◽

Cited By ~ 30

Author(s):

I. Cartwright ◽

U. Morgenstern

Keyword(s):

Transit Time ◽

Headwater Streams ◽

Stream Water ◽

Mean Transit Time ◽

Significant Proportion ◽

Low Flow ◽

Flow Conditions ◽

Headwater Catchments ◽

Flow Models ◽

Transit Times

Abstract. Headwater streams contribute a significant proportion of the total flow to many river systems, especially during summer low-flow periods. However, despite their importance, the time taken for water to travel through headwater catchments and into the streams (the transit time) is poorly understood. Here, 3H activities of stream water are used to define transit times of water contributing to streams from the upper reaches of the Ovens River in south-east Australia at varying flow conditions. 3H activities of the stream water varied from 1.63 to 2.45 TU, which are below the average 3H activity of modern local rainfall (2.85 to 2.99 TU). The highest 3H activities were recorded following higher winter flows and the lowest 3H activities were recorded at summer low-flow conditions. Variations of major ion concentrations and 3H activities with streamflow imply that different stores of water from within the catchment (e.g. from the soil or regolith) are mobilised during rainfall events rather than there being simple dilution of an older groundwater component by event water. Mean transit times calculated using an exponential-piston flow model range from 4 to 30 years and are higher at summer low-flow conditions. Mean transit times calculated using other flow models (e.g. exponential flow or dispersion) are similar. There are broad correlations between 3H activities and the percentage of rainfall exported from each catchment and between 3H activities and Na and Cl concentrations that allow first-order estimates of mean transit times in adjacent catchments or at different times in these catchments to be made. Water from the upper Ovens River has similar mean transit times to the headwater streams implying there is no significant input of old water from the alluvial gravels. The observation that the water contributing to the headwater streams in the Ovens catchment has a mean transit time of years to decades implies that these streams are buffered against rainfall variations on timescales of a few years. However, impacts of any changes to land use in these catchments may take years to decades to manifest themselves in changes to streamflow or water quality.

Download Full-text

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

Hydrology and Earth System Sciences ◽

10.5194/hess-22-635-2018 ◽

2018 ◽

Vol 22 (1) ◽

pp. 635-653 ◽

Cited By ~ 6

Author(s):

William Howcroft ◽

Ian Cartwright ◽

Uwe Morgenstern

Keyword(s):

Land Use ◽

Major Ions ◽

Stream Water ◽

Shallow Groundwater ◽

Drainage Density ◽

Headwater Catchments ◽

Southeastern Australia ◽

Lumped Parameter ◽

Transit Times ◽

Major Ion Geochemistry

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 (3H) 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. 3H activities of stream water ranged from 0.20 to 2.14 TU, which are significantly lower than the annual average 3H activity of modern local rainfall, which is between 2.4 and 3.2 TU. The 3H 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 3H 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 3H 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 3H 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.

Download Full-text

Importance of vegetation, topography and flow paths for water transit times of base flow in alpine headwater catchments

Hydrology and Earth System Sciences ◽

10.5194/hess-17-1661-2013 ◽

2013 ◽

Vol 17 (4) ◽

pp. 1661-1679 ◽

Cited By ~ 36

Author(s):

M. H. Mueller ◽

R. Weingartner ◽

C. Alewell

Keyword(s):

Stable Isotope ◽

Vegetation Cover ◽

Stream Water ◽

Mean Transit Time ◽

Snow Accumulation ◽

Base Flow ◽

Swiss Alps ◽

Flow Paths ◽

Headwater Catchments ◽

Transit Times

Abstract. The mean transit time (MTT) of water in a catchment gives information about storage, flow paths, sources of water and thus also about retention and release of solutes in a catchment. To our knowledge there are only a few catchment studies on the influence of vegetation cover changes on base flow MTTs. The main changes in vegetation cover in the Swiss Alps are massive shrub encroachment and forest expansion into formerly open habitats. Four small and relatively steep headwater catchments in the Swiss Alps (Ursern Valley) were investigated to relate different vegetation cover to water transit times. Time series of water stable isotopes were used to calculate MTTs. The high temporal variation of the stable isotope signals in precipitation was strongly dampened in stream base flow samples. MTTs of the four catchments were 70 to 102 weeks. The strong dampening of the stable isotope input signal as well as stream water geochemistry points to deeper flow paths and mixing of waters of different ages at the catchments' outlets. MTTs were neither related to topographic indices nor vegetation cover. The major part of the quickly infiltrating precipitation likely percolates through fractured and partially karstified deeper rock zones, which increases the control of bedrock flow paths on MTT. Snow accumulation and the timing of its melt play an important role for stable isotope dynamics during spring and early summer. We conclude that, in mountainous headwater catchments with relatively shallow soil layers, the hydrogeological and geochemical patterns (i.e. geochemistry, porosity and hydraulic conductivity of rocks) and snow dynamics influence storage, mixing and release of water in a stronger way than vegetation cover or topography do.

Download Full-text

Contributions of streamflow variability, concentration-discharge shifts and forested wetlands to terrestrial-aquatic solute export in Precambrian Shield headwater catchments

Ecohydrology ◽

10.1002/eco.244 ◽

2011 ◽

Vol 5 (5) ◽

pp. 596-612 ◽

Cited By ~ 9

Author(s):

Murray C. Richardson

Keyword(s):

Forested Wetlands ◽

Headwater Catchments ◽

Precambrian Shield ◽

Streamflow Variability

Download Full-text

Transit times from rainfall to baseflow in headwater catchments estimated using tritium: the Ovens River, Australia

Hydrology and Earth System Sciences Discussions ◽

10.5194/hessd-12-5427-2015 ◽

2015 ◽

Vol 12 (6) ◽

pp. 5427-5463 ◽

Cited By ~ 2

Author(s):

I. Cartwright ◽

U. Morgenstern

Keyword(s):

Transit Time ◽

Headwater Streams ◽

Stream Water ◽

Mean Transit Time ◽

Significant Proportion ◽

Low Flow ◽

Flow Conditions ◽

Headwater Catchments ◽

Flow Models ◽

Transit Times

Abstract. Headwater streams contribute a significant proportion of the total flow to many river systems, especially during summer low-flow periods. However, despite their importance, the time taken for water to travel through headwater catchments and into the streams (the transit time) is poorly constrained. Here, 3H activities of stream water are used to define transit times of water contributing to streams from the upper reaches of the Ovens River in southeast Australia at varying flow conditions. 3H activities of the stream water varied from 1.63 to 2.45 TU, which are below the average 3H activity of modern local rainfall (~3 TU). The highest 3H activities were recorded following higher winter flows and the lowest 3H activities were recorded at summer low-flow conditions. Variations of major ion concentrations and 3H activities with streamflow imply that different stores of water from within the catchment (e.g. from the soil or regolith) are mobilised during rainfall events rather than there being simple dilution of an older groundwater component by event water. Mean transit times calculated using an exponential-piston flow model range between 5 and 31 years and are higher at summer low-flow conditions. Mean transit times calculated using other flow models (e.g. exponential flow or dispersion) are similar. There are broad correlations between 3H activities and the percentage of rainfall exported from each catchment and between 3H activities and Na and Cl concentrations that allow first-order estimates of mean transit times in adjacent catchments or at different times in these catchments to be made. Water from the upper Ovens River has similar mean transit times to the headwater streams implying there is no significant input of old water from the alluvial gravels. The observation that the water contributing to the headwater streams in the Ovens catchment has a mean transit time of years to decades implies that these streams are buffered against rainfall variations on timescales of a few years. However, impacts of any changes to landuse in these catchments may take years to decades to manifest itself in changes to streamflow or water quality.

Download Full-text

Determining water transit times in dynamic environments

10.5194/egusphere-egu2020-1874 ◽

2020 ◽

Author(s):

Ian Cartwright

Keyword(s):

Hydrological Cycle ◽

Shallow Groundwater ◽

Dynamic Environments ◽

Near Surface ◽

Geochemical Tracers ◽

Headwater Catchments ◽

Transit Times ◽

Hydrological Cycles ◽

Pass Through ◽

Stream Waters

Determining the time taken for water to pass through catchments from where it is recharged to where it discharges into streams or is sampled from within the soils or aquifers (the transit time) is vital for understanding catchment functioning. Near-surface environments are dynamic and transit times are likely to vary at different stages of the hydrological cycle. Because of the lower input of bomb-pulse tritium in the southern hemisphere it is possible to determine transit times from individual tritium measurements. Additionally, because tritium is radioactive, transit times can be estimated where the catchment is not stationary. While the transit times are subject to uncertainties, this approach allows transit times at different stages of the hydrological cycles in dynamic environments to be determined.In several southeast Australian headwater catchments, the mean transit times of stream waters at low flows range from several years to decades. The tritium activities increase at higher flows, implying that there is an input of younger water at that time. However, the tritium activities generally remain below those of recent rainfall implying that simple dilution by recent rainfall is not occurring; that conclusion is consistent with the variation in the concentrations of other geochemical tracers at different streamflows. Rather, the variations in geochemistry are consistent with shallower younger stores of water from the soils and regolith being progressively mobilised as the catchments wet up during winter. These younger water stores typically have mean transit times of at least a few years. The generally long transit times imply that the southeast Australian headwater catchments have large storage capacities, probably due to the catchments being unglaciated and deeply weathered. The observation that the transit times at high flows are still relatively long suggest that, even though they may only be active for part of the year, the shallow water stores also have relatively large volumes.Understanding the transit times improves our ability to predict the behaviour and management of these catchments. The large storage capacities result in the catchments being resilient to year-on-year variations in rainfall and many of the headwater streams in southeast Australia have continued to flow through recent droughts. Similarly, the streams are less susceptible to inputs of surface contamination but contaminants stored in the soil water or shallow groundwater may impact the streams over prolonged periods. As the bomb-pulse tritium decays over the next few decades, determining mean transit times from single tritium measurements will become possible in northern hemisphere catchments. This will enable a better global understanding of catchment functioning in a wider range of environments.

Download Full-text