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

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.

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Relating stable isotope and geochemical data to conclude on water residence times in four small alpine headwater catchments with differing vegetation cover

Hydrology and Earth System Sciences Discussions ◽

10.5194/hessd-9-11005-2012 ◽

2012 ◽

Vol 9 (9) ◽

pp. 11005-11048 ◽

Cited By ~ 1

Author(s):

M. H. Mueller ◽

R. Weingartner ◽

C. Alewell

Keyword(s):

Stable Isotope ◽

Vegetation Cover ◽

Stream Water ◽

Base Flow ◽

Swiss Alps ◽

Geochemical Data ◽

Residence Times ◽

Headwater Catchments ◽

Water Ph ◽

Flow Pathways

Abstract. The mean water residence time (MRT) in a catchment gives information about storage, flow pathways, sources of water and thus also about retention and release of solutes in a catchment. To our knowledge there are no catchment studies on the influence of vegetation cover change on base flow mean water residence times. 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 catchments in the Swiss Alps (Ursern valley) were investigated to relate different vegetation cover to water residence times and geochemical behaviour of runoff. Time series of water stable isotopes were used to calculate mean water residence times. The high temporal variation of the stable isotope signals in precipitation was strongly dampened in stream base flow samples. Mean water residence times of the four catchments were 64–98 weeks. The strong dampening of our input signal might point to deeper flow paths and mixing of waters of different ages at the catchments outlets. Parent geological materials are mainly gneisses and schists but they can contain dolomite, carbonate or gypsum rich zones. The major part of the quickly infiltrating precipitation likely percolates through these deeper zones. Relatively high stream water pH, Ca and SO42− concentrations in micro catchment outlets support this conclusion. We conclude that in mountainous headwater catchments with relatively thin soil layers the geological and topographical situation and snow dynamics influence storage, mixing and release of meteoric waters and its geochemistry in a stronger way than vegetation cover or catchment size do.

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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.

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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.

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A porewater – based stable isotope approach for the investigation of subsurface hydrological processes

Hydrology and Earth System Sciences Discussions ◽

10.5194/hessd-8-9089-2011 ◽

2011 ◽

Vol 8 (5) ◽

pp. 9089-9112 ◽

Cited By ~ 1

Author(s):

J. Garvelmann ◽

C. Külls ◽

M. Weiler

Keyword(s):

Stable Isotope ◽

Input Signal ◽

Stream Water ◽

Vertical Movement ◽

Base Flow ◽

Topographic Wetness Index ◽

Flow Paths ◽

Depth Profiles ◽

Isotope Profile ◽

Fall Line

Abstract. Predicting and understanding subsurface flowpaths is still a crucial issue in hydrological research. We present an experimental approach to reveal present and past subsurface flowpaths of water in the unsaturated and saturated zone. Two hillslopes in a humid moutainous catchment have been investigated. The H2O(liquid) – H2O(vapor) equilibration laser spectroscopy method was used to obtain high resolution δ2H vertical depth profiles of porewater at various points along a fall line of a pasture hillslope in the southern Black Forest, Germany. The Porewater Stable Isotope Profile (PSIP) approach was developed to use the integrated information of several vertical depth profiles of deuterium along two transects at the hillslopes. Different shapes of depth profiles were observed in relation to hillslope position. The statistical variability (inter-quartile range and standard deviation) of each profile was used to characterize different types of depth profiles. The profiles upslope or with a weak affinity for saturation as indicated by a low topographic wetness index preserve the isotopic input signal by precipitation with a distinct seasonal variability. These observations indicate mainly vertical movement of soil water in the upper part of the hillslope before sampling. The profiles downslope or at locations with a strong affinity for saturation do not show a similar seasonal isotopic signal. The input signal is erased in the foothills and a large proportion of pore water samples are close to the isotopic values of δ2H in stream water during base flow. Near the stream indications for efficient mixing of water from lateral subsurface flow paths with vertical percolation are found.

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Water flow pathways and the water balance within a head-water catchment containing a dambo: inferences drawn from hydrochemical investigations

Hydrology and Earth System Sciences ◽

10.5194/hess-3-581-1999 ◽

1999 ◽

Vol 3 (4) ◽

pp. 581-591 ◽

Cited By ~ 9

Author(s):

M. P. McCartney ◽

C. Neal

Keyword(s):

Water Balance ◽

Stream Water ◽

Management Strategies ◽

Dry Season ◽

Sub Saharan Africa ◽

Flow Paths ◽

Headwater Catchments ◽

Sub Saharan ◽

Flow Pathways ◽

Water Catchment

Abstract. Dambos, seasonally saturated wetlands, are widespread in headwater catchments in sub-Saharan Africa. It is widely believed that they play an important role in regional hydrology but, despite research conducted over the last 25 years, their hydrological functions remain poorly understood. To improve conceptualisation of hydrological flow paths and investigate the water balance of a small Zimbabwean catchment containing a single dambo, measurements of alkalinity and chloride in different water types within the catchment have been used as chemical markers. The temporal variation in alkalinity is consistent with the premise that all stream water, including the prolonged dry season recession, is derived predominantly from shallow sources. The proposition that dry season recession flows are maintained by water travelling at depth within the underlying saprolite is not substantiated. There is evidence that a low permeability clay lens, commonly present in many dambos, acts as a barrier for vertical water exchange. However, the highly heterogeneous chemical composition of different waters precludes quantitative hydrograph split-ting using end member mixing analysis. Calculation of the chloride mass-balance confirms that, after rainfall, evaporation is the largest component of the catchment water budget. The study provides improved understanding of the hydrological functioning of dambos. Such understanding is essential for the development and implementation of sustainable management strategies for this landform.

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Understanding the sources and transit times of water contributing streamflow from intermittent headwater catchments in semi-arid areas

10.5194/egusphere-egu2020-2030 ◽

2020 ◽

Author(s):

Shovon Barua ◽

Ian Cartwright ◽

Edoardo Daly ◽

Uwe Morgenstern

Keyword(s):

Land Use ◽

Riparian Zone ◽

Stream Water ◽

Land Use Changes ◽

Significant Proportion ◽

Catchment Management ◽

Stream Health ◽

Headwater Catchments ◽

Transit Times ◽

Semi Arid

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.

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Factors influencing stream water transit times in tropical montane watersheds

Hydrology and Earth System Sciences Discussions ◽

10.5194/hessd-12-10975-2015 ◽

2015 ◽

Vol 12 (10) ◽

pp. 10975-11011 ◽

Cited By ~ 2

Author(s):

L. E. Muñoz-Villers ◽

D. R. Geissert ◽

F. Holwerda ◽

J. J. McDonnell

Keyword(s):

Land Use ◽

Stream Water ◽

Spatial Scales ◽

Cloud Forest ◽

Mean Transit Time ◽

Field Measurements ◽

Drainage Density ◽

Soil Water Retention ◽

Transit Times ◽

Retention Characteristics

Abstract. Stream water mean transit time (MTT) is a fundamental hydrologic parameter that integrates the distribution of sources, flow paths and storages present in catchments. However, in the tropics little MTT work has been carried out, despite its usefulness for providing important information on watershed functioning at different spatial scales in (largely) ungauged basins. In particular, very few studies have quantified stream MTTs and related to catchment characteristics in tropical montane regions. Here we examined topographic, land use/cover and soil hydraulic controls on baseflow transit times for nested watersheds (0.1–34 km2) within a humid mountainous region, underlain by volcanic soil (Andisols) in central Veracruz (eastern Mexico). We used a 2 year record of bi-weekly isotopic composition of precipitation and stream baseflow data to estimate MTT. Land use/cover and topographic parameters (catchment area and form, drainage density, slope gradient and length) were derived from GIS analysis. Soil water retention characteristics, and depth and permeability of the soil–bedrock interface were obtained from intensive field measurements and laboratory analysis. Results showed that baseflow MTT ranged between 1.2 and 2.7 years across the 12 study catchments. Overall, MTTs across scales were mainly controlled by catchment slope and the permeability observed at the soil–bedrock interface. In association with topography, catchment form, land cover and the depth to the soil–bedrock interface were also identified as important features influencing baseflow MTTs. The greatest differences in MTTs were found at the smallest (0.1–1.5 km2) and the largest scales (14–34 km2). Interestingly, longest stream MTTs were found in the headwater cloud forest catchments.

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Land use alters dominant water sources and flow paths in tropical montane catchments in East Africa

10.5194/hess-2018-61 ◽

2018 ◽

Author(s):

Suzanne R. Jacobs ◽

Edison Timbe ◽

Björn Weeser ◽

Mariana C. Rufino ◽

Klaus Butterbach-Bahl ◽

...

Keyword(s):

Land Use ◽

Transit Time ◽

Stream Water ◽

Mean Transit Time ◽

Water Sources ◽

Natural Forest ◽

Smallholder Agriculture ◽

Catchment Hydrology ◽

Flow Paths ◽

Tea Plantation

Abstract. Conversion of natural forest to other land uses could lead to significant changes in catchment hydrology, but the nature of these changes has been insufficiently investigated in tropical montane catchments, especially in Africa. To address this knowledge gap, we identified stream water sources and flow paths in three tropical montane sub-catchments (27–36 km2) with different land use (natural forest, smallholder agriculture and commercial tea plantations) within a 1 021 km2 catchment in the Mau Forest Complex, Kenya. Weekly samples were collected from stream water, precipitation and soil water for 75 weeks and analysed for stable water isotopes (δ2H and δ18O) for mean transit time estimation, whereas trace element samples from stream water and potential end members were collected over a period of 55 weeks for end member mixing analysis. Stream water mean transit time was similar (~ 4 years) in the three sub-catchments, and ranged from 3.2–3.3 weeks in forest soils and 4.5–7.9 weeks in pasture soils at 15 cm depth to 10.4–10.8 weeks in pasture soils at 50 cm depth. The contribution of springs and wetlands to stream discharge increased from 18, 1 and 48 % during low flow to 22, 51 and 65 % during high flow in the natural forest, smallholder agriculture and tea plantation sub-catchments, respectively. The dominant stream water source in the tea plantation sub-catchment was spring water (56 %), while precipitation was dominant in the smallholder agriculture (59 %) and natural forest (45 %) sub-catchments. These results confirm that catchment hydrology is strongly influenced by land use, which could have serious consequences for water-related ecosystem services, such as provision of clean water.

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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.

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Assessment of hydrological pathways in East African montane catchments under different land use

Hydrology and Earth System Sciences ◽

10.5194/hess-22-4981-2018 ◽

2018 ◽

Vol 22 (9) ◽

pp. 4981-5000 ◽

Cited By ~ 12

Author(s):

Suzanne R. Jacobs ◽

Edison Timbe ◽

Björn Weeser ◽

Mariana C. Rufino ◽

Klaus Butterbach-Bahl ◽

...

Keyword(s):

Land Use ◽

Transit Time ◽

Stream Water ◽

Mean Transit Time ◽

Natural Forest ◽

Smallholder Agriculture ◽

Tea Plantation ◽

Water Fraction ◽

Transit Times ◽

Water Isotope

Abstract. Conversion of natural forest (NF) to other land uses could lead to significant changes in catchment hydrology, but the nature of these changes has been insufficiently investigated in tropical montane catchments, especially in Africa. To address this knowledge gap, we aimed to identify stream water (RV) sources and flow paths in three tropical montane sub-catchments (27–36 km2) with different land use (natural forest, NF; smallholder agriculture, SHA; and commercial tea and tree plantations, TTP) within a 1021 km2 catchment in the Mau Forest complex, Kenya. Weekly samples were collected from stream water, precipitation (PC) and mobile soil water for 75 weeks and analysed for stable isotopes of water (δ2H and δ18O) for mean transit time (MTT) estimation with two lumped parameter models (gamma model, GM; and exponential piston flow model, EPM) and for the calculation of the young water fraction. Weekly samples from stream water and potential endmembers were collected over a period of 55 weeks and analysed for Li, Na, Mg, K, Rb, Sr and Ba for endmember mixing analysis (EMMA). Solute concentrations in precipitation were lower than in stream water in all catchments (p < 0.05), whereas concentrations in springs, shallow wells and wetlands were generally more similar to stream water. The stream water isotope signal was considerably damped compared to the isotope signal in precipitation. Mean transit time analysis suggested long transit times for stream water (up to 4 years) in the three sub-catchments, but model efficiencies were very low. The young water fraction ranged from 13 % in the smallholder agriculture sub-catchment to 15 % in the tea plantation sub-catchment. Mean transit times of mobile soil water ranged from 3.2–3.3 weeks in forest soils and 4.5–7.9 weeks in pasture soils at 15 cm depth to 10.4–10.8 weeks in pasture soils at 50 cm depth. The contribution of springs and wetlands to stream discharge increased from a median of 16.5 (95 % confidence interval: 11.3–22.9), 2.1 (−3.0–24.2) and 50.2 (30.5–65.5) % during low flow to 20.7 (15.2–34.7), 53.0 (23.0–91.3) and 69.4 (43.0–123.9) % during high flow in the natural forest, smallholder agriculture and tea plantation sub-catchments, respectively. Our results indicate that groundwater is an important component of stream water, irrespective of land use. The results further suggest that the selected transit time models and tracers might not be appropriate in tropical catchments with highly damped stream water isotope signatures. A more in-depth investigation of the discharge dependence of the young water fraction and transit time estimation using other tracers, such as tritium, could therefore shed more light on potential land use effects on the hydrological behaviour of tropical montane catchments.

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