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

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.

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

2018 ◽  
Vol 22 (9) ◽  
pp. 4981-5000 ◽  
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.

scholarly journals Factors influencing stream water transit times in tropical montane watersheds

2015 ◽  
Vol 12 (10) ◽  
pp. 10975-11011 ◽  
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.

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

2015 ◽  
Vol 19 (9) ◽  
pp. 3771-3785 ◽  
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.

Estimating groundwater mean transit time from SF6 in stream water: field example and planning metrics for a reach mass-balance approach

2022 ◽  
Author(s):  
Craig R. Jensen ◽  
David P. Genereux ◽  
Troy E. Gilmore ◽  
D. Kip Solomon ◽  
Aaron R. Mittelstet ◽  
...  
Keyword(s):  
Mass Balance ◽  
Transit Time ◽  
Stream Water ◽  
Mean Transit Time ◽  
Balance Approach ◽  
Water Field ◽  
Mass Balance Approach

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

2013 ◽  
Vol 17 (4) ◽  
pp. 1661-1679 ◽  
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.

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

2015 ◽  
Vol 12 (6) ◽  
pp. 5427-5463 ◽  
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.

scholarly journals Dating of streamwater using tritium in a post-bomb world: continuous variation of mean transit time with streamflow

2010 ◽  
Vol 7 (4) ◽  
pp. 4731-4760 ◽  
Author(s):  
U. Morgenstern ◽  
M. K. Stewart ◽  
R. Stenger
Keyword(s):  
New Zealand ◽  
Transit Time ◽  
Time Series Data ◽  
Stream Water ◽  
Mean Transit Time ◽  
Low Flow ◽  
Series Data ◽  
Transit Times ◽  
Bomb Test ◽  
The Mean

Abstract. Tritium measurements of streamwater draining the Toenepi catchment, a small dairy farming area in Waikato, New Zealand, have shown that the mean transit time of the water varies with the flow of the stream. Mean transit times through the catchment are 2–5 years during high baseflow conditions (in winter), becoming older as streamflow decreases (in summer), and then quite dramatically older during drought conditions, with ages of more than 100 years. Older water seems to be gained in the lower reaches of the stream, compared to younger water in the headwater catchment. The groundwater store supplying baseflow was estimated from the mean transit time and average baseflow to be 15.4×106 m3 of water, about 1 m water equivalent over the catchment and 2.3 times total annual streamflow. Nitrate from recent intensified land use is relatively high at normal streamflow, but is low at times of low flow with old water. This reflects both lower nitrate loading in the catchment several decades ago, and active denitrification processes in older groundwater. Silica, leached from the aquifer material and accumulating in the water in proportion to contact time, is high at times of low streamflow. There was a good correlation between silica and streamwater age, which potentially allows silica concentrations to be used as a proxy for age when calibrated by tritium measurements. This study shows that tritium dating of stream water is possible with single tritium measurements now that bomb-test tritium has effectively disappeared from hydrological systems in New Zealand, without the need for time-series data.

scholarly journals Dating of streamwater using tritium in a post nuclear bomb pulse world: continuous variation of mean transit time with streamflow

2010 ◽  
Vol 14 (11) ◽  
pp. 2289-2301 ◽  
Author(s):  
U. Morgenstern ◽  
M. K. Stewart ◽  
R. Stenger
Keyword(s):  
New Zealand ◽  
Transit Time ◽  
Time Series Data ◽  
Stream Water ◽  
Mean Transit Time ◽  
Dairy Farming ◽  
Low Flow ◽  
Series Data ◽  
Transit Times ◽  
The Mean

Abstract. Tritium measurements of streamwater draining the Toenepi catchment, a small dairy farming area in Waikato, New Zealand, have shown that the mean transit time of the water varies with the flow rate of the stream. Mean transit times through the catchment are 2–5 years during high baseflow conditions in winter, increasing to 30–40 years as baseflow decreases in summer, and then dramatically older water during drought conditions with mean transit time of more than 100 years. Older water is gained in the lower reaches of the stream, compared to younger water in the headwater catchment. The groundwater store supplying baseflow was estimated from the mean transit time and average baseflow to be 15.4 × 106 m3 of water, about 1 m water equivalent over the catchment and 2.3 times total annual streamflow. Nitrate is relatively high at higher flow rates in winter, but is low at times of low flow with old water. This reflects both lower nitrate loading in the catchment several decades ago as compared to current intensive dairy farming, and denitrification processes occurring in the older groundwater. Silica, leached from the aquifer material and accumulating in the water in proportion to contact time, is high at times of low streamflow with old water. There was a good correlation between silica concentration and streamwater age, which potentially allows silica concentrations to be used as a proxy for age when calibrated by tritium measurements. This study shows that tritium dating of stream water is possible with single tritium measurements now that bomb-test tritium has effectively disappeared from hydrological systems in New Zealand, without the need for time-series data.

scholarly journals Application of Water Stable Isotopes for Hydrological Characterization of the Red River (Asia)

Water ◽  
2021 ◽  
Vol 13 (15) ◽  
pp. 2051
Author(s):  
Nho Lan Nguyen ◽  
Thu Nga Do ◽  
Anh Duc Trinh
Keyword(s):  
Transit Time ◽  
Mean Transit Time ◽  
River System ◽  
Red River ◽  
Catchment Hydrology ◽  
Natural Conditions ◽  
Reservoir Volume ◽  
The Mean ◽  
Hydrological Station

Fraction of young water (Fyw) and mean transit time (MTT, τ) calculated from water isotope profiles are valuable information for catchment hydrological assessment, especially in anthropogenically impacted region where natural conditions may not be decisive to catchment hydrology. The calculation of Fyw and MTT were performed on three subsets of δ18O_H2O data collected at the Hanoi meteo-hydrological station, Red River, in three periods; 2002–2005, 2015, and 2018–2019. The mean (min and max) values of δ18O_H2O in rainwater over the three periods are, respectively, −5.3‰ (−11.0 and −1.2‰), −5.4‰ (−10.7 and −1.4‰), and −4.5‰ (−13.9 and 1.7‰). The corresponding values in river water are −8.4‰ (−9.8 and −6.9‰), −8.5‰ (−9.1 and −7.7‰), and −8.4‰ (−9.5 and −7.2‰), respectively. The mean of Fyw calculated from the δ18O_H2O data for different periods is 22 ± 9%, 10 ± 5%, and 8 ± 3%. Mean transit time is 4.69 ± 15.57, 1.65 ± 1.53, and 2.06 ± 1.87 years. The calculated Fyw (MTT) is negatively (positively) proportional to change in reservoir volume over the three periods, which is logical, since reservoirs tend to keep more water in the catchment and slower down water flow. The strong variation of Fyw and τ, two essential variables characterizing the catchment hydrology, represents an anthropogenic impact in the Red River system.

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