Tracer-based estimation of temporal variation of water sources: an insight from supra- and subglacial environments

ABSTRACT

The temporal variations in electrical conductivity and the stable isotopes of water, δD and δ¹⁸O, were examined at Chhota Shigri Glacier, India, to understand water sources and flow paths to discharge. Discharge is highly influenced by supraglacially derived meltwater during peak ablation, and subglacial meltwaters are more prominent at the end of the melt season. The slope of the best fit linear regression line for δD versus δ¹⁸O, for both supraglacial and runoff water, is lower than that for precipitation (snow and rain) and surface ice, indicating strong isotopic fractionation associated with the melting processes. The slope of the local meteoric water line (LMWL) is close to that of the global meteoric water line (GMWL), reflecting that the moisture source is predominantly oceanic. The d-excess variation in rainwater confirms that the southwest monsoon is the main contributor during summer while the remainder including winter is mostly influenced by westerlies.     

Characteristics of soil moisture in permafrost observed in East Siberian taiga with stable isotopes of water

Soil moisture and its isotopic composition were observed at Spasskaya Pad experimental forest near Yakutsk, Russia, during summer in 1998, 1999, and 2000. The amount of soil water (plus ice) was estimated from volumetric soil water content obtained with time domain reflectometry. Soil moisture and its δ¹⁸O showed large interannual variation depending on the amount of summer rainfall. The soil water δ¹⁸O decreased with soil moisture during a dry summer (1998), indicating that ice meltwater from a deeper soil layer was transported upward. On the other hand, during a wet summer (1999), the δ¹⁸O of soil water increased due to percolation of summer rain with high δ¹⁸O values. Infiltration after spring snowmelt can be traced down to 15 cm by the increase in the amount of soil water and decrease in the δ¹⁸O because of the low δ¹⁸O of deposited snow. About half of the snow water equivalent (about 50 mm) recharged the surface soil. The pulse of the snow meltwater was, however, less important than the amount of summer rainfall for intra‐annual variation of soil moisture. Excess water at the time just before soil freezing, which is controlled by the amount of summer rainfall, was stored as ice during winter. This water storage stabilizes the rate of evapotranspiration. Soil water stored in the upper part of the active layer (surface to about 120 cm) can be a water source for transpiration in the following summer. On the other hand, once water was stored in the lower part of the active layer (deeper than about 120 cm), it would not be used by plants in the following summer, because the lower part of the active layer thaws in late summer after the plant growing season is over. Copyright © 2002 John Wiley & Sons, Ltd.     

Factors controlling diurnal variation in the isotopic composition of atmospheric water vapour observed in the taiga,eastern Siberia

Deciduous forest covers vast areas of permafrost under severe dry climate in eastern Siberia. Understanding the water cycle in this forest ecosystem is quite important for climate projection. In this study, diurnal variations in isotopic compositions of atmospheric water vapour were observed in eastern Siberia with isotope analyses of precipitation, sap water of larch trees, soil water, and water in surface organic layer during the late summer periods of 2006, 2007, and 2008. In these years, the soil moisture content was considerably high due to unusually large amounts of summer rainfall and winter snowfall. The observed sap water δ¹⁸O ranged from ?17.9‰ to ?13.3‰, which was close to that of summer precipitation and soil water in the shallow layer, and represents that of transpired water vapour. On sunny days, as the air temperature and mixing ratio rose from predawn to morning, the atmospheric water vapour δ¹⁸O increased by 1‰ to 5‰ and then decreased by about 2‰ from morning to afternoon with the mixing ratio. On cloudy days, by contrast, the afternoon decrease in δ¹⁸O and the mixing ratio was not observed. These results show that water vapour that transpired from plants, with higher δ¹⁸O than the atmospheric water vapour, contributes to the increase in δ¹⁸O in the morning, whereas water vapour in the free atmosphere, with lower δ¹⁸O, contributes to the decrease in the afternoon on sunny days. The observed results reveal the significance of transpired water vapour, with relatively high δ¹⁸O, in the water cycle on a short diurnal time scale and confirm the importance of the recycling of precipitation through transpiration in continental forest environments such as the eastern Siberian taiga. Copyright © 2012 John Wiley & Sons, Ltd.     

From precipitation to runoff: stable isotopic fractionation effect of glacier melting on a catchment scale

Stable isotope variability and fractionation associated with transformation of precipitation/accumulation to firn to glacial river water is critical in a variety of climatic, hydrological and paleoenvironmental studies. This paper documents the modification of stable isotopes in water from precipitation to glacier runoff in an alpine catchment located in the central Tibetan Plateau. Isotopic changes are observed by sampling firnpack profiles, glacier surface snow/ice, meltwater on the glacier surface and catchment river water at different times during a melt season. Results show the isotopic fractionation effects associated with glacier melt processes. The slope of the δD‐δ¹⁸O regression line and the deuterium excess values decreased from the initial precipitation to the melt‐impacted firnpack (slope from 9.3 to 8.5 and average d‐excess from 13.4‰ to 7.4‰). The slope of the δD‐δ¹⁸O line further decreased to 7.6 for the glacier runoff water. The glacier surface snow/ice from different locations, which produces the main runoff, had the same δD‐δ¹⁸O line slope but lower deuterium excess (by 3.9‰) compared to values observed in the firnpack profile during the melt season. The δD‐δ¹⁸O regression line for the river water exhibited a lower slope compared to the surface snow/ice samples, although they were closely located on the δD‐δ¹⁸O plot. Isotope values for the river and glacier surface meltwater showed little scatter around the δD‐δ¹⁸O regression line, although the samples were from different glaciers and were collected on different days. Results indicate a high consistency of isotopic fractionation in the δD‐δ¹⁸O relationships, as well as a general consistency and temporal covariation of meltwater isotope values at the catchment scale. Copyright © 2013 John Wiley & Sons, Ltd.     

Preferential exchange rate effect of isotopic fractionation in a melting snowpack

Stable isotope exchange processes between solid and liquid phases of a natural melting snowpack are investigated in detail by separating the liquid water from snow grains at different depths of the snowpack and collecting the bottom discharge using a lysimeter. In the melting–freezing mass exchange process between the two phases, the theoretical slope of the δD? δ¹⁸O line for newly refrozen ice is calculated to be nearly that of pore water. However, based on observations of the isotopic evolution and snow grain coarsening of the snowpack, it is demonstrated that the slope of the δD? δ¹⁸O line for newly refrozen ice is equal to that of the original ice. This is proved to be due to preferential water flow in the snowpack, which leads to relatively more deuterium and less oxygen‐18 in the mobile water than the immobile water because of the kinetic effect. Higher mass exchange rate in the mobile water region results in excess deuterium in the bulk refrozen ice, compared with the fractionation of uniform fractionation factors and exchange rate. This effect, which is termed the ‘preferential exchange rate effect of isotopic fractionation’, is shown to be larger in the lower part than the upper part of the snowpack. Copyright © 2008 John Wiley & Sons, Ltd.     

Characterization of lake water and ground water movement in the littoral zone of Williams Lake,a closed‐basin lake in north central Minnesota

Williams Lake, Minnesota is a closed‐basin lake that is a flow‐through system with respect to ground water. Ground‐water input represents half of the annual water input and most of the chemical input to the lake. Chemical budgets indicate that the lake is a sink for calcium, yet surficial sediments contain little calcium carbonate. Sediment pore‐water samplers (peepers) were used to characterize solute fluxes at the lake‐water–ground‐water interface in the littoral zone and resolve the apparent disparity between the chemical budget and sediment data. Pore‐water depth profiles of the stable isotopes δ¹⁸O and δ²H were non‐linear where ground water seeped into the lake, with a sharp transition from lake‐water values to ground‐water values in the top 10 cm of sediment. These data indicate that advective inflow to the lake is the primary mechanism for solute flux from ground water. Linear interstitial velocities determined from δ²H profiles (316 to 528 cm/yr) were consistent with velocities determined independently from water budget data and sediment porosity (366 cm/yr). Stable isotope profiles were generally linear where water flowed out of the lake into ground water. However, calcium profiles were not linear in the same area and varied in response to input of calcium carbonate from the littoral zone and subsequent dissolution. The comparison of pore‐water calcium profiles to pore‐water stable isotope profiles indicate calcium is not conservative. Based on the previous understanding that 40–50 % of the calcium in Williams Lake is retained, the pore‐water profiles indicate aquatic plants in the littoral zone are recycling the retained portion of calcium. The difference between the pore‐water depth profiles of calcium and δ¹⁸O and δ²H demonstrate the importance of using stable isotopes to evaluate flow direction and source through the lake‐water–ground‐water interface and evaluate mechanisms controlling the chemical balance of lakes. Published in 2003 by John Wiley & Sons, Ltd.     

Stability of massive ground ice bodies in University Valley,McMurdo Dry Valleys of Antarctica: Using stable O–H isotope as tracers of sublimation in hyper-arid regions

To date, studies of the stability of subsurface ice in the McMurdo Dry Valleys of Antarctica have been mainly based on climate-based vapor diffusion models. In University Valley (1800 m), a small glacier is found at the base of the head of the valley, and adjacent to the glacier, a buried body of massive ice was uncovered beneath 20–40 cm of loose cryotic sediments and sandstone boulders. This study assesses the origin and stability of the buried body of massive ice by measuring the geochemistry and stable O–H isotope composition of the ice and applies a sublimation and molecular diffusion model that accounts for the observed trends. The results indicate that the buried massive ice body represents an extension of the adjacent glacier that was buried by a rock avalanche during a cold climate period. The contrasting δ¹⁸O profiles and regression slope values between the uppermost 6 cm of the buried massive ice (upward convex δ¹⁸O profile and S_D-18O = 5.1) and that below it (progressive increase in δ¹⁸O and S_D-18O = 6.4) suggest independent post-depositional processes affected the isotope composition of the ice. The upward convex δ¹⁸O profile in the uppermost 6 cm is consistent with the ice undergoing sublimation. Using a sublimation and molecular diffusion model, and assuming that diffusion occurred through solid ice, the sublimation rate needed to fit the measured δ¹⁸O profile is 0.2 ? 10^? 3 mm yr^? 1, a value that is more similar to net ice removal rates derived from ³He data from cobbles in Beacon Valley till (7.0 ? 10^? 3 mm yr^? 1) than sublimation rates computed based on current climate (0.1–0.2 mm yr^?1). We suggest that the climate-based sublimation rates are offset due to potential ice recharge mechanisms or to missing parameters, particularly the nature and thermo-physical properties of the overlying sediments (i.e., temperature, humidity, pore structure and ice content, grain size).     

Distribution of tritium and stable isotopes in precipitation in Syria

Abstract

The paper discusses aspects of the isotopic composition (tritium and stable isotopes) of precipitation, which was monitored from 2000 to 2003 at 12 stations in Syria. The seasonal variations in δ¹⁸O are smaller at the western stations than at the eastern ones due to low seasonal temperature variations. A good correlation between δ²H and δ¹⁸O was obtained for each station, and the slopes of the local meteoric water lines are significantly lower than the Global Meteoric Water Line. Values of d-excess decrease from 19‰ at the western stations to 13‰ at the eastern ones, indicating the influence of precipitation generated by air masses coming from the Mediterranean Sea. A reliable altitude effect represented by depletion of heavy stable isotopes (δ¹⁸O and δ²H), of about??0.21‰ and??1.47‰ per 100 m elevation, respectively, was observed. Monthly tritium contents in precipitation, and seasonal variations, are less at the western stations than at the eastern ones. The weighted mean tritium values are between 3 and 9 TU, and increase with distance from the Syrian coast by 1 TU/100 km.

Citation Al Charideh, A. R. & Abou Zakhem, B. (2010) Distribution of tritium and stable isotopes in precipitation in Syria. Hydrol. Sci. J. 55(5), 832–843.     

Factors controlling spatial and seasonal distributions of precipitation δ18O in China

Utilising datasets from the Global Network of Isotopes in Precipitation of the International Atomic Energy Agency, and previous isotopic studies, we investigated δ¹⁸O spatial and temporal patterns in Chinese precipitation. Significantly positive relationships existed between precipitation δ¹⁸O and air temperature above the north of 35°N and in high altitude regions above 32°N. Significantly negative relationships between precipitation δ¹⁸O and the precipitation amount existed below south of 35°N. These temperature and precipitation effects became stronger with increasing altitude except in high altitude regions between 32°N and 35°N. The NCEP/NCAR reanalysis data from 1980 to 2004 showed that variations in spatial and seasonal wind fields at 700 hpa and total precipitable water from the ground to the top of the atmosphere were correlated with the monthly spatial distribution of precipitation δ¹⁸O. Basing on this relationship, we established quantitative correlations between the mean monthly precipitation δ¹⁸O and both latitude and temperature in different seasons. We found that spatial variations in precipitation δ¹⁸O could be described well using the Bowen–Wilkinson model and second‐order equations developed during the present study only in winter (from December to February). During the rest of the year, patterns were too complex to predict using simple models. The results suggest that it is difficult to demonstrate variations of precipitation δ¹⁸O throughout the year and for all regions of China using a single model. Moreover, the new models for the relationships among precipitation, latitude, and temperature were better able to depict the variations in precipitation δ¹⁸O than the Bowen–Wilkinson model. Copyright © 2011 John Wiley & Sons, Ltd.     

How do non-halophyte locust trees thrive in temperate coastal regions: A study of salinity and multiple environmental factors on water uptake patterns

Understanding plant water use patterns is crucial for comprehending the dynamics of the soil–plant-atmosphere continuum and evaluating the adaptability of plants across diverse ecosystems. However, there remains a gap in our comprehension of non-halophyte plants' water uptake patterns and driving factors in temperate coastal regions. For this reason, we used locust trees (a widely planted non-halophyte tree species in northern China) as a study subject. We collected water isotope data (δ²H and δ¹⁸O) for locust trees xylem and soil over two consecutive growing seasons. The MixSIAR model was used along with five distinct sets of input data (single isotopes, uncorrected dual isotopes, and corrected dual isotopes incorporating δ²H data obtained by soil water line or cryogenic vacuum distillation methods) to infer water utilization patterns. The results indicated that locust trees primarily absorb shallow soil water (0–20 cm, 29.4% ± 16.9%) and deep soil water (120–180 cm, 24.7% ± 5.8%). Pearson's correlation analysis revealed the key driving factors behind water uptake patterns were vegetation transpiration and soil salinity. Remarkably, the build up of salts in the lower soil layer (60–120 cm) hinders the absorption of water by plants. To prevent high salt concentrations from affecting water uptake in non-halophyte plants, we recommend implementing sufficient irrigation from March to April each year to meet the water needs of plant growth and regulate the accumulation of salts in various soil layers. This study reveals the dynamic water utilization strategy of non-halophyte plants in temperate coastal regions, offering valuable information for water resources management.     

Seasonal changes in the water use strategies of three co‐occurring desert shrubs

Water is a major limiting factor in desert ecosystems. In order to learn how plants cope with changes in water resources over time and space, it is important to understand plant–water relations in desert region. Using the oxygen isotopic tracing method, our study clarified the seasonal changes in the water use strategies of three co‐occurring desert shrubs. During the 2012 growing season, δ¹⁸O values were measured for xylem sap, the soil water in different soil layers between 0 and 300 cm depth and groundwater. Based on the similarities in δ¹⁸O values for the soil water in each layer, three potential water sources were identified: shallow soil water, middle soil water and deep soil water. Then we calculated the percentage utilization of potential water sources by each species in each season using the linear mixing model. The results showed that the δ¹⁸O values of the three species showed a clear seasonal pattern. Reaumuria songarica used shallow soil water when shallow layer was relatively wet in spring, but mostly took up middle soil water in summer and autumn. Nitraria tangutorum mainly utilized shallow and middle soil water in spring, but mostly absorbed deep soil water in summer and autumn. Tamarix ramosissima utilized the three water sources evenly in spring and primarily relied on deep soil water in summer and autumn. R. songarica and N. tangutorum responded quickly to large rainfall pulses during droughts. Differential root systems of the three species resulted in different seasonal water use strategies when the three competed for water. Copyright © 2013 John Wiley & Sons, Ltd.     

Calibration method affects the measured δ2H and δ18O in soil water by direct H2Oliquid–H2Ovapour equilibration with laser spectroscopy

The direct H₂O_liquid–H₂O_vapour equilibration method utilizing laser spectroscopy (DVE-LS) is a way to measure soil pore water stable isotopes. Various equilibration times and calibration methods have been used in DVE-LS. Yet little is known about their effects on the accuracy of the obtained isotope values. The objective of this study was to evaluate how equilibration time and calibration methods affect the accuracy of DVE-LS. We did both spiking and field soil experiments. For the spiking experiment, we applied DVE-LS to four soils of different textures, each of which was subjected to five water contents and six equilibration times. For the field soil experiment, we applied three calibration methods for DVE-LS to two field soil profiles, and the results were compared with cryogenic vacuum distillation (CVD)-LS. Results showed that DVE-LS demonstrated higher δ²H and δ¹⁸O as equilibration time increased, but 12 to 24 hr could be used as optimal equilibration time. For field soil samples, DVE-LS with liquid waters as standards led to significantly higher δ²H and δ¹⁸O than CVD-LS, with root mean square error (RMSE) of 8.06‰ for δ²H and 0.98‰ for δ¹⁸O. Calibration with soil texture reduced RMSE to 3.53‰ and 0.72‰ for δ²H and δ¹⁸O, respectively. Further, calibration with both soil texture and water content decreased RMSE to 3.10‰ for δ²H and 0.73‰ for δ¹⁸O. Our findings conclude that the calibration method applied may affect the measured soil water isotope values from DVE-LS.     

Spatial variation of isotope composition in precipitation in a tropical environment: a case study from the Deduru Oya river basin,Sri Lanka

The stable isotope composition (¹⁸O and ²H) in the tropical precipitation collected from 18 locations throughout the Deduru Oya river basin in Sri Lanka, has been studied during August and September 2001, in order to characterize the isotopic composition of precipitation in the dry and intermediate climatic zones of Sri Lanka. The isotope compositions are described with respect to the distance from the coast and the altitude. The analyses show that δ¹⁸O vary from ? 5·11 to 1·39‰ and δD vary from ? 35·71 to 12·55‰. The d‐excess values range from ? 0·65 to 13·17 with an average value of ～7. Regression for the δ¹⁸O ? δD is y = 6·8x + 4·9 (R² = 0·9) which is compatible with the precipitation in other tropical regions. The lower slope in the regression line and the lower d‐excess value indicate high temperature events which were possibly aided by concentration through successive evaporation within the atmosphere. The spatial variation of isotope composition indicates two different cloud contributions for the rain events, of which one may be linked to the Indian Ocean contribution and the other to the high altitude condensation. Copyright © 2008 John Wiley & Sons, Ltd.     

Role of bedrock groundwater in the rainfall–runoff process in a small headwater catchment underlain by volcanic rock

The role of bedrock groundwater in rainfall–runoff processes is poorly understood. Hydrometric, tracer and subsurface water potential observations were conducted to study the role of bedrock groundwater and subsurface flow in the rainfall–runoff process in a small headwater catchment in Shiranui, Kumamoto prefecture, south‐west Japan. The catchment bedrock consists of a strongly weathered, fractured andesite layer and a relatively fresh continuous layer. Major chemical constituents and stable isotopic ratios of δ¹⁸O and δD were analysed for spring water, rainwater, soil water and bedrock groundwater. Temporal and spatial variation in SiO₂ showed that stream flow under the base flow condition was maintained by bedrock groundwater. Time series of three components of the rainstorm hydrograph (rainwater, soil water and bedrock groundwater) separated by end member mixing analysis showed that each component fluctuated during rainstorm, and their patterns and magnitudes differed between events. During a typical mid‐magnitude storm event, a delayed secondary runoff peak with 1·0 l s⁻¹ was caused by increase in the bedrock groundwater component, whereas during a large rainstorm event the bedrock groundwater component increased to ≈ 2·5 l s⁻¹. This research shows that the contribution of bedrock groundwater and soil water depends strongly on the location of the groundwater table, i.e. whether or not it rises above the soil–bedrock interface. Copyright © 2010 John Wiley & Sons, Ltd.     

Hydrological processes controlling ground and surface water flow from a hypermaritime forest–peatland complex,Diana Lake Provincial Park,British Columbia,Canada

The proposed harvesting of previously undeveloped forests in north coastal British Columbia requires an understanding of hydrological responses. Hydrometric and isotopic techniques were used to examine the hydrological linkages between meteoric inputs to the surface‐groundwater system and runoff response patterns of a forest‐peatland complex. Quickflow accounted for 72–91% of peak storm discharge. The runoff ratio was lowest for open peatland areas with thick organic horizons (0·02–0·05) due to low topographic gradients and many surface depressions capable of retaining surface water. Runoff ratio increased comparatively for ephemeral surface seep flows (0·06–0·40) and was greatest in steeply sloping forest communities with more permeable soils (0·33–0·69). The dominant mechanism for runoff generation was saturated shallow subsurface flow. Groundwater fluxes from the organic horizon of seeps (1·70–1·72 m³ day^?1 m^?1) were an important component of quickflow. The homogeneous δ²H? δ¹⁸O composition of groundwater indicated attenuation of the seasonal rainfall signal by mixing during recharge. The positive correlation (r² = 0·64 and 0·38, α = 0·05) between slope index and δ¹⁸O values in groundwater suggests that the spatial pattern in the δ¹⁸O composition along the forest‐peatland complex is influenced by topography and provides evidence that topographic indices may be used to predict groundwater residence time. Copyright © 2006 John Wiley & Sons, Ltd.     

Origins of groundwater inferred from isotopic patterns of the Badain Jaran Desert, northwestern China

There are many viewpoints about the sources of groundwater in the Badain Jaran Desert (BJD), such as precipitation and snowmelt from the Qilian Mountains (the upper reaches [UR] of the Heihe River Basin [HRB]) and precipitation from the BJD and the Yabulai Mountains. To understand the source of the groundwater of the BJD and their possible associations with nearby bodies of water, we analyzed variations of stable isotope ratios (δD and δ(18) O) and the deuterium excess (d-excess) of groundwater and precipitation in the BJD, of groundwater, precipitation, river and spring water in the UR, and of groundwater and river water in the middle and lower reaches (MR and LR) of the HRB. In addition, the climatic condition under which the groundwater was formed in the BJD was also discussed. We found obvious differences in δD, δ(18) O, and d-excess among groundwater in the BJD, nearby water bodies and the HRB. The groundwater δD-δ(18) O equation for the BJD was δD = 4.509δ(18) O-30.620, with a slope and intercept similar to that of nearby areas (4.856 and -29.574), indicating a strong evaporation effect in the BJD and its surrounding areas. The equation's slope of the BJD was significantly lower than those of HRB groundwater (6.634), HRB river water (6.202), precipitation in the BJD and Youqi (7.841), and the UR of the HRB (7.839). The d-excess (-17.5‰) of the BJD was significantly lower than those of nearby groundwater (-7.4‰), HRB groundwater (12.1‰), precipitation in the BJD (5.7‰) and in the UR of the HRB (15.2‰), and HRB river water (14.4‰). The spatial patterns of δ(18) O and d-excess values in the BJD suggest mixing and exchange of groundwater between the BJD and neighboring regions, but no hydraulic relationship between the BJD groundwater and water from more distant regions except Outer Mongolia, which is north of the BJD. Moreover, we conclude that there is little precipitation recharge to groundwater because of the obvious d-excess difference between groundwater and local precipitation, low precipitation, and high evaporation rates. The abnormally negative d-excess values in groundwater of the BJD indicate that this water was formed in the past under higher relative humidity and lower temperatures than modern values.     

Testing the ‘two water worlds’ hypothesis under variable preferential flow conditions

Widespread observations of ecohydrological separation are interpreted by suggesting that water flowing through highly conductive soil pores resists mixing with matrix storage over periods of days to months (i.e., two ‘water worlds’ exist). These interpretations imply that heterogeneous flow can produce ecohydrological separation in soils, yet little mechanistic evidence exists to explain this phenomenon. We quantified the separation between mobile water moving through preferential flow paths versus less mobile water remaining in the soil matrix after free-drainage to identify the amount of preferential flow necessary to maintain a two water world's scenario. Soil columns of varying macropore structure were subjected to simulated rainfall of increasing rainfall intensity (26 mm h⁻¹, 60 mm h⁻¹, and 110 mm h⁻¹) whose stable isotope signatures oscillated around known baseline values. Prior to rainfall, soil matrix water δ²H nearly matched the known value used to initially wet the pore space whereas soil δ¹⁸O deviated from this value by up to 3.4‰, suggesting that soils may strongly fractionate ¹⁸O. All treatments had up to 100% mixing between rain and matrix water under the lowest (26 mm h⁻¹) and medium (60 mm h⁻¹) rainfall intensities. The highest rainfall intensity (110 mm h⁻¹), however, reduced mixing of rain and matrix water for all treatments and produced significantly different preferential flow estimates between columns with intact soil structure compared to columns with reduced soil structure. Further, artificially limiting exchange between preferential flow paths and matrix water reduced bypass flow under the most intense rainfall. We show that (1) precipitation offset metrics such as lc-excess and d-excess may yield questionable interpretations when used to identify ecohydrological separation, (2) distinct domain separation may require extreme rainfall intensities and (3) domain exchange is an important component of macropore flow.     

Kinetic processes and stable isotopes in cave dripwaters as indicators of winter severity

We examine how the stable isotope composition of meteoric water is transmitted through soil and epikarst to dripwaters in a cave in western Romania. δ²H and δ¹⁸O in precipitation at this site are influenced by temperature and moisture sources (Atlantic and Mediterranean), with lower δ¹⁸O in winter and higher in summer. The stable isotope composition of cave dripwaters mimics this seasonal pattern of low and high δ¹⁸O, but the onset and end of freezing conditions in the winter season are marked by sharp transitions in the isotopic signature of cave dripwaters of approximately 1 ‰. We interpret these shifts as the result of kinetic isotopic fractionation during the transition phase from water to ice at the onset of freezing conditions and the input of meltwater to the cave at the beginning of the spring season. This process is captured in dripwaters and therefore speleothems from Ur?ilor Cave, which grew under such dripping points, may have the potential to record past changes in the severity of winters. Similar isotopic changes in dripwaters driven by freeze–thaw processes can affect other caves in areas with winter snow cover, and cave monitoring during such changes is essential in linking the isotopic variability in dripwaters and speleothems to surface climate.     

Sediment movement rates and processes on cinder cones in the maritime Subantarctic (Marion Island)

Sediment transport in the scoria areas of Marion Island is primarily the result of needle‐ice‐induced frost creep associated with diurnal soil frost cycles. Clasts move most rapidly in ?ne textured areas (532 mm a^?1; SD 382), more slowly in stony areas (161 mm a^?1; SD 179), and most slowly in blocky areas (26 mm a^?1; SD 23). Movement rates increase with increasing frost susceptibility of sediments, slope angle and altitude. The heave of dowels indicates that frost heave is active in all the scoria areas examined. The depth of effective frost heave increases with increasing altitude, with frost heave being restricted to the upper 100 mm of the soil in low altitude areas (<200 m). The heave of 150 mm dowels at the higher altitude sites provides evidence for segregation ice formation at depths greater than those associated with needle ice and diurnal soil frost cycles. Vertical movement pro?les show a concave downslope pro?le, with sediment movement rates being most rapid at the soil surface and decreasing rapidly with depth. This pro?le shape is typical of areas dominated by diurnal freeze–thaw cycles and needle ice. The capture of sediments moving downslope in troughs and the sampling of material lifted by needle ice, suggest that sediment transport by needle ice under present conditions is extremely effective. Observations suggest that although both ?ne material and clasts are transported downslope, some preferential transport of clasts occurs. Experiment results and observations of soil frost processes suggest that frost creep associated with needle ice activity is the dominant slope process in the scoria areas of Marion Island. Other slope processes such as slopewash and debris ?ows appear to play a relatively minor and localized role in sediment transport. It is suggested that needle ice activity is likely to be the dominant geomorphic agent in other areas of the Subantarctic with similar climatic characteristics to Marion Island. Copyright © 2004 John Wiley & Sons, Ltd.     

Isotope hydrology of the Allt a' Mharcaidh catchment,Cairngorms, Scotland: implications for hydrological pathways and residence times

The hydrology of oxygen‐18 (¹⁸O) isotopes was monitored between 1995 and 1998 in the Allt a' Mharcaidh catchment in the Cairngorm Mountains, Scotland. Precipitation (mean δ¹⁸O=−7·69‰) exhibited strong seasonal variation in δ¹⁸O values over the study period, ranging from −2·47‰ in the summer to −20·93‰ in the winter months. As expected, such variation was substantially damped in stream waters, which had a mean and range of δ¹⁸O of −9·56‰ and −8·45 to −10·44‰, respectively. Despite this, oxygen‐18 proved a useful tracer and streamwater δ¹⁸O variations could be explained in terms of a two‐component mixing model, involving a seasonally variable δ¹⁸O signature in storm runoff, mixing with groundwater characterized by relatively stable δ¹⁸O levels. Variations in soil water δ¹⁸O implied the routing of depleted spring snowmelt and enriched summer rainfall into streamwaters, probably by near‐surface hydrological pathways in peaty soils. The relatively stable isotope composition of baseflows is consistent with effective mixing processes in shallow aquifers at the catchment scale. Examination of the seasonal variation in δ¹⁸O levels in various catchment waters provided a first approximation of mean residence times in the major hydrological stores. Preliminary estimates are 0·2–0·8 years for near‐surface soil water that contributes to storm runoff and 2 and >5 years for shallow and deeper groundwater, respectively. These ¹⁸O data sets provide further evidence that the influence of groundwater on the hydrology and hydrochemistry of upland catchments has been underestimated. Copyright © 2000 John Wiley & Sons, Ltd.