The proposed approach aims to estimate the flood extent and soil wetness using AMSR-E passive microwave data. The approach is applied over the Mackenzie River Basin, which is situated in northwestern Canada. The methodology is based on the polarization ratio index (PR), which is computed using AMSR-E 37 GHz, vertically and horizontally polarized brightness temperature values. The water surface fraction (WSF), which represents the fraction of flooded soil, was derived on a pixel-per-pixel basis. The fractional vegetation cover was added to the WSF calculation in order to take into account the temporal variation of the vegetation shading effect. The WSF derived from AMSR-E data, WSF(AMSR-E), was compared to those derived from the Moderate-resolution Imaging Spectroradiometer Terra instrument (MODIS-Terra) images (250 m), WSF(MODIS). A rating curve relationship was developed between the observed discharge and WSF(MODIS). It was noted that the WSF obtained from AMSR-E images systematically exceed those from MODIS, as they are formed from a combination of different contributions, including open water surface, flooded area and wetlands, which are abundant in the northern climates. Therefore, a wetness index was defined based on the difference between passive microwave and visible image responses. This index was able to qualitatively describe the temporal evolution of the wetness over the Mackenzie River Basin. The availability of discharge observations and passive microwave data leads to the definition of a consistent wetness index and soil moisture monitoring over the Mackenzie River Basin. A satisfactory agreement was noted between the wetness index, the precipitation, and the temperature values. The wetness index agrees well with the measured soil moisture.
In large scale field experiments over the past 10 years, long
wavelength microwave radiometers have been used to map surface soil
moisture with considerable success. These experiments, which include
PIPE (87 & 89), Monsoon 90, Washita 92 and 94, and HAPEX-Sahel,
covered a wide range of climatic regimes. The results from all of them
have shown that the microwave emission at the 21-cm wavelength is a
strong function of surface (0 to 5-cm) soil moisture. The salient
results from these experiments are presented and compared. The use of
the surface soil moisture data to determine geophysical parameters such
as evaporative fraction, soil evaporation and soil hydraulic properties
Theoretical considerations are discussed, and design criteria and fabrication of a physical hydrologic model including a storm-simulating device are described. Data from several preliminary experimental tests indicate that the use of physical models of actual watersheds merits further investigation.
Construction and spatial and temporal properties for a 0.25° resolution gridded data set of monthly Penman–Monteith reference evapotranspiration estimates over the territory of the PR China (including Tibet) and adjacent areas (15°N–55°N, 65°E–135°E) for the period 1951–1990 are described. To account for the interaction between climate and the rugged topography of the study area the REGEOTOP procedure was used to incorporate the effects of relief forms into the interpolation.Evapotranspiration rates over much of China show a range of values (annual rates from 550–2300 mm) and variability comparable to precipitation. Monthly evapotranspiration rates are distributed more evenly over the year than precipitation, are out of phase with the summer precipitation peak and in some cases may reach winter rates comparable to those in summer. Hydrological studies based on idealized regular seasonal variation of evapotranspiration may contain considerable errors due to inherent seasonal fluctuations as compared to precipitation.High resolution gridded PET data that account for the influence of topography on climate are required to resolve the spatial heterogeneity of topography and land use in order to allow precise estimates of actual evapotranspiration and run-off. The spatial distribution of runoff appears to have remained fairly constant over most of China during 1951–1990 which stands in contrast to the anticipated increase in hydrological activity under global warming conditions.
A time series model of the ARMA class for seasonal, multisite applications is presented. Methods of estimating model parameters are developed and two methods (maximum likelihood and the method of moments) are compared. For model verification, a data generation experiment provides diagnostic checks at three levels: (1) properties of the model residuals; (2) preservation of short-term statistics (serial and cross-correlation); and (3) preservation of long-term characteristics (drought and reservoir storage properties). The model is then used to model bivariate, monthly river flow on the Yampa and White Rivers in northwest Colorado.
Hydrological modifications frequently result in wetland loss and degradation while wetland management, restoration and creation schemes rely upon further hydrological manipulations. These schemes can benefit from models which can accurately represent often complex wetland hydrological situations. Although the potential of the physically based, distributed model MIKE SHE to model wetlands has been demonstrated, a number of inadequacies in its channel flow component have been identified. These include difficulties in representing control structures and simulating inundation from channels. A coupling has been developed between MIKE SHE and the MIKE 11 hydraulic modelling system. This paper reports a coupled MIKE SHE/MIKE 11 model developed for a lowland wet grassland, the Elmley Marshes, in southeast England. Long term monitoring, supplemented by selected secondary sources, provided the necessary input, calibration and validation data. A procedure was developed to evaluate evaporation from ditch surfaces which could not be represented dynamically within MIKE 11. Two consecutive 18-month periods were used for model calibration and validation which were based upon comparisons of observed and simulated groundwater depths and ditch water levels. Model results were generally consistent with the observed data and reproduced the seasonal dynamics of groundwater and ditch water. The close association between flooding and both groundwater and ditch water levels was demonstrated. Topographic depressions are important for the initiation of flooding and are responsible for much of the shallow surface water in areas isolated from ditches. Deeper flooding occurs in areas which are inundated from these ditches. Results suggested that improvements could be made to the MIKE SHE bypass flow routine to enable it to more accurately represent macropore flow associated with soil cracking and swelling. Dynamic calculation of evaporation from ditch water surfaces would enhance the ability of the model to explore alternative water level management and climate change scenarios. The potential use of the model to investigate these scenarios is outlined.
Poor agreement between 3H/3He ages and CFC-11 and CFC-12 ages suggests that CFCs may not be conservative tracers in the Everglades National Park. 3H/3He ages were used to calculate the expected concentration of CFC-11 and CFC-12 in groundwater from wells 2 to 73 m deep. The expected concentrations of CFCs were compared to the measured concentrations and plots of the % CFC-12 and CFC-11 remaining offered no evidence that significant CFC removal was occurring in the groundwater at depths ≥2 m, suggesting that CFC removal occurs at shallower depths. Except where CFC contamination was suspected, CFC-11, CFC-12 and CFC-113 concentrations in fresh surface water were nearly always below solubility equilibrium with the atmosphere. Measurements of CFC-11, CFC-12 and CFC-113 in pore water indicate a 50–90% decrease in concentration 5 cm below the groundwater–surface water (GW–SW) interface. In the same 5 cm interval CH4 concentrations increased by 300–1000%. This suggested that CFCs were removed at the GW–SW interface, possibly by methane-producing bacteria. CFC derived recharge ages should therefore be viewed with caution when recharging water percolates through anoxic methanogenic sediments.
The distribution of groundwater and dynamic fluctuations in groundwater levels have direct impacts on the eco-environment of arid areas. Investigations of groundwater recharge in arid areas are typically limited by a lack of adequate meteorological and hydrogeological records. This study focuses on groundwater recharge in a seasonally flooded arid area within the Tarim Basin, China, with the aim of analyzing the influence of groundwater and topography on the response characteristics of overland flow. We conducted a simulation using a coupled MIKE SHE/MIKE 11 model over 112 days of the flood season and calculated the average water balance. Based on the properties of the multi-scale recognition of wavelets and the self-comparability of fractals in analyzing the detailed characteristics of groundwater diffusion and fluctuations in groundwater levels, a hybrid fractal–wavelet method was used to explain the recharge response associated with overland flow, distance from midstream, topography, and flooding depth. The results of the model simulations are generally consistent with observed data, indicating that the hybrid fractal–wavelet method is able to recognize the detailed characteristics of the groundwater response. Furthermore, the groundwater levels show a significant relationship with the orientation of the riverway prior to flooding. When flooding occurred, the groundwater levels showed a rapid response to changes in the depth of long-term overland flow. A total of 71.31% of the study area showed a strong correlation between groundwater level and the distance from midstream. The results demonstrate that the relationship exists for fluctuations in groundwater levels of more than 1.4 m. Variations in the height of the water table were significantly influenced by topographic elevation; in contrast, slope and aspect had little effect. In conclusion, the above results indicate that the proposed methodology is applicable for the management of water resources in arid regions. The modeling and hybrid fractal–wavelet method study allowed quantification of the processes affecting groundwater levels and provided an insight into their implications in exploring groundwater level management.
We describe field observations and numerical simulations of the environmental tracers 3H, 85Kr, CFC-113 (C2Cl3F3), SF6 and Ne in groundwater. The field site is a well-characterised shallow aquifer in central Germany, consisting of basalts of Miocene age, overlain by up to 15 m of loess deposits. A two-dimensional numerical model was used to simulate tracer transport at the study site. Simulated and observed tracer concentrations show acceptable agreement for most wells and tracers. Due to the variable thickness of the loess cover, residence times of 3H in the unsaturated zone are highly variable with values ranging from 1 to more than 30 years. This effect explains the observed variability of 3H in the saturated zone. Excess air in groundwater requires correction of measured concentrations of the dissolved gas tracers. A maximum excess of the SF6 content in water compared to the theoretical solubility equilibrium concentration of 28% was observed. A novel iterative method is used to correct for excess air. CFC-113 transport seems to be retarded. On the basis of the effective porosity for SF6, 85Kr and 3H transport, a retardation factor of R=1.5 for CFC-113 with respect to SF6, 3H and 85Kr can be derived. Together with non-adsorbing tracers, such as 85Kr or SF6, CFC-113 can therefore serve as a reactive retardation tracer exploring mean sorption characteristics of the aquifer material. Extrapolation of CFC-113 sorption characteristics to organic contaminants with comparable physicochemical properties (e.g. chlorinated hydrocarbons) offers an opportunity for improved assessment of the behaviour of this important group of contaminants in groundwater.
The most complete historical series of instrumental data available, spanning more than a century, on rainfall, temperature and outflow of a karst spring obtained from gauging stations in the south of the Iberian peninsula were analysed by means of spectral and correlation analyses and continuous wavelet analyses. Annual periodicity of the rainfall and temperature distributions was constant over more than 100 years, although weaker (6-month) periodicities have also been observed, as well as rainfall and temperature periodicities of 5 and 2.5 years, which have also been recorded in other areas of Europe. These multiannual scale components can be explained by climatic variations or effects described in the literature in connection with the North Atlantic Oscillation (NAO) and are likely to be the same as the climate variability at decadal to annual scale detected in several proxy data from geological records. No long-term trends in the distribution of precipitation and temperature were detected.
On the 12th and the 13th November 1999, the Aude region received amounts of rainfall that locally exceeded 500 mm. This extreme rainfall event resulted in one of the century's most significant floods in the Aude river catchment area and produced remarkable flash floods in some of its tributaries. Peak flood discharges from many upstream watersheds (area smaller than 100 km2) exceeded 10 m3/s/km2. An extensive post-flood investigation was carried out in 2000 to collect information concerning the floods in seven of these upstream tributaries and to analyse the hydrological behaviour of the watersheds during this extreme rainfall event. As well as analysing the available rainfall data and estimating peak discharges on the basis of high water marks, information related to the timing of the floods, as provided by eyewitnesses, was gathered. A simple hydrological model, based on the SCS method and the kinematic wave equation, was used to guide the hydrological interpretation. This study revealed some interesting aspects of the hydrological rainfall-runoff relation during flash flooding that could be important for estimating flood frequencies and flood forecasting: (1) around 200 mm of rainwater were retained in the catchments without contributing to the flood flow, (2) the response of the watersheds to the intense storm bursts was late and relatively sudden, (3) no significant effects resulting from different types of land use could be identified. These conclusions, based on inaccurate data, were validated for the entire Aude river catchment. They also concur with some other reported flash flood data.
A method for estimating flash flood peak discharge, hydrograph, and volume in poorly gauged basins, where the hydrological characteristics of the flood are partially known, due to stage gauge failure, is presented. An empirical index is used to generate missing hourly rainfall data and hydrologic and hydraulic models performs the basin delineation, flood simulation, and flood inundation. The peak discharge, hydrograph, and volume, derived from the analysis of measured hydrographs in a number of non-flood causing rainfall events with operating stage gauge, were used for calibration and verification of the simulated stage-discharge hydrographs. An empirical equation was developed in order to provide the peak discharge as a function of the total precipitation, its standard deviation, and storm duration. The peak discharge for a flash flood case based on the empirical equation was in close agreement with the results from a number of consolidated methods. These methods involved hydrological and hydraulic modeling and peak flow estimates based on Manning’s equation and post flash flood measurements of the maximum water level observed at the control cross-section, for the 13–14 January 1994 flash flood in the Giofiros basin on the island of Crete, Greece. This method can be applied to other poorly gauged basins for floods with a stage higher than that defined by the rating curve.
Recent concern for problems of soil degradation and the offsite impacts of accelerated erosion has highlighted the need for improved methods of estimating rates and patterns of soil erosion by water. The use of environmental radionuclides, particularly caesium-137 (137Cs), as a means of estimating rates of soil erosion and deposition is attracting increasing attention and the approach has now been recognised as possessing several important advantages. However, one important uncertainty associated with the use of 137Cs measurements to estimate soil erosion rates is the need to employ a calibration relationship to convert the measured 137Cs inventory to an estimate of the erosion rate. Existing calibration procedures are commonly subdivided into empirical relationships, based on independent measurements of soil loss, and theoretical models, that make use of existing understanding of the fate and behaviour of fallout radionuclides in eroding soils to derive a relationship between erosion rate and the reduction in the 137Cs inventory relative to the local reference value. There have been few attempts to validate these theoretical calibration models and there is an important need for such validation if the 137Cs approach is to be more widely applied. This paper reports the results of a study aimed at validating the use of a simple exponential profile distribution model to convert measurements of 137Cs inventories on uncultivated soils to estimates of soil erosion rates. It is based on a small (1.38 ha) catchment in Calabria, southern Italy, for which measurements of sediment output are available for the catchment outlet. Because there is no evidence of significant deposition within the catchment, a sediment delivery ratio close to 1.0 can be assumed. It is therefore possible to make a direct comparison between the estimate of the mean annual erosion rate within the catchment derived from 137Cs measurements and the measured sediment output. In undertaking this comparison, account was taken of the different periods covered by the measured sediment output and the erosion rate estimated using 137Cs measurements. The results of the comparison show close agreement between the estimated and the measured erosion rates and therefore provide an effective validation of the use of the 137Cs approach and, more particularly, a profile distribution calibration model, to estimate soil erosion rates in this small catchment. Further studies are required to extend such independent validation to other environments, including cultivated soils, and to different calibration procedures.
The turnover of radioactive caesium was studied experimentally and theoretically in a forested catchment that was covered by snow during the wet deposition of radioactive nuclides from the Chernobyl accident. The study spans from 1 week before to 8 years after the deposition event. A fraction of the catchment is covered by a mire (16010). From the edge of the mire a stream channel runs to the outlet of the catchment. Two phases of decreasing activity concentration in the stream water were found in addition to a positive influence of runoff on the activity concentration in the stream. The half-lives for 137CS in the stream water corresponding to the early and the later phase were estimated by non-linear regression to be 6.5 days and 4 years, respectively. During the first phase, which corresponded to the initial snow melt in 1986, 6.8% of the deposition was lost from the catchment, whereas the slow secondary loss during the following 8 years was 1.8%. The main contribution to the yearly discharge of 137Cs occurred during spring and autumn when the areal contribution to saturated surface runoff was highest. The remaining deposition in soil a few years after the fallout was significantly lower in the mire than in the surrounding forest. By using the calculated activity concentration of 137Cs in the stream water together with the remaining deposition in the different biotopes and information on stream flow for the catchment it was concluded that the loss originated from the mire. During the initial phase 44% of the deposition was lost from the mire, and during the following years the yearly loss was 30% from the fraction that constantly undergoes saturated surface runoff and 2% from the drier fractions of the mire. Until the end of the study it was not possible to demonstrate any loss from the recharge areas (podzol and cambisol soils), which means that physical decay will govern the decrease in activity in these areas.
A 13C and 14C isotope subroutine called ISOTOP has been written for WATEQF, a FORTRAN IV version of the water-analysis treatment program (WATEQ) translated and modified by Plummer et al. from the original PL1 version by Truesdell and Jones. This subroutine performs a series of simulations on water samples where 13C and 14C data are available by assuming various initial pH— −log (PCO2) values under open-system conditions and computing the δ13C of the CO2 gas phase to be compared with the actual water sample's value and an age for each simulation applied. Some of the models use total Ca plus Mg as a measure of whole-rock carbonate dissolved under closed-system conditions while others make corrections for gypsum dissolution and ion-exchange reactions.In addition, the subroutine may be used to compute age differences between pairs of water samples along the same flow path using congruent and incongruent carbonate dissolution models. In the simulations applied, carbonate and bicarbonate ion pairing and complexing are considered.
Stable isotope data for dissolved inorganic carbon (DIC), carbonate shell material and cements, and microbial CO2 were combined with organic and inorganic chemical data from aquifer and confining-bed pore waters to construct geochemical reaction models along a flowpath in the Black Creek aquifer of South Carolina. Carbon-isotope fractionation between DIC and precipitating cements was treated as a Rayleigh distillation process. Organic matter oxidation was coupled to microbial fermentation and sulfate reduction. All reaction models reproduced the observed chemical and isotopic compositions of final waters. However, model 1, in which all sources of carbon and electron-acceptors were assumed to be internal to the aquifer, was invalidated owing to the large ratio of fermentation CO2 to respiration CO2 predicted by the model (5–49) compared with measured ratios (two or less). In model 2, this ratio was reduced by assuming that confining beds adjacent to the aquifer act as sources of dissolved organic carbon and sulfate. This assumption was based on measured high concentrations of dissolved organic acids and sulfate in confining-bed pore waters (60–100 μM and 100–380 μM, respectively) relative to aquifer pore waters (from less than 30 μM and 2–80 μM, respectively). Sodium was chosen as the companion ion to organic-acid and sulfate transport from confining beds because it is the predominant cation in confining-bed pore waters. As a result, excessive amounts of Na-for-Ca ion exchange and calcite precipitation (three to four times more cement than observed in the aquifer) were required by model 2 to achieve mass and isotope balance of final water. For this reason, model 2 was invalidated. Agreement between model-predicted and measured amounts of carbonate cement and ratios of fermentation CO2 to respiration CO2 were obtained in a reaction model that assumed confining beds act as sources of DIC, as well as organic acids and sulfate. This assumption was supported by measured high concentrations of DIC in confining beds (2.6–2.7 mM). Results from this study show that geochemical models of confined aquifer systems must incorporate the effects of adjacent confining beds to reproduce observed groundwater chemistry accurately.
Measurements of the carbon-14 concentration of water in 33 boreholes in a saline unconfined aquifer in southern Australia have been interpreted to show that local recharge under natural conditions has a mean annual value which falls between 0.1 and 0.2 mm yr−1.The chloride concentration of groundwater is log-normally distributed suggesting that its source is rainfall. The estimate of mean annual recharge to the aquifer made using chloride is 0.25 mm yr−1.
Delineation of the spatial distribution of ground water that is several thousands of years old can provide an important piece of the puzzle in the evaluation of long-term, ground water resource sustainability under pumping conditions. Ground water for municipal and local water supplies within the Palouse Basin of eastern Washington and northern Idaho are derived almost entirely from two basalt aquifer systems. Decades of continual water level declines in the deeper aquifer system in response to interstate pumping have suggested that this high transmissivity, low storativity aquifer system is being mined. Average water level declines in the deeper aquifer system have been on the order of 0.46 m (1.5 ft) per year for the past 40-plus years. Carbon-isotopic measurements on dissolved inorganic carbon were used to provide information on the relative ages of the ground water pumped from various locations within the basin. Mass balance modeling of hydrochemical reactions together with steep, downward vertical gradients throughout the basin suggest that the ground water being withdrawn currently required several thousand years to traverse spatially variable sequences of loess, fluvial sediments, and basalt flows with associated sedimentary interbeds, that overlie the primary producing zones in the basin. Restrictions to the vertical movement of recharge water such as low permeability, overburden deposits, well-developed fragipans, and thick, low permeability, sedimentary interbeds increase in the eastern portion of the Palouse Basin compared to the western portion. This has resulted in accumulation of the oldest ground water in the topographically highest areas of the basin adjacent to the basin divide.
Helium isotope and concentration characteristics were determined for a suite of groundwater samples from the Amadeus Basin in Central Australia. Two study areas include a wellfield south of Alice Springs, and the Dune Plains and Mututjulu aquifers near Uluru. Measurements of 36Cl/Cl and 14C on the same sample suite enable us to assess the relative applicability of the three groundwater chronometers over a range of anticipated groundwater residence times (ages), and to investigate possible causes of discordant ‘ages’ derived from the different groundwater dating techniques.
Repetitive sampling for radioactive isotope analysis can be used to more accurately estimate the age of groundwater. Groundwater was sampled in wells along the Coastal Aquifer of Israel for tritium, 14C, and 13C analysis in the 1970s as well as the 1990s.Examination of the tritium values from two sampling periods on the tritium bomb curve enabled us to overcome the ambiguity of tritium data, in which a single value could yield two ages, and allowed a better age estimate. Therefore, repetitive sampling over an extended time interval was found to be effective in assisting calculations of groundwater ages through the use of the tritium curve over the 20-year period. The age, obtained by repetitive sampling can be used to determine the dilution factor (Q) of the initial 14C value in the aquifer. This corrected initial value can, thus, be utilized for better age determination of older samples in the deeper sub-aquifers.In most cases, based on the presence of detectable levels of tritium throughout most of the aquifer, the groundwater in the coastal aquifer was determined to contain a young component with a transit time of less than forty years. However, the 14C levels of most of the same groundwater samples were found to be lower than atmospheric levels (34–102 vs. >100 pMC), probably due to water–rock interactions rather than decay due to old age. Some older waters exist in the lower sub-aquifers of the deeper wells (120–140 m) where both lower 14C values (42–69 pMC) and very low levels of tritium were found (0.1–0.7 TU).
In the absence of pure piston-flow, the 14C decay ages of groundwater can be related to groundwater residence times only in the context of a flow or mixing model. Discrete-state compartment models offer a means of constructing flow models that can simulate flow regimes ranging from pure piston-flow to perfect mixing and can thus be used to relate 14C decay ages to residence times. A three-dimensional steady-state flow model of a portion of the Tucson Basin aquifer, south-central Arizona, U.S.A., was constructed using the theory of discrete-state compartment models and calibrated with the spatial distribution of adjusted 14C decay ages. The model provides estimates of groundwater residence times (about 100–15,000 yr.), vertical flow in the aquifer and long-term average annual recharge (0.048 km3). Although the amount of time spanned by the model is long and presents problems with respect to the constancy of hydrologic processes, it may suggest a novel use for such models: that of delineating the paleohydrology of a particular region.
Estimation of groundwater recharge in arid and semi-arid areas is difficult due to the low amount and variability of recharge. A combination of radiotracers investigation based on simple mixing models allows direct investigation of relatively long-term renewal rates of an aquifer. The recharge process of the shallow Continental Terminal aquifer in the Iullemeden basin (Niger) was investigated using a geochemical and isotopic approach. This study investigates the area in the one degree square of Nianey (13–14°N, 2–3°E). In this area, recharge is highly heterogeneous and mainly occurs through a drainage system of temporary streams and pools during the rainy season. Heterogeneity of the recharge is reflected through the wide variation in electrical conductivity and oxygen-18 content of the groundwater. The carbon-14 activity range for most of the groundwater falls between 69 and 126 pmc showing pre and post-aerial thermonuclear test recharge. Two renewal rate models have been investigated: the first one models a well-mixed reservoir and the second one is derived from a piston flow model, in which mixing is in equal proportions. Major ions in tritium data analyses allow exclusion of non-representative samples and confirm the carbon-14 renewal rate estimations. Both models give similar results for the relatively low renewal rate investigated in the area. Using carbon-14, the mean annual rates of groundwater renewal range from 3 to 0.03% of the aquifer volume with a median of 0.1%. Assuming the median is representative of the overall renewal rate of the area, the recharge rate is in the order of 5 mm a−1. The shallow aquifer recharge extends from the last small humid period (around 4000 a) up to now. High recharge rates are found in depressions whereas low recharge occurs below the plateaux.
Stable isotopes (δ2H, δ18O and δ13C) and radiocarbon (14C) have been used in conjunction with chemical data to evaluate recharge mechanisms, groundwater residence time and palaeohydrology within the confined Dilwyn sand aquifer in the Gambier Embayment of the Otway Basin. This aquifer does not receive recharge down-gradient of the hydraulic hinge-line and data have been interpreted along two discrete flow lines. The mean residence time of groundwater (determined by 14C) in the confined aquifer from the hydraulic hinge position to the sea (a distance of about 50 km) along an inferred flow path is approximately 12 800 years. The corresponding hydraulic travel time calculated from Darcy's law is approximately 49 000 years. The apparent discrepancy may be a result of eustatic sea-level lowering during the last glacial. Because the groundwater system is hydraulically connected to the sea, lower sea-level would result in increased gradients and a decrease in groundwater residence time. Variations in stable isotopic composition along flow lines suggest a number of recharge mechanisms. Stable isotope data indicate progressive depletion of 2H and 18O in the groundwaters over the past 30 000 years. Groundwaters older than about 10 000 years B.P. were recharged either during a cooler climate climatic regime (lower precipitation/evapotranspiration and temperature) and/or the vapour source(s) had travelled over greater continental mass than those recharged over the past 10 000 years. Lower chloride concentrations in some of the older groundwaters indicate lower evapotranspiration rates in the recharge areas before 10 000 years ago.
The calculation of model 14C ages for groundwater often relies on a correction being made for the incongruent dissolution of aquifer carbonate. Correction equations are particularly sensitive, in certain ranges, to variation in the δ13C value input for dissolving carbonate. This study uses a combined and δ13C approach to define the source and mean δ13C value of dissolving carbonate in the Lincolnshire Limestone aquifer of eastern England. This δ13C value is most appropriately used in 14C correction equations and yields the most potentially accurate groundwater model ages.
This paper investigates the link between vegetation types and long-term water balance in catchment areas. We focus on the most widely used water balance formulas – or models – that relate long-term annual streamflow to long-term annual rainfall and long-term potential evapotranspiration estimates. Our investigation seeks to assess whether long-term streamflow can be explained by land cover attributes. As all but one of these formulas do not use land cover information, we develop a methodology to introduce land cover information into the models’ formulations. Then, the modified formulas are compared to the original ones in terms of performance and a sensitivity analysis is performed, with a special focus on the parameters representing vegetation characteristics. In line with the global coverage of long-term water balance models, we base our work on as many basins as possible (1508) representing as large a hydroclimatic variety as possible.Results show that introducing additional degrees of freedom within the original formulas improves overall model efficiency, and that land cover information makes only a small but nonetheless significant contribution to this improvement.
The formation of a model to represent surface and subsurface flows of water for a catchment in central England is described. A finite-difference model is used to represent groundwater flows in an aquifer with surface water flows being represented by flow balance techniques. Three hydrologically significant land types are identified and the model contains three components which correspond to these. The purpose of the model is to assess the long-term water resources of the catchment. River flows are calculated for over 120 locations within the catchment. The results show that it is possible to formulate a comprehensive model, of a complete catchment, based upon measurable, physical parameters with the inflow of water being calculated solely from rainfall and evapotranspiration estimates.
Stable N isotopes are used to examine the source, flow and fate of N at scales ranging from greenhouse pots to landscapes. There are two main approaches: the 15N-enriched method applies an artificially enriched source of 15N and the 15N natural abundance (δ15N) method uses natural 15N differences between N sources and sinks.The δ15N method is good for semi-quantitative estimates of N flow in undisturbed ecosystems, for analyzing patterns, and for developing new hypotheses, particularly when spatial variability across a landscape or watershed can be explained. The spatial variability of δ15N across a landscape is often non-random, following predictable spatial patterns. Topographic features control the rate of various hydrological and biological processes, resulting in significantly different δ15N between lower and upper slope positions. However, if the difference between source-δ15N and sink-δ15N is small due to inherent background variability and/or if fractionating processes have a large effect on the isotopic signature of the N to be traced, δ15N will not work as a tracer.With the 15N-enriched method, the isotopic signature of the enriched tracer can be pre-determined to ensure a significant difference in atom%15N between source and background levels, even when fractionation occurs. In most situations, the 15N-enriched method can be successfully used as a tracer to test hypotheses and to quantify N cycling through the landscape, regardless of background variability in δ15N. Limitations of the 15N-enriched method include the cost associated with applying an enriched tracer, especially at the landscape scale, and the potentially confounding effects of applying N to a previously undisturbed landscape.
Hydrologic changes associated with the 17 January 1995 Kobe Earthquake occurred in Awajishima Island very close to the epicenter. These included: (1) large drop of water table in the mountainous area; (2) rapid increase of discharge along active faults; and (3) change of chemistry of discharged water. A simple horizontal flow model was constructed to explain the observed changes; and optimal sets of specific yield and the change of hydraulic conductivity were estimated. Results suggest that this model can explain the observed phenomena consistently. The hydraulic conductivity is estimated to increase at least five times than that before the Earthquake; however, quantitative measurement of the increase of discharge just after the earthquake would constrain better the change of the hydraulic conductivity. The specific yield of the unconfined aquifer is between 0.3 and 1.7% depending on the assumed recharge rate but independent of the assumed depth to the impermeable basement. The change in chemical composition of the discharged water could be explained by the upward movement of deeper water due to the invasion of saltwater into the aquifer.
Seasonal variations of the stable isotope, oxygen-18 (18O), were used to estimate mean residence times (MRTs) of water in two Valley and Ridge Province watersheds in central Pennsylvania. MRT was estimated by applying 18O input precipitation signatures to response function models that describe flow conditions in the subsurface system. Precipitation 18O is usually seasonally periodic; however, during this study period (March 1998 to June 1999), unusual weather conditions and severe drought caused an abnormal seasonal signature in precipitation 18O. The atypical input precipitation amounts and 18O signatures required that adjustments be made using recharge factors and an extended past input function to represent the varying fraction of input water that contributed to turnover within the watershed during the study period. Oxygen-18 variations were investigated in output waters from tension and zero-tension soil water lysimeters, shallow wells, and streamflow. Soil water 18O signatures were more responsive to precipitation 18O variations than streamflow 18O signatures. Theoretical models based on exponential piston-flow and dispersion flow processes fit data better than did other groundwater age distribution models (i.e. pure piston flow or pure exponential models). Analysis suggested that during drought conditions, larger portions of older water dominated streamflow 18O composition, and that the current year's precipitation 18O signature became more attenuated in the subsurface flow system. MRTs obtained for streamflow at each site were 9.5 and 4.8 months for a 14 and 100 ha watershed, respectively, and soil water MRT at 100 cm depth was on the order of a couple months; indicating a relatively rapid response of shallow groundwater to precipitation.
The depth distributions of environmental chloride, deuterium and oxygen-18 in the deep sands (> 20 m) were monitored with a view to estimate average recharge to groundwater. The investigated area is located on the southwestern coast of Western Australia and supports a perennial native woodland vegetation.By using a steady state model based on the conservation of chloride, the average areal recharge was estimated to be approximately 15% of the average precipitation (775 mm yr−1). Whilst the Cl− concentration of groundwater was relatively uniform, there was a considerable spatial variability in the depth distribution of Cl− in the unsaturated zone. An approximate method of data analysis is developed. Application of this to Cl− data shows that possibly some 50% of the total annual recharge occurs via movement of water through preferred pathways, thus bypassing the soil matrix.Stable isotope (deuterium and oxygen-18) data show that there has been some isotopic enrichment due to evaporation of the rainfall, but the extent of enrichment appears to be too small to estimate recharge rates reliably. It is argued that relatively small isotopic enrichment occurs in the system studied since the direct evaporation from soil is a rather small component of the total evapotranspiration. The observations on the depth distribution of isotopes qualitatively support the presence of a bimodal system for water movement through the profile, as was suggested by the analysis of chloride data.
The stable isotope compositions (δ18O and δD) of water have been used to determine the relative contributions of different water sources to the groundwater in a barrier sand-bar that separates a coastal freshwater marsh from Lake Erie, Canada. An extensive groundwater study was initiated by Environment Canada at Point Pelee National Park after elevated nutrient concentrations were measured in the park’s marsh, located on a spit of land that extends 15 km south into Lake Erie. As part of this larger study, which includes groundwater and nutrient modelling, the stable isotope compositions (δ18O and δD) of water have been used to independently determine the nature and extent of groundwater flow within sand deposits that separate the marsh from Lake Erie. One of the two study sites chosen for this investigation is located near a large septic-system tile-bed that receives human waste from a public toilet facility within the park, and could potentially release nutrients to the marsh via the groundwater. Both transects studied are well suited to isotopic investigation because they are located between two potentially recharging surface water bodies with temporally variable isotopic compositions, and because the transects are subject to recharge by local precipitation, which exhibits large seasonal isotopic variations. These differing isotopic compositions, when considered spatially and temporally, make it possible to establish the source and movement of groundwater within the transects. The oxygen and hydrogen isotopic compositions of surface waters from the Point Pelee marsh and Lake Erie lie on an evaporation line with a slope of 5.7. The isotopic composition of Lake Erie remained relatively stable for over 21 months (δ18O=−7.5‰ to −6.7‰) whereas the marsh exhibited considerable spatial and temporal variability (δ18O=−8.4‰ to −0.1‰). The δD and δ18O values of precipitation samples vary seasonally with local surface temperatures, and constitute a local meteoric water line of δD=8.0(δ18O)+9.8. While influx of precipitation is evident and similar in the upper two metres of both transects, the relative contribution to groundwater from Lake Erie and the Point Pelee marsh appears to differ. The average δ18O and δD groundwater values measured for the narrower transect were −7.0‰ and −54‰ respectively, and reflect significant groundwater recharge from both Lake Erie (∼67%) and the marsh (∼28%). Precipitation constitutes the primary source of groundwater recharge to the wider transect, located near the large septic-system tile-bed, with average groundwater values of δ18O=−9.1‰ and δD=−65‰. The isotopic profile of groundwater in this area is very stable, both with depth and over time, and contrasts sharply with the narrower northern transect which is much more dynamic. These findings suggest that the septic-system tile-bed is well situated in the wider portion of the barrier sand-bar, where a larger head difference between the lake and the marsh would be required to produce groundwater flow equivalent to that observed in the narrower transect. The differences in groundwater recharge between the two sites arise primarily from the difference in width, and account for the 2‰ and 10‰ differences in δ18O and δD values, respectively, measured in the base-flow groundwater of the two transects.
A Mesoscale Convective System in North-Western Slovenia produced up to 350–400 mm in 12 h on 18 September, 2007. The region impacted by the storm shows significant differences in climatic and geologic properties at short distances. Owing to such variability, extreme flooding concentrated over the Selška Sora watershed at Železniki (103.3 km2), outside the area which received the highest precipitation. Hydrometeorological analyses of the storm are based on accurate analysis of C-band weather-radar observations and data from a rain gauge network. Detailed surveys of high-water marks and channel/floodplain geometry, carried out two months after the flood, are used for hydrologic analyses of the Selška Sora flood. These include estimation of peak discharge at 21 sites. Unit peak discharges range from 5 to 7 m3 s−1 km−2 in basins characterised by size up to approximately 25 km2. Higher unit peak discharges (>10 m3 s−1 km−2), estimated in a few smaller basins, are influenced by intense sediment transport. Observed rainfall, estimated peak discharges, and observer notes on timing of peak discharge are used along with a distributed hydrologic model to reconstruct hydrographs at multiple locations. Examination of the rainfall distribution and flood response shows that the extent and the position of the karst terrain provided a major control on flood response in the region impacted by the storm. Use of the distributed hydrological model together with the post-flood survey observations is shown to provide an accurate description of the flood. Water balance and response time characteristics are examined for selected catchments, showing that event runoff coefficient ranged between 17% and 24% for different catchments. The quality of the peak discharge simulation at the 21 surveyed sites is substantially degraded when using spatially-uniform rainfall over the area covering all the surveyed sub-catchments, mainly due to rainfall volume errors introduced by using the spatially uniform value. On the other hand, the influence of rainfall spatial averaging at the scale of the sub-catchments generally has a very limited effect on runoff modelling, showing that rainfall spatial organisation was not able to overcome the catchment dampening effect for this flood.
The area under investigation concerns a 0.3 km2 island in the Danube River near Passau (F.R.G.). Measurements of 18O-content were used to determine the portion of bank infiltrated river water making up the groundwater of the island. Values of 77 and 96% have been found for different measuring points. Applying mathematical flow models to the environmental tracer data, mean transit times of 48—120 days for the bank infiltrated water and dispersion parameter (D/vx) of about 0.12 have been calculated. These estimates permitted prediction of the migration of possible pollution from the river to the observation and production wells.
Oxygen-18 tracer studies of snowmelt runoff in the Turkey Lakes watershed, Ontario, Canada indicate that 50–60% of the peak runoff is derived from premelt groundwater in storage in the surficial deposits. Groundwater in the stream valleys has relatively high alkalinity concentrations from silicate and minor detrital carbonate dissolution which neutralize much of the acidity in the meltwater component of runoff when it is discharged into the stream during meltwater infiltration. However shallow groundwater on the hillslopes is susceptible to acidification from infiltrating meltwater and rapid flushing to small, first-order streams. For this reason first-order headwater streams experience greater pH depression than do higher-order streams in well defined stream channels, despite having similar premelt components.
Three karstic systems in the Pyrenees (France) have been subjected to a monthly systematic sampling in view of isotopic analysis of oxygen-18. In addition, a higher frequency sampling in overflow conditions has been made in one of the systems which is also continuously studied as regards hydrodynamics, geochemistry and biology.After analysing the sample representativity, an attempt has been made to find out the meaning of the variations of the contents in oxygen-18 in function of time. In the case of these systems of the Pyrenees, a relation between the altitude of the catchment area and the contents in oxygen-18 of their risings has been established which does not agree with Dansgaard's relation.The variations of the isotopic contents in function of the hydrogeologic events confirm the heterogeneity of the submerged zone and the importance of the part played by the percolation, chiefly by considering the connection with the surface saturated zone, named “epikarstic aquifer”. These results, obtained with those of hydrodynamic, geochemical and biological studies of the karst, are not likely to be interpreted without the help of these disciplines.RésuméTrois systèmes karstiques des pyrénées (France0 ont été soumis à un échantillonnage systématique mensuel en vue d'analyses isotopiques de l'oxygène-18. En outre, sur l'un des systèmes, qui fait par ailleurs l'objet d'études en continu d'ordre hydrodynamique, géochimique et biologique, un en échantillonnage plus fréquent en période de crue a été effectué.Aprés une analyse de la représentativité des échantillons, un essai d'interprétation de la signification des variations de teneur en oxygène-18 au cours temps a été tneté. Une émergences a été établie dans le cas de ces systèmes pyrénéen et ne correspond pas à la relation de Dansgaard.
Deuterium and oxygen-18 were used as natural tracers to investigate the hydraulic relationship between the Columbia River and the Blue Lake gravel aquifer near Portland, Oregon. A time series of stable-isotope data collected from surface and ground waters during a March 1990 aquifer test confirms that the river and aquifer are hydraulically connected. Calculations based on simple mixing show that the river contributed 40–50% of the yield of three wells after 5–6 days of pumping. Data collected during August 1990, show that the river contributed 65–80% of the yield of one well after 22 days of pumping and indicate that the contribution of the river was still increasing.
The paper presents and discusses an historical series of evaporation losses, both potential and actual, and differences between precipitation and evaporation. The series were calculated using temperature and rainfall records for the Radcliffe Meteorological Station (Oxford) where an unbroken series of daily observations began in 1815. It is demonstrated that during the last decade potential evaporative losses have been above the long-term average; differences between precipitation and potential evaporation, an index which shows availability of water for runoff, have noticeably declined. These trends have been particularly marked in summer months. The summer soil moisture deficits observed during the last 20 years have been the largest on record; moisture deficits have persisted into the late autumn delaying seasonal recovery in river flow.
Lake levels of three hydrologically closed maar lakes, Lakes Keilambete, Gnotuk and Bullenmerri in Western Victoria, Australia, have declined since the first recorded observations in 1841. High levels were previously sustained for several centuries with the lakes in climatic equilibrium. Historical survey and field investigations provide a detailed picture of both historical land-use changes and of the geological and hydrogeological influences on the water budget. Groundwater components include baseflow from deep percolation within the catchment and discharge from a surrounding low-yield aquifer. A perched watertable at Lake Keilambete helped maintain high lake levels. Climate records back to 1859 were reconstructed; inhomogeneities from 1863 were removed creating a high quality instrumental record. A water balance model simulating the historical decline demonstrates important features. (1) Regional climate expressed as a lake precipitation/evaporation (P/E) ratio remains the over-riding influence on lake levels. (2) The lakes fell in response to a change in climate. (3) This climate change pre-dated instrumental records. (4) Land-use change did not contribute to declining water levels. The fall in water levels was initiated by a decrease in P/E ratio from a pre-1840 value of 0.94–0.96 to a historical value of 0.79. This change probably involved a decrease in rainfall, possibly associated with increases in solar radiation and decreases in cloud cover. Temperature (T) may also have increased but the likelihood of an altered temperature–evaporation relationship means that a quantified estimate is not possible. The ability of the lakes to reflect climate independently of land-use change is highly unusual, both in Australia and elsewhere. These lakes provide a rare opportunity to discriminate between human impact on regional hydrology and climate change.
The Changjiang (Yangtze River) has been effectively gauged since the 1950s and demonstrates the transformation of a river system due to intensified human activities in its drainage basin over the past 50 yr. However, the 50-yr measurements of water and sediment are inadequate to show the long-term trend of sediment flux from the river to the sea or to capture the transition from natural to human dominance over the sediment flux. In this study we used the existing water discharge and sediment load records (1950s–2005) at the Hankou gauging station, together with water discharge recorded since 1865 at the same station, to reconstruct the changes of sediment flux to the sea since the 1860s. We established rating curves between stream discharge and suspended sediment concentration from the recent 50-yr data sets, which show that human disturbances have had a substantial impact on rating parameters. The commissioning of dams and undertaking of soil-conservation works have decreased sediment supply, leading to a decrease in the rating coefficient a of the rating curve equation Cs = aQb. The decreases in suspended sediment concentration have increased the erosive power of the river, and hence increased the rating exponent b. In particular, the commissioning of the Three Gorges Reservoir in 2003 resulted in a further increase of b, and channel scour in the middle and lower reaches has increased sediment flux to the sea to a level higher than sediment supply from the upper reaches. Our results suggest that the rating curves derived from 1954 to 1968 data are appropriate for estimating sediment loads for the period from 1865 to 1953, since both were periods of minimal human disturbance. This approach provides a time series of sediment loads from 1865 to 2005 at Hankou gauging station, which yields a time series of sediment flux from the Changjiang to the sea over the past 140 yr. The estimated mean annual sediment flux to the sea between 1865 and 1968 was ∼488 Mt/yr, a comparable result to the previously published estimate from Milliman and Syvitski [Milliman, J.D., Syvitski, J.P.M., 1992. Geomorphic/tectonic control of sediment discharge to the ocean: the importance of small mountainous rivers. Journal of Geology 100, 525–544] and to that from an equation proposed by Syvitski and Morehead [Syvitski, J.P.M, Morehead, M.D., 1999. Estimating river-sediment discharge to the ocean: application to the Eel margin, northern California. Marine Geology 154, 13–28]. The long-term variation of annual sediment flux from the Changjiang to the sea shows a transition from a river system mostly dominated by nature (the monsoon-dominated period, 1865–1950s) to one strongly affected by human activities (the human-impacted period, 1950s–present).
A combined simulation of runoff dynamics and distributed transport of the stable isotope oxygen-18 (18O) was performed in the mountainous Dreisam basin (258 km2) in the southwest of Germany. For this purpose, a mixing cell solute routing scheme was implemented in the fully distributed, process-orientated catchment model TACD. Perfect mixing in the catchment reservoirs was assumed in order to keep the model simple. In addition, effects like fractionation, diffusion into and out of immobile phases, as well as kinematic effects were neglected. Although results show a generally good agreement between simulated and measured concentrations of 18O in stream discharges, we discovered that the initial objective of validating the process representation of TACD by incorporating conservative solute transport cannot be achieved with the given data. Simulation misfits cannot be clearly attributed to process descriptions, mainly due to large uncertainties in the input regionalization of precipitation and 18O, and due to the lack of data for model initialization. Nevertheless, we critically evaluate the model’s conceptualization and parameterization, in order to deliver insight to the potential and limitations of distributed modelling of 18O signals on the meso-scale. In addition, the large influence of surface runoff generated on a small fraction of the total catchment area, and fast interflow components on the 18O values in total stream discharge could be demonstrated for the investigated events.This paper documents the first attempt to simulate the distributed 18O balance in a meso-scale catchment with temporal resolution on an hourly basis in order to optimize future measurement campaigns and modelling attempts.
Four major storms occurring on a 7.4 km2 Pennsylvania watershed during May–June 1990 were used to examine methods for isotopically separating surface and subsurface stormflow components when the 18O variability in rainfall is known. Rainfalls and the resulting stormflows were incrementally sampled and analyzed for 18O, Si, Na, Cl, Mg, and NO3. Compared with conventionally used methods that input rainfall as a whole-storm value, analysis of the 18O data by peak rainfall periods rather than by total storm changed the separations the most. In contrast, substituting incremental for total-storm averaging of rainfall 18O input caused much less change. When the peak and incremental averaging approaches for 18O were combined into a single method, the 18O-based separations corresponded to the silica-based separations for all storms and Na-based separations for the largest storms. The peak-based methods estimate subsurface contributions to stormflow to be substantially larger than would be estimated by conventionally used total-storm methods.
A mathematical model is developed describing the shape of H218O and HDO depth profiles which result from evaporation of water from dry soil under quasi-steady state conditions. Typically, isotope concentrations rise from a minimum at the soil surface to a maximum a short distance beneath the surface, and then decrease approximately exponentially to constant concentrations at depth. The model predicts that for isothermal conditions, the slope of the relationship between 18O and deuterium δ-values of samples of the soil water will be ∼30% lower for a dry soil than for a wet soil evaporating under the same conditions. It is concluded that low slopes should be indicative of soil water or groundwater recharged under arid or semi-arid conditions. Using the shape of the 18O and deuterium profiles, three independent methods for estimating evaporation for dry soils are developed. When water loss occurs by both transpiration and evaporation, the slope of the 18O-D relationship should be slightly lower than that for a site where water loss occurs by evaporation alone.
A model is developed which predicts the shape of H218O and HDO depth profiles which result from evaporation of water from a soil under non-isothermal and quasi-steady-state conditions. The profiles have similar form to those which develop under isothermal conditions. However, when the soil at depth is cooler than the soil surface — the condition under which most evaporation takes place — a minimum develops in the isotope profile some distance beneath the evaporating front. This is due to the diffusive flux of isotopes in the vapour phase partially offsetting the convective isotope flux in the liquid phase. It is shown that for coarse-textured soils under isothermal conditions the diffusive vapour flux of isotopes may be the dominant mechanism in balancing convective effects in the liquid phase, even when no net movement of water vapour occurs.The theory is extended to enable partitioning of the water flux into liquid and vapour components under both isothermal and non-isothermal conditions.
A detailed examination of the relationship between synoptic patterns and the oxygen isotopic composition (δ18O) of rainfall is made using a 5-year long dataset of daily-event based rainfall samples from Tasmania, southern Australia. Rainfall δ18O varies inversely with rainfall amount and has only a weak relationship with site surface temperature. Events with extreme rainfall and δ18O values were identified from monthly anomalies. The synoptic patterns for these events were averaged using NCEP/DOE reanalyses-2 mean sea-level pressure data to define the major atmospheric circulation features. Large amounts of rainfall with low δ18O values occur when mid-latitude Southern Ocean low pressure systems pass close to the southeast of Tasmania in a circulation pattern with a strong meridional component. By contrast, small amounts of rainfall with high δ18O values occur when the centre of these mid-latitude cyclonic systems pass much further south of Tasmania. In the latter, only the distal limits of the fronts associated with these low pressure systems cross Tasmania and airflow has a stronger zonal component. In addition, synoptic patterns causing advection of air from lower latitudes towards Tasmania may raise the δ18O value of precipitation. Oxygen isotope records from southern Australia should not be interpreted exclusively as proxies for temperature changes.
Isotope data (tritium and 18O) for rivers and ground water, acquired since 1968, are combined with geological, hydrological and hydrochemical evidence to determine sources and flow patterns of ground water in gravel aquifers of the Wairau coastal plain and its southern tributary valleys. Determination of the components of groundwater origin depends on a strong west-east gradient of 18O concentration between mountain catchment precipitation feeding rivers flowing to the Plain and rainwater falling directly on the Plain. The main aquifer is postglacial alluvial material deposited by Wairau River, deriving its water from that river; locally infiltrated precipitation contributes at most only a few percent to the flow in the shallowest gravels downstream of the infiltration zone. In the south of the Plain, the unconfined aquifer comprises material deposited during the last glaciation; its ground water has both local precipitation and inflow from southern tributary streams as appreciable sources. The main aquifer is confined near the coast. where it is overlain by up to 60 m of marine sediments deposited during postglacial marine transgression and subsequent coastal progradation. Tritium concentrations show that ground water in the shallowest geological stratum (Rapaura gravels) reaches the confined zone boundary less than 1 year after recharge from the river, with much shallow flow being discharged at the surface just upstream of the boundary; in contrast, ground water in the underlying unit (Speargrass gravels) requires at least 10 years to reach this point. Analysis of tritium concentrations in the confined zone shows a central zone through which the faster-flowing Rapaura component dominates as far as the coast, but also areas to the north and south where ground water in the Rapaura gravels has a dominant older component. The combined evidence reveals that the confined ground water is leaking into the overlying sediments, allowing water to flow from the lower Speargrass gravels into the Rapaura gravels. Because Wairau River is the dominant source of ground water for the main aquifer, its response to bomb-tritium input to its mountain catchments is given detailed consideration. Much of the river water derives promptly from bare catchment areas, but the analysis suggests that about 50% is retained for about 8 years on average before release to the river. Large deposits of scree in higher-lying, formerly glacial catchments appears to be the major factor in the surprisingly large quantity of stored water implied by this result.
Experimental data show that the slope of the relationship between deuterium and 18O δ-values can fall as low as 2 for soil water in unsaturated sand subjected to evaporation. This is considerably lower than the value of ∼5 obtained during evaporation of free water. The effect is explained in terms of increased thickness of the laminar layer through which evaporating water molecules escape. This work suggests that arid-zone groundwaters replenished by local recharge, should be characterised by low 18O-D slopes.
18O and T have been measured in the precipitation, ground water, springs and a brook on the slope of an open-cast coal-mine in the region of the Krus̆né Hory mountains. Evaluation of the runoff components indicates a varying contribution from precipitation during the year. Melted snow forms up to 65% of the stream water during the high-discharge period (spring), and the groundwater component predominates during the low-discharge period (autumn and winter). The calculated mean retention time of the ground water is about 4 years in the area observed.
The Darling River faces environmental pressures from both climate change and anthropogenic influences leading to a reduction in fresh water availability for the river system. This study uses temporal hydrochemical and stable isotope data (18O and 2H) that has been collected over a five-year period (2002 to 2007), as part of the Global Network for Isotopes in Rivers (GNIR) programme, which is aimed at monitoring hydrological processes in large river systems throughout the world. Daily stream flow, monthly stable isotope and major ion chemistry data is presented for sampling locations along the Darling River at Bourke, Louth and Wilcannia, as well as additional more detailed data from locations near Glen Villa. The hydrochemical data is used to partition groundwater influx that is not readily separable by using only the available isotopic data. Individual flow events in the river were found to be isotopically distinct but the Local Evaporation Lines (LELs) that develop after these events have a similar slope indicating similar climatic conditions across this region. After a storm event, fresh waters that are isotopically depleted are introduced to the system and d-excess (d) values return towards meteoric values. During low flow, the Cl−, Na+, Mg2+, SO42−, δ18O and δ2H values all increase systematically, and d values become more negative. Hydrochemical and isotopic tracers in conjunction with high resolution sampling strategies have been used to quantify the contribution of evaporation, bank storage release and saline groundwater influx to the evolution of the river waters. Fractional contributions (% of volume) of groundwater to the river water were calculated for different reaches using Cl− concentrations, δ18O and d values and it was found that river waters comprised of approximately 60–99% saline groundwater during zero flow. The reduced water levels in the river during the drought conditions experienced in the period of this study had detrimental impacts on the surface water system by providing a pathway for saline groundwaters to discharge into the river system. Persistent drought and continued over-abstraction of surface waters will lead to further saline groundwater intrusion along this reach of the river. This work shows that a suite of hydrochemical and isotopic tracers are needed on spatially and temporally significant scales to unravel the hydrological complexities of dryland river systems such as the Darling River.
Stable isotope approaches are often used for estimating water balances of lakes. Such studies require regional background information about hydrogen and oxygen isotope variability of lakes and their potential inflows. Here, a stable isotope database (δ2H and δ18O) is presented for estimating evaporation to inflow ratios (E/I) of lakes in semi-arid southern Patagonia. Water samples of 23 lakes and ponds located in the Patagonian steppe at about 52°S were sampled during three subsequent austral summers. Two deep crater lakes, Laguna Azul and Laguna Potrok Aike, were studied in more detail during a two-years monitoring. Furthermore, precipitation, groundwater and atmospheric water vapor were sampled for isotope analyses. Presented data imply that the isotopic composition of rainfall in southeastern Patagonia is predominantly determined by precipitation amount and moisture source area. For the investigated area, the first meteoric water and evaporation lines in δ2H vs. δ18O space are presented. The database was further used to estimate the water balances of the two crater lakes, Laguna Azul and Laguna Potrok Aike, which are in the focus of recent paleoclimatic investigations. According to that approach about 50% and 60%, respectively, of the water entering Laguna Azul and Laguna Potrok Aike via surface and subsurface inflow evaporates. These results testify a considerable flow of lake waters into the groundwater.