Soil profile evolution following land‐use change: implications for groundwater quantity and quality

Soil and vadose zone profiles are used as an archive of changes in groundwater recharge and water quality following changes in land use in an area of the Loess Plateau of China. A typical rain‐fed loess‐terrace agriculture region in Hequan, Guyuan, is taken as an example, and multiple tracers (chloride mass balance, stable isotopes, tritium and water chemistry) are used to examine groundwater recharge mechanisms and to evaluate soil water chloride as an archive for recharge rate and water quality. Results show that groundwater recharge beneath natural uncultivated grassland, used as a baseline, is about 94–100 mm year^?1 and that the time it takes for annual precipitation to reach water table through the thick unsaturated zone is from decades to hundreds of years (tritium free). This recharge rate is 2–3 orders of magnitude more than in the other semiarid areas with similar annual rainfall but with deep‐rooted vegetation and relatively high temperature. Most of the water that eventually becomes recharge originally infiltrated in the summer months. The conversion from native grassland to winter wheat has reduced groundwater recharge by 42–50% (50–55 mm year^?1 for recharge), and the conversion from winter wheat to alfalfa resulted in a significant chloride accumulation in the upper soil zone, which terminated deep drainage. The paper also evaluates the time lag between potential recharge and actual recharge to aquifer and between increase in solute concentration in soil moisture and that in the aquifer following land‐use change due to the deep unsaturated zone. Copyright © 2012 John Wiley & Sons, Ltd.     

Groundwater recharge in an arid grassland as indicated by soil chloride profile and multiple tracers

Previous studies have shown that shallow groundwater in arid regions is often not in equilibrium with near‐surface boundary conditions due to human activities and climate change. This is especially the case where the unsaturated zone is thick and recharge rate is limited. Under this nonequilibrium condition, the unsaturated zone solute profile plays an important role in estimating recent diffuse recharge in arid environments. This paper combines evaluation of the thick unsaturated zone with the saturated zone to investigate the groundwater recharge of a grassland in the arid western Ordos Basin, NW China, using the soil chloride profiles and multiple tracers (²H, ¹⁸O, ¹³C, ¹⁴C, and water chemistry) of groundwater. Whereas conventional water balance and Darcy flux measurements usually involve large errors in recharge estimations for arid areas, chloride mass balance has been widely and generally successfully used. The results show that the present diffuse recharge beneath the grassland is 0.11–0.32 mm/year, based on the chloride mass balance of seven soil profiles. The chloride accumulation age is approximately 2,500 years at a depth of 13 m in the unsaturated zone. The average Cl content in soil moisture in the upper 13 m of the unsaturated zone ranges from 2,842 to 7,856 mg/L, whereas the shallow groundwater Cl content ranges from 95 to 351 mg/L. The corrected ¹⁴C age of shallow groundwater ranges from 4,327 to 29,708 years. Stable isotopes show that the shallow groundwater is unrelated to modern precipitation. The shallow groundwater was recharged during the cold and wet phases of the Late Pleistocene and Holocene humid phase based on palaeoclimate, and consequently, the groundwater resources are nonrenewable. Due to the limited recharge rate and thick unsaturated zone, the present shallow groundwater has not been in hydraulic equilibrium with near‐surface boundary conditions in the past 2,500 years.     

Climate,vegetation, and soil controls on hydraulic redistribution in shallow tree roots

Hydraulic redistribution defined as the translocation of soil moisture by plant root systems in response to water potential gradients is a phenomenon widely documented in different climate, vegetation, and soil conditions. Past research has largely focused on hydraulic redistribution in deep tree roots with access to groundwater and/or winter rainfall, while the case of relatively shallow (i.e., ≈1–2 m deep) tree roots has remained poorly investigated. In fact, it is not clear how hydraulic redistribution in shallow root zones is affected by climate, vegetation, and soil properties. In this study, we developed a model to investigate the climate, vegetation, and soil controls on the net direction and magnitude of hydraulic redistribution in shallow tree root systems at the growing season to yearly timescale. We used the model to evaluate the effect of hydraulic redistribution on the water stress of trees and grasses. We found that hydraulic lift increases with decreasing rainfall frequency, depth of the rooting zone, root density in the deep soil and tree leaf area index; at the same time for a given rainfall frequency, hydraulic lift increases with increasing average rainstorm depth and soil hydraulic conductivity. We propose that water drainage into deeper soil layers can lead to the emergence of vertical water potential gradients sufficient to explain the occurrence of hydraulic lift in shallow tree roots without invoking the presence of a shallow water table or winter precipitation. We also found that hydraulic descent reduces the water stress of trees and hydraulic lift reduces the water stress of grass with important implications on tree–grass interactions.     

Evaluation of the soil water balance in an alluvial flood plain with a shallow groundwater table

Abstract

Many of the hydrological and ecological functions of alluvial flood plains within watersheds depend on the water flow exchanges between the vadoze soil zone and the shallow groundwater. The water balance of the soil in the flood plain is investigated, in order to evaluate the main hydrological processes that underlie the temporal dynamics of soil moisture and groundwater levels. The soil moisture and the groundwater level in the flood plain were monitored continuously for a three-year period. These data were integrated with the results derived from applying a physically-based numerical model which simulated the variably-saturated vertical water flow in the soil. The analysis indicated that the simultaneous processes of lateral groundwater flow and the vertical recharge from the unsaturated zone caused the observed water table fluctuations. The importance of these flows in determining the rises in the water table varied, depending on soil moisture and groundwater depth before precipitation. The monitoring period included two hydrological years (September 2009–September 2011). About 13% of the precipitation vertically recharged the groundwater in the first year and about 50% in the second. The difference in the two recharge coefficients was in part due to the lower groundwater levels in the recharge season of the first hydrological year, compared to those observed in the second. In the latter year, the shallow groundwater increased the soil moisture in the unsaturated zone due to capillary rise, and so the mean hydraulic conductivity of the unsaturated soil was high. This moisture state of soil favoured a more efficient conversion of infiltrated precipitation into vertical groundwater recharge. The results show that groundwater dynamics in the flood plain are an important source of temporal variability in soil moisture and vertical recharge processes, and this variability must be properly taken into account when the water balance is investigated in shallow groundwater environments.

Citation Pirastru, M. and Niedda, M., 2013. Evaluation of the soil water balance in an alluvial flood plain with a shallow groundwater table. Hydrological Sciences Journal, 58 (4), 898–911.     

Water uptake by saltcedar (Tamarix ramosissima) in a desert riparian forest: responses to intra‐annual water table fluctuation

There is considerable interest in naturalizing flow regime on managed rivers to slow the spread of saltcedar (Tamarix ramosissima) invasion in southwestern USA or to preserve riparian forests dominated by saltcedar and other species in northwestern China. However, little is known about the responses of established saltcedar in water sources to frequent intra‐annual fluctuation of water table resulting from this new, more dynamic flow regime. This study investigates how saltcedar at a riparian site in the middle reaches of the Heihe River, northwest China, responds in water sources use to intra‐annual water table fluctuations. Stable oxygen isotope was employed to determine accurate depth at which saltcedar obtains its water supply, and soil moisture monitoring was used to determine sources of plant‐available soil water. We found that the primary zone of water uptake by saltcedar were stable at 25–60 cm depth, but the water sources used by saltcedar switched between groundwater and soil moisture with the water table fluctuations. Saltcedar derived its water from groundwater when water table was at depth less than 60 cm but switched to soil moisture at 25–60 cm depth when water table declined. It is supposed that the well‐developed clay layer at 60–80 cm depth constrained lateral roots of saltcedar to the soil layers above 60 cm, while the fine‐textured soils at this site, which were periodically resaturated by rising groundwater before the stored soil moisture had become depleted, provided an important water reservoir for saltcedar when groundwater dropped below the primary zone of fine roots. The root distribution of saltcedar may also be related to local groundwater history. The quick decline in water table in the early 1980s when the riparian saltcedar had established may strand its roots in the shallow unsaturated zone. We suggested that raising the water table periodically instead of maintaining it invariably above the rooting depth could sustain desired facultative phreatophytes while maximizing water deliveries. Copyright © 2015 John Wiley & Sons, Ltd.     

Groundwater Recharge at Five Representative Sites in the Hebei Plain,China

Accurate estimates of groundwater recharge are essential for effective management of groundwater, especially when supplies are limited such as in many arid and semiarid areas. In the Hebei Plain, China, water shortage is increasingly restricting socioeconomic development, especially for agriculture, which heavily relies on groundwater. Human activities have greatly changed groundwater recharge there during the past several decades. To obtain better estimates of recharge in the plain, five representative sites were selected to investigate the effects of irrigation and water table depth on groundwater recharge. At each site, a one‐dimensional unsaturated flow model (Hydrus‐1D) was calibrated using field data of climate, soil moisture, and groundwater levels. A sensitivity analysis of evapotranspirative fluxes and various soil hydraulic parameters confirmed that fine‐textured surface soils generally generate less recharge. Model calculations showed that recharge on average is about 175 mm/year in the piedmont plain to the west, and 133 mm/year in both the central alluvial and lacustrine plains and the coastal plain to the east. Temporal and spatial variations in the recharge processes were significant in response to rainfall and irrigation. Peak time‐lags between infiltration (rainfall plus irrigation) and recharge were 18 to 35 days in the piedmont plain and 3 to 5 days in the central alluvial and lacustrine plains, but only 1 or 2 days in the coastal plain. This implies that different time‐lags corresponding to different water table depths must be considered when estimating or modeling groundwater recharge.     

Nitrate transport in the unsaturated zone: a case study of the riverbank filtration system Karany,Czech Republic

Nitrate transport in the unsaturated zone of a riverbank filtration (RBF) system in Karany, Czech Republic, was studied. Previous study of the system estimated RBF recharge as 60% riverbank filtrate and 40% local groundwater contaminated by nitrates. Nitrate concentrations observed in RBF recently cannot be explained by simple groundwater contamination and a new conception of groundwater recharge is suggested. A two‐component model based on water ¹⁸O data modelled recharge of local groundwater. One component of groundwater recharge is rainfall and irrigation water moving through the unsaturated zone of the Quaternary sediments in piston flow. The second component is groundwater from the Cretaceous deposits with a free water table. Both the components of groundwater recharge have different nitrate concentrations, and resulting contamination of groundwater depends on the participation of water from Quaternary and Cretaceous deposits. Nitrates' origins and their mixing in the subsurface were traced by ¹⁵N data. Nitrate transport from the unsaturated zone is important and time variable source of groundwater contamination. Copyright © 2011 John Wiley & Sons, Ltd.     

Soil water dynamics and water balance on a tropical coral island

Understanding soil water dynamics and the water balance of tropical coral islands is important for the utilization and management of their limited freshwater resources, which is only from rainfall. However, there is a significant knowledge gap in the influence of soil water on the water cycle of coral islands. Soil water dynamics and the water balance of Zhaoshu Island, Xisha Archipelago were thus investigated using soil moisture measurements and the Hydrus-1D model from October 2018 to September 2019. Over the study period, vegetation transpiration, soil evaporation, groundwater recharge and storage in the vadose zone were approximately 196, 330, 365 and 20 mm, occupying 22%, 36%, 40% and 2% of annual rainfall total (911 mm), respectively. For the wet season (from May to October) these values became 75, 202, 455 and 40 mm, occupying 10%, 26% and 59% and 5% of the seasonal rainfall total (772 mm), respectively. During the dry season (from November to April), a dry soil layer between 40 and 120 cm depth of the soil profile was identified that prevented water exchange between the upper soil layers and the groundwater resulting in the development of deep roots so that vegetation could extract groundwater to supplement their water requirements. Vegetation not only consumes all dry season rainfall (140 mm) but extracts water deeply from groundwater (90 mm) as well as from the vadose layer (20 mm). As such, the vegetation appears to be groundwater-dependent ecosystems. The research results aid us to better understand the process of water dynamics on coral islands and to protect coral island ecosystems.     

Field‐Scale Relationships Among Soil Properties and Shallow Groundwater Quality

It is important to understand the link between land surface/soil properties and shallow groundwater quality. To that end, soil properties and near‐water‐table groundwater chemistry of a shallow, unconfined aquifer were measured on a 100‐m grid on a 64‐ha irrigated field in southeastern North Dakota. Soil properties and hydrochemistry were compared via multivariate analysis that included product‐moment correlations and factor analysis/principal component analysis. Topographic low areas where the water table was in close proximity to the soil surface generally had higher apparent electrical conductivity (EC_a) and higher percent silt and clay than higher positions on the landscape. The majority of the groundwater was characterized by Ca‐ and Mg‐HCO₃ type water and was associated with topographic high areas with lower EC_a and net groundwater recharge. Small topographic depressions were areas of higher EC_a (net groundwater discharge) where salts that precipitated via evapotranspiration and evaporative discharge dissolved and leached to the groundwater during short‐term depression‐focused recharge events. At this site, groundwater quality and soil EC_a were related to surface topography. High‐resolution topography and EC_a measurements are necessary to characterize the land surface/soil properties and surficial groundwater quality at the field‐scale and to delineate areas where the shallow groundwater is most susceptible to contamination.     

Impact of seasonal variations in hydrological stresses and spatial variations in geologic conditions on a TCE plume at an industrial complex in Wonju,Korea

Spatial and temporal variations in a trichloroethylene (TCE) plume at an industrial complex in Wonju, Korea, were examined based on hydrogeological data and seven rounds of groundwater quality data collected over a year. The site has considerable vertical heterogeneities; the top layer of soil is covered by impermeable paving material at several locations, followed by a series of reclaimed or residual soil layers, and with weathered rocks to the crystalline biotite granite at the bottom. Areal heterogeneity in the surface conditions plays an important role in controlling groundwater recharge. The heterogeneity structure is influenced by complex surface conditions paved with asphalt and concrete. Owing to the presence of limited recharge area and concentrated summer precipitation events, the effects of seasonal variations on groundwater hydraulics tend to diminish with distance from the recharge area. This result was established by analysing the influence of the contrasting surface recharge conditions between the near‐source zone and the far zone, and the seasonally concentrated precipitation on the transport patterns of a TCE plume. In addition, variations in the plume's downstream contaminant flux levels were also analysed along a transect line near the source zone. The results show that the general tendency of the TCE plume contaminant concentration and mass discharges were reproducible if we account for seasonal recharge variations and the associated changes in the groundwater level. During recharge events, the TCE concentration variations appear to be influenced by leaching of the residual dense non‐aqueous‐phase liquid (DNAPL) TCE trapped in the unsaturated zone. This result shows that seasonal variations in contaminant plume near the source zone is inevitable at this site, and that these variations indicate the presence of residual DNAPL at or above the water table, at least in some isolated locations. Copyright © 2011 John Wiley & Sons, Ltd.     

Water use strategies of two co‐occurring tree species in a semi‐arid karst environment

The flow of precipitation from the surface through to groundwater in karst systems is a complex process involving storage in the unsaturated zone and diffuse and preferential recharge pathways. The processes associated with this behaviour are not well understood, despite the prevalence of karst aquifers being used as freshwater supplies. As a result, uncertainty regarding the ecohydrological processes in this geological setting remains large. In response to the need to better understand the impact of woody vegetation on groundwater recharge, annual evapotranspiration (ET) rates and tree water sources were measured for two years above a shallow, fresh karst aquifer. Water use strategies of the co‐occurring Eucalyptus diversifolia subsp. diversifolia Bonpl. and Allocasuarina verticillata (Lam.) L. Johnson were investigated using a monthly water balance approach, in conjunction with measurement of the stable isotopes of water, leaf water potentials and soil matric potentials. The results suggest that it is unlikely groundwater resources are required to sustain tree transpiration, despite its shallow proximity to the soil surface, and that similarities exist between ET losses and the estimated long‐term average rainfall for this area. Irrespective of stand and morphological differences, E. diversifolia and A. verticillata ET rates showed remarkable convergence, demonstrating the ability of these co‐occurring species to maximise their use of the available precipitation, which avoids the requirement to differentiate between these species when estimating ET at a landscape scale. We conclude that the water holding capacity of porous geological substrates, such as those associated with karst systems, will play an important role in equilibrating annual rainfall variability and should be considered when assessing ecohydrological links associated with karst systems. Copyright © 2013 John Wiley & Sons, Ltd.     

Modelling soil water dynamics considering measurement uncertainty

In shallow water table‐controlled environments, surface water management impacts groundwater table levels and soil water dynamics. The study goal was to simulate soil water dynamics in response to canal stage raises considering uncertainty in measured soil water content. Water and Agrochemicals in the soil, crop and Vadose Environment (WAVE) was applied to simulate unsaturated flow above a shallow aquifer. Global sensitivity analysis was performed to identify model input factors with the greatest influence on predicted soil water content. Nash–Sutcliffe increased and Root Mean Square Error reduced when uncertainties in measured data were considered in goodness‐of‐fit calculations using measurement probability distributions and probable asymmetric error boundaries, implying that appropriate model performance evaluation should be carried out using uncertainty ranges instead of single values. Although uncertainty in the experimental measured data limited evaluation of the absolute predictions by the model, WAVE was found a useful exploratory tool for estimating temporal variation in soil water content. Visual analysis of soil water content time series under proposed changes in canal stage management indicated that sites with land surface elevation of less than 2.0‐m NGVD29 were predicted to periodically experience saturated conditions in the root zone and shortening of the growing season if canal stage is raised more than 9 cm and maintained at this level. The models developed could be combined with high‐resolution digital elevation models in future studies to identify areas with the greatest risk of experiencing saturated root zone. The study also highlighted the need to incorporate measurement uncertainty when evaluating performance of unsaturated flow models. Copyright © 2014 John Wiley & Sons, Ltd.     

Nitrate fluctuations at the water table: implications for recharge processes and solute transport in the Chalk aquifer

This study investigates fluctuations in nitrate concentration at the water table to improve understanding of unsaturated zone processes in the Chalk aquifer. Sampling was conducted using a novel multi‐level sampler during periods of water table rise over 5 years at a vertical resolution of 0.05 m. Nitrate concentration increased as the water table seasonally recovered, with similar inter‐annual trends with depth. The rising water table activated horizontal fractures facilitating the delivery of water elevated by up to 10 mg/l of nitrate with respect to the adjacent groundwater below. These fractures are considered to activate via piston displacement of water from the adjoining matrix. Hydrograph analysis identified 16 events which significantly perturbed the water table within 24–48 h of rainfall. Consistent nitrate concentrations indicate recharge through persistent fracture flow from the surface was not generally the primary driver of the rapid water table response during these events. Instead, the response was attributed to the piston displacement of porewater immediately above the water table. However, a single event in November 2012 delivered relatively dilute recharge indicating rapid persistent fracture flow following rainfall was possible to a depth of 14–15 m. Decreases in porewater nitrate concentration around fracture horizons and the dilution of many groundwater samples with respect to porewaters indicate a fresher source of water at depth. This was considered most likely to be a result of near surface water bypassing the matrix because of widespread mineralization on fracture surfaces, which retard water and solute exchange. Therefore, persistent fracture flow maybe considered a frequent process, operating independently of the matrix, and is not necessarily event driven. Copyright © 2015 John Wiley & Sons, Ltd.     

A large weighing lysimeter for evapotranspiration and soil‐water–groundwater exchange studies

A large weighing lysimeter was installed at Yucheng Comprehensive Experimental Station, north China, for evapotranspiration and soil‐water–groundwater exchange studies. Features of the lysimeter include the following: (i) mass resolution equivalent to 0·016 mm of water to accurately and simultaneously determine hourly evapotranspiration, surface evaporation and groundwater recharge; (ii) a surface area of 3·14 m² and a soil profile depth of 5·0 m to permit normal plant development, soil‐water extraction, soil‐water–groundwater exchanges, and fluctuations of groundwater level; (iii) a special supply–drainage system to simulate field conditions of groundwater within the lysimeter; (iv) a soil mass of about 30 Mg, including both unsaturated and saturated loam. The soil consists mainly of mealy sand and light loam. Monitoring the vegetated lysimeter during the growing period of winter wheat, from October 1998 through to June 1999, indicated that during the period groundwater evaporation contributed 16·6% of total evapotranspiration for a water‐table depth from 1·6 m to 2·4 m below ground surface. Too much irrigation reduced the amount of upward water flow from the groundwater table, and caused deep percolation to the groundwater. Data from neutron probe and tensiometers suggest that soil‐water‐content profiles and soil‐water‐potential profiles were strongly affected by shallow groundwater. Copyright © 2000 John Wiley & Sons, Ltd.     

Importance of unsaturated zone flow for simulating recharge in a humid climate

Transient recharge to the water table is often not well understood or quantified. Two approaches for simulating transient recharge in a ground water flow model were investigated using the Trout Lake watershed in north-central Wisconsin: (1) a traditional approach of adding recharge directly to the water table and (2) routing the same volume of water through an unsaturated zone column to the water table. Areas with thin (less than 1 m) unsaturated zones showed little difference in timing of recharge between the two approaches; when water was routed through the unsaturated zone, however, less recharge was delivered to the water table and more discharge occurred to the surface because recharge direction and magnitude changed when the water table rose to the land surface. Areas with a thick (15 to 26 m) unsaturated zone were characterized by multimonth lags between infiltration and recharge, and, in some cases, wetting fronts from precipitation events during the fall overtook and mixed with infiltration from the previous spring snowmelt. Thus, in thicker unsaturated zones, the volume of water infiltrated was properly simulated using the traditional approach, but the timing was different from simulations that included unsaturated zone flow. Routing of rejected recharge and ground water discharge at land surface to surface water features also provided a better simulation of the observed flow regime in a stream at the basin outlet. These results demonstrate that consideration of flow through the unsaturated zone may be important when simulating transient ground water flow in humid climates with shallow water tables.     

Water balance of a tropical woodland ecosystem, Northern Australia: a combination of micro-meteorological, soil physical and groundwater chemical approaches

A combination of micro-meteorological, soil physical and groundwater chemical methods enabled the water balance of a tropical eucalypt savanna ecosystem in Northern Australia to be estimated. Heat pulse and eddy correlation were used to determine overstory and total evapotranspiration, respectively. Measurements of soil water content, matric suction and water table variations were used to determine changes in soil moisture storage throughout the year. Groundwater dating with chlorofluorocarbons was used to estimate net groundwater recharge rates, and stream gauging was used to determine surface runoff. The wet season rainfall of 1585 mm is distributed as: evapotranspiration 810 mm, surface runoff (and shallow subsurface flow) into the river 410 mm, groundwater recharge 200 mm and increase in soil store 165 mm. Of the groundwater recharge, 160 mm enters the stream as baseflow in the wet season, 20 mm enters as baseflow in the dry season, and the balance (20 mm) is distributed to and used by minor vegetation types within the catchment or discharges to the sea. In the dry season, an evapotranspiration of 300 mm comprises 135 mm rainfall and 165 mm from the soil store. Because of the inherent errors of the different techniques, the water balance surplus (estimated at 20 mm) cannot be clearly distinguished from zero. It may also be as much as 140 mm. To our knowledge, this is the first time that such diverse methods have been combined to estimate all components of a catchment's water balance.     

Water percolation in a thick unsaturated loess layer considering the ground‐atmosphere interaction

The key objective of this paper is to advance our present understanding of how surface water infiltrates in thick unsaturated loess, which is found in arid and semiarid regions of the world, considering the ground‐atmosphere interaction. In situ data for a period of 1 year in thick loess layer at a site in the Loess Plateau of China that has groundwater table at 97.5 m depth were collected for achieving this objective. Climate factors, mainly rainfall and actual evaporation, were measured. In addition, variations of soil temperature and water content at different depths in the unsaturated zone were also measured. The data were used to interpret the water percolation characteristics by dividing the thick unsaturated zone into three zones; namely, (i) surface zone, which constitutes the top 1.0 m, (ii) unsteady zone, which is from 1.0 to 7.0 m, and (iii) steady zone, which is below 7.0 m. In the surface zone, soil temperature and water content are sensitive to climate factors. There is a variation of water content associated with the cumulative influence of infiltration and evaporation in the precipitation and nonprecipitation periods, respectively. In the unsteady zone, the water content is relatively constant; however, temperature varies in different seasons. Water percolation in this zone is both in liquid and vapour phases. In the steady zone, both soil temperature and water content are constant during the entire investigation period. The percolation velocity in this zone is approximately 1.23 × 10^?8 m/s or 0.39 m/year, which suggests that it will take approximately 230.8 years for surface water to pass through the thick unsaturated zone and recharge the groundwater.     

An experimental study of water table recharge by seepage losses from a ditch with intermittent flow

Farmed catchments in the Mediterranean area often exhibit dense networks of ditches which are also preferential zones of water table recharge, and thereby of groundwater contamination. This study presents an experimental analysis of seepage losses and related groundwater recharge patterns during a typical Mediterranean runoff event at the scale of a ditch located above a shallow water table. The objectives were (i) to evaluate the patterns of water table recharge by seepage in a ditch, (ii) to study the main flow processes occurring during recharge, and (iii) to estimate solute propagation in case of contaminated flow in the ditch. The field observation indicated three major points. Firstly, they showed that seepage losses during a runoff event in a ditch can rapidly lead to a significant recharge of a shallow water table. Secondly, the recharge induces a groundwater mound much larger than the event plume. The infiltrated water and the accompanying solutes remained in the vicinity of the ditch. The patterns of groundwater recharge and contamination appeared very different. Lastly, both unsaturated and saturated‐piston flow processes were observed which suggests that a variably‐saturated flow modelling approach ought to be used to simulate the ditch‐water shallow table interaction. Finally, the study indicates that the patterns of water table recharge and contamination in Mediterranean catchments with dense ditches network vary largely in space and time, and will require dense monitoring networks to estimate the evolution of the average contamination levels. Copyright © 2008 John Wiley & Sons, Ltd.     

Temporal variation of stable isotopes in a precipitation–groundwater system: implications for determining the mechanism of groundwater recharge in high mountain–hills of the Loess Plateau,China

The groundwater in shallow loess aquifers in high mountain–hills in the western Loess Plateau in China is almost the sole water resource for local residents. However, the question about how the loess groundwater naturally circulates in these high mountain–hills, characterized by low precipitation and high potential evaporation, remains unclear. The objectives of this study are to evaluate the application of hydrogen and oxygen isotopes to (1) examine temporal variations of the isotopic composition of precipitation and shallow groundwater and (2) uncover the mechanism of groundwater recharge in high mountain–hills. Results from 2 years of monitoring data show a difference in the stable isotopes for groundwater and local precipitation between the winter and summer periods. Similar to precipitation, stable isotopes in groundwater are observed to be depleted in winter and enriched in summer, particularly in oxygen isotope. A prominent characteristic is that H and O isotopes of groundwater show a very clear response to strong precipitation in the rainy season in 2013. The results highlight that local precipitation is the likely recharge source for groundwater in shallow loess aquifers. Annual recharge from local precipitation maintains the groundwater resource in the shallower loess aquifer. The mechanisms governing shallow loess groundwater recharge in high mountain–hills were evaluated. In addition to possible vertical slow percolation of soil water through the unsaturated zone, rapid groundwater recharge mechanisms have been identified as temporal preferential infiltration through sinkholes, slip surface or landslide surface and through the interface of loess layer and palaeo‐soils. Most groundwater can be recharged after a heavy rainy season. Copyright © 2015 John Wiley & Sons, Ltd.     

Predicting Water Table Response to Rainfall Events,Central Florida

A rise in water table in response to a rainfall event is a complex function of permeability, specific yield, antecedent soil‐water conditions, water table level, evapotranspiration, vegetation, lateral groundwater flow, and rainfall volume and intensity. Predictions of water table response, however, commonly assume a linear relationship between response and rainfall based on cumulative analysis of water level and rainfall logs. By identifying individual rainfall events and responses, we examine how the response/rainfall ratio varies as a function of antecedent water table level (stage) and rainfall event size. For wells in wetlands and uplands in central Florida, incorporating stage and event size improves forecasting of water table rise by more than 30%, based on 10 years of data. At the 11 sites studied, the water table is generally least responsive to rainfall at smallest and largest rainfall event sizes and at lower stages. At most sites the minimum amount of rainfall required to induce a rise in water table is fairly uniform when the water table is within 50 to 100 cm of land surface. Below this depth, the minimum typically gradually increases with depth. These observations can be qualitatively explained by unsaturated zone flow processes. Overall, response/rainfall ratios are higher in wetlands and lower in uplands, presumably reflecting lower specific yields and greater lateral influx in wetland sites. Pronounced depth variations in rainfall/response ratios appear to correlate with soil layer boundaries, where corroborating data are available.