Transient Water Table Research Articles

Numerical modeling of transient water table in shallow unconfined aquifers: A hyperbolic theory and well-balanced finite volume scheme

We present a new methodology capable of modeling transient motion of shallow phreatic surface of groundwater in unconfined aquifers. This methodology is founded on a new and comprehensive theory for water table motion and a corresponding efficient numerical scheme. In the theoretical aspect, we derived a new set of governing equations constituted by a depth-averaged continuity equation and momentum equations based on unsteady Darcy’s law. The derived governing equations are of the hyperbolic type and possess stiff terms in the momentum equations due to the inertia motion in a characteristic time scale that is relatively shorter than the time scale of seepage motion. To effectively solve the derived hyperbolic system with stiff terms, in the numerical aspect, we utilize f-wave propagation algorithm, an explicit finite volume method, that can ensure numerical convergence and well-balancing solutions when momentum is rapidly relaxing to an equilibrium of steady state. Verification is successfully performed by comparing the results with analytic solutions to the classic problem of multidimensional spreading of a groundwater mound. This study demonstrates that the proposed methodology can accurately and satisfactorily simulate the spatiotemporal distribution of shallow water table and its wetting front in unconfined aquifers.

Revegetation impacts on runoff generation processes in a typical loess catchment

AbstractSince 1999, large‐scale ecosystem restoration has been implemented in the Loess Plateau, effectively increasing regional vegetation coverage. Vegetation restoration has significantly elevated the saturated hydraulic conductivity (Ks) of the near‐surface soil layers and increased the vertical heterogeneity of the Ks profile. Many studies have examined the change of runoff due to revegetation, yet the impacts of Ks profile on the soil moisture distribution and runoff generation processes were less explored. In this study, numerical simulations were conducted to investigate how changes in the Ks profile caused by vegetation restoration influenced the hydrological responses at event scale. The numerical simulation results show that the increase of surface Ks caused by vegetation restoration can effectively reduce runoff at event scale. Moreover, the enhancement of vertical heterogeneity of Ks profiles can significantly change the vertical profile of soil water content, prompting more water to percolate into the deep soil layer. When rainfall exceeds a threshold, the accumulation of soil water above the relatively less permeable layer can cause short‐term saturation in shallow soil layers, resulting in a transient perched water table. As a result, after the vegetation restoration in the Loess Plateau, though Horton overland flow is still the main runoff generation mechanism, there is a possibility of the emergence of Dunne overland flow under the high vegetation coverage (e.g., NDVI larger than 0.5). This emergence of new runoff generation mechanism, saturation excess runoff, in the Loess Plateau due to the vegetation restoration could provide scientific guidance for water and sediment movement, soil and water conservation practices, and desertification control in the Loess Plateau.

From rainfall to runoff: The role of soil moisture in a mountainous catchment

Soil moisture is essential in runoff generation, chemical transport, vegetation growth, and geological hazards like landslides in mountainous regions. However, due to its difficulty of measurement and its significant spatial heterogeneity and temporal variability, the dynamics and influence of soil moisture in the rainfall-runoff process are still not fully understood. In this study, precipitation, runoff, and soil moisture at multiple depths were measured and analyzed in a small, post-seismic mountainous catchment. Our results suggest that the dynamics of soil moisture could be explained by the characteristics of soil. The drastic change and the anisotropy of saturated hydraulic connectivity in the upper soil of the loose deposits in our catchment would lead to soil stratification at depths between 30 cm and 50 cm. The temporal variability of soil moisture was more highly correlated within the soil layers above and beneath the interface. Above the interface, besides vertical infiltration, occurrences of preferential flow were also frequent, transient perched water table may emerge during the wet period, making it a more active runoff contributor than the soil layers below the interface. When the level of rainfall reached the threshold, the correlation between rainfall and runoff became linear. This threshold response was more significant at soil layers above the interface, especially at the site with large topographic gradient. High antecedent soil moisture could skew the partitioning of precipitation into discharge, leading to an earlier flow peak. Our findings provide guidance on observation spot selection and the soil properties needed for the interpretation of future measurements, and may help develop flood early warnings system in small ungauged mountainous catchments with high flood risk.

Assessment of Spatiotemporal Groundwater Recharge Distribution Using SWAT-MODFLOW Model and Transient Water Table Fluctuation Method

Recharge is a crucial section of water balance for both surface and subsurface models in water resource assessment. However, quantifying its spatiotemporal distribution at a regional scale poses a significant challenge. Empirical and numerical modeling are the most commonly used methods at the watershed scales. However, integrated models inherently contain a vast number of unknowns and uncertainties, which can limit their accuracy and reliability. In this work, we have proposed integrated SWAT-MODFLOW and Transient Water Table Fluctuation Method (TWTFM) to evaluate the spatiotemporal distribution of groundwater recharge in Anyang watershed, South Korea. Since TWTFM also uses SWAT model percolation output data, calibration was performed for individual models and a coupled model. The coupled model was calibrated using daily streamflow and hydraulic head. The SWAT-MODFLOW model performed well during the simulation of streamflow compared to the SWAT model. The study output showed that the study watershed had significant groundwater recharge variations during the simulated period. A significant amount of recharge happens in the wet season. It contributes a significant amount of the average annual precipitation of the region. The direct flow components (surface and lateral) showed significant contributions when the water balance components were evaluated in the region. TWTFM showed a glimpse to estimate recharge, which requires representative monitoring wells in the study region. Comprehensively, the SWAT-MODFLOW model estimated groundwater recharge with reasonable accuracy in the region.

A Richards’ equation-based model for wave-resolving simulation of variably-saturated beach groundwater flow dynamics

This study introduces a model based on Richards’ equation to describe variably-saturated beach groundwater flow. The surface wave propagation is computed by the phase-resolving non-hydrostatic SWASH code. The SWASH data are used to make a suitable dynamic boundary condition at the beach face to force Richards’ equation. The latter is solved by a weighted discontinuous Galerkin method together with adaptive mesh refinement. The model is validated by comparison with a laboratory experiment of a transient water table recharge problem. Then, the BARDEX II prototype-scale experiment is considered to assess the model abilities for beach groundwater dynamics. The barrier beach is studied for three cases with different lagoon levels. Steady-state results with no-wave conditions show excellent agreement. Transient waves simulations are evaluated in terms of pressure heads, saturations, water table position and groundwater velocities for time-averaged, swash-resolving and spectral analysis. Results bring interesting insights about beach groundwater modelling by comparison with the experimental data as well as a Darcy’s equation-based model. A first investigation is carried out to assess the groundwater effect on the bed sediment dynamics through the modification of sediment relative weight.

Influence of Subsurface Pipe Perforations on Transient Water Table Response and Steady Soil-Water Flux under Subirrigation

HighlightsWater table rise was slower under a sparsely perforated pipe as compared to a pipe that was densely perforated.Response time increased as buried pipe depth decreased for both combinations of perforation characteristics.Fluxes from buried pipes spaced at 15 m in loamy sand satisfied the water demand for cereal/grain crops (6 mm d-1).Increasing perforation density on the pipe wall is the most effective way to reduce exit head losses.Abstract. It is important to consider the water table rise into the unsaturated zone when designing subsurface irrigation systems that consist of buried perforated corrugated pipes. Corrections to pipe spacing equations for head losses due to perforations are based on the entrance resistance, which is derived for buried pipes operating in drainage mode. However, the boundary condition at the soil-pipe interface when operated in subsurface irrigation mode requires the use of the exit resistance to accurately account for perforations in the pipe wall. Simulations of a subsurface irrigation system with four soil textures, three lateral pipe spacings, and three buried depths were carried out, with the numerical model COMSOL, to demonstrate the effects of variable perforation characteristics on the transient water table rise into the unsaturated zone. Soil-water fluxes were also computed for the system when operated under steady-state conditions. The simulated water table rise exceeded or approached a predetermined target water level of 1.65 m above the buried pipes in coarse-textured soils (loamy sands and silt loams) with lateral pipe spacings as large as 10 m. Increasing the number and length of rectangular slots in the pipe wall (densely perforated) can reduce the water table response time to reach the target level by 15 h in a silt loam soil and 25 h in a clay soil. The optimum pipe spacing needed to sustain a soil-water flux of 6 mm d-1 (common to eastern Canada and the U.S. Midwest) can be increased from 15.1 to 16.2 m in loamy sands by using densely perforated pipes. In contrast, increasing perforations in pipes buried in clays and sandy clays will result in pipe spacing increases of less than 0.2 m because these fine-textured soils have lower hydraulic conductivity. This study has demonstrated that variable perforation characteristics can be included in the analysis and design of subsurface drainage systems for improving water management. Keywords: Corrugated perforated pipes, Exit resistance, Head loss, Subirrigation, Unsaturated zone, Water table response.

Field infiltration of artificial irrigation into thick loess

Frequent agricultural irrigation triggers numerous landslides in the loess platform, northwest China. We conducted a field infiltration experiment with a diameter of 20 m on the South Jingyang tableland in Shaanxi province, China to better understand the infiltration process in thick unsaturated loess. Soil moisture probes, tensiometers, and differential pressure transducers were installed at various depths to monitor the infiltration process. The results showed that the initially high infiltration rate gradually decreased and finally approached a constant value less than that of the saturated hydraulic conductivity of the top soil. Matric suction decreased rapidly, and the volumetric water content increased to a nearly saturated state with the arrival of the wetting front. The soil water characteristic curve obtained from field monitoring data agreed with that from laboratory tests performed on undisturbed specimens. Preferential flow associated with vertical cracks parallel to the margin of the platform was observed in the late Pleistocene Loess layer (L1), and an empirical model was developed from monitoring recordings to explain preferential flow formation. A transient perched water table formed above the lower part of the first paleosol layer (S1). The monitored pore-air pressure increased and then fluctuated before the arrival of the wetting front. The maximum value of pore-air pressure at different depths was less than 5 kPa and tended to increase linearly with dry density. The results of this research help understand the migration of irrigation water in thick unsaturated loess and the recharge mechanism of the deep groundwater table.

Estimating the Temporal Distribution of Groundwater Recharge by Using the Transient Water Table Fluctuation Method and Watershed Hydrologic Model

HighlightsThe transient water table fluctuation method (TWTFM) is revisited.A novel application of linking SWAT model and TWTFM is suggested.A method is proposed to estimate daily groundwater recharge distribution.The method is demonstrated for the Jeju Island in Korea.Abstract. Estimating groundwater recharge remains a difficult but necessary task as part of managing available groundwater supplies. For example, the average groundwater recharge rate of Jeju Island is 54%, which is considerably higher than the inland recharge rate (~15%) in Korea. Although groundwater is the main water source of this and many other islands, quantifying temporal groundwater recharge for water resources planning remains difficult. To estimate groundwater recharge based on rainfall, a simple and straightforward method is proposed that uses an application of the Transient Water Table Fluctuation Method (TWTFM) linked with the Soil and Water Assessment Tool (SWAT). By using the computed annual percolation from the SWAT as input, two parameters (reaction factor and specific yield) could be estimated by assuming that the sum of daily recharge via the TWTFM was approximately equal to the annual percolation near the water table. This methodology was applied to the Hancheon watershed of Jeju Island, South Korea. Runoff time series data for two years (2009 and 2010) were used to calibrate SWAT and another two years of data were used to validate computed discharges from SWAT. For the calibration of the combined SWAT and TWTFM model, groundwater level data from 2009 and 2010 were used, and then data from 2011 and 2012 were used to predict groundwater recharge using the calibrated TWTFM parameters. The proposed methodology can be used as an efficient tool for estimating the temporal distribution of groundwater recharge using only groundwater data and the annual percolation rate. This methodology can be beneficial for regions where the vadose zone depth is deeply formed and temporal recharge predictions are essential for water management. Keywords: Reaction factor, Specific yield, SWAT, Transient water table fluctuation method (TWTFM).

Modelling of Water Flow and Nitrate Dynamics Through Partially Saturated Porous Media in Laboratory Condition using Hydrus-2D

Due to the reducing availability of water resources and the growing competition for water between residential, industrial, and agricultural users, increasing irrigation efficiency, by several methods like drip irrigation, is a demanding concern for agricultural experts. The understanding of the water and contaminants flow through the subsurface is needed for the sustainable irrigation water management, pollution assessment, remediation, and groundwater recharge. In this study, the Windows-based computer software package HYDRUS-2D, which numerically simulates water and solute movement in two-dimensional, variably-saturated porous media, was used to evaluate the distribution of water and Nitrate in the sand tank. The laboratory and simulation experiments were conducted to evaluate the role of drainage, recharge flux, and infiltration on subsurface flow condition and subsequently, on nitrate movement in the subsurface. The water flow in the unsaturated zone was model by Richards’ equation, which was highly nonlinear and its parameters were largely dependent on the moisture content and pressure head of the partially saturated zone. Following different cases to be considered to evaluate- a) applying drainage and recharge flux to study domains, b) transient infiltration in a vertical soil column and c) subsequently, nitrate movement in a 2D sand tank setup. A single porosity model was used for the simulation water and nitrate flow in the study domain. The results indicate the transient water table position decreases as the time increase significantly by applying drainage flux at the bottom. Similarly, the water table positions in study domains increasing in the domain by applying recharge flux. Likewise, the water flow profile shows the decreasing water table elevation with increasing water content in the vertical domain. Furthermore, the nitrate transport dynamics was dominated by advective flux and highly affected by the recharge flux in the vertical direction. The findings of the study help to enhance the understanding of the sustainable soil-water resources management and agricultural practices.

Ephemeral and intermittent runoff generation processes in a low relief, highly weathered catchment

AbstractMost field‐based approaches that address runoff generation questions have been conducted in steep landscapes with shallow soils. Runoff generation processes in low relief landscapes with deep soils remain less understood. We addressed this by characterizing dominant runoff generating flow paths by monitoring the timing and magnitude of precipitation, runoff, shallow soil moisture, and shallow and deep groundwater dynamics in a 3.3 ha ephemeral‐to‐intermittent drainage network in the Piedmont region of North Carolina, USA. This Piedmont region is gently sloped with highly weathered soils characterized by shallow impeding layers due to decreases in saturated hydraulic conductivity with depth. Our results indicated two dominant catchment storage states driven by seasonal evapotranspiration. Within these states, distinct flow paths were activated, resulting in divergent hydrograph recessions. Groundwater dynamics during precipitation events with different input characteristics and contrasting storage states showed distinct shallow and deep groundwater flow path behavior could produce similar runoff magnitudes. During an event with low antecedent storage, activation of a shallow, perched, transient water table dominated runoff production. During an event with high antecedent storage, the deeper water table activated shallow flow paths by rising into the shallow transmissive soil horizons. Despite these differing processes, the relationship between active surface drainage length (ASDL) and runoff was consistent. Hysteretic behavior between ASDL and runoff suggested that while seasonal ASDLs can be predicted based on runoff, the mechanisms and source areas producing flow can be highly variable and not easily estimated from runoff alone. These processes and flow paths have significant implications for stream chemistry across seasons and storage states.

Modeling of transient water table response to managed aquifer recharge: a lagoon in Muscat, Oman

Natural and anthropogenic conditions in the Gulf countries lead to substantial changes in groundwater (GW) dynamics. GW mound under a lagoon supplied by treated wastewater (TWW) from a municipal sewage treatment plant (STP) in Muscat, Oman, is studied using analytical and numerical modeling techniques. The water table mound in an unconfined aquifer, laterally bounded by the sea, receives recharge from a lagoon fed by a STP in a seasonally varying regime. The onset and duration of the filling period and, correspondingly, infiltration from the lagoon bed is dictated by the wastewater processing and TWW use. Following the period of recharge-induced growth, the GW mound decays when recharge ceases, and the void space above the water table becomes available for future injection in the next annual cycle of infiltration and TWW subsurface storage. The Green-Ampt model application illustrates that the first phase of the process (vertical infiltration with front propagating from the lagoon bed to the regional flat water table) is typically short (several hours). The second phase, when TWW gets in a direct hydraulic contact with GW, is the longest and with a mound spreading to hundreds of meters during winter months. No analytical solutions to this moving boundary-value problem are known and approximate models, based on the Boussinesq approximation and its linearizations, are utilized. The Hantush-type drawup of the water table in the vicinity of the lagoon is studied by the Watson analytical solution to the diffusion equation for a vertically averaged hydraulic head in an unbounded domain, with an instantaneous jump of the head over a disk-shaped zone. This drawup is compared with MODFLOW simulations, which take into account a finite size of the flow domain (catchment) and discharge into the sea through a constant-head segment. The Mishra-Guyonnet’s formula assumes a spatially uniform flux from the lagoon bed (disk in a model plain) into a growing mound and is used for calculation of the total discharge, which progressively decreases from the early stages of the Green-Ampt infiltration to a theoretically zero value if the second phase lasts indefinitely long. We assess characteristic times and phases of the mound dynamics during the rise and fall stages using regional hydrogeological parameters, site geometry, and typical patterns of wastewater disposal. Sensitivity analysis of the locus of the phreatic surface at a given instance and location is carried out, with storativity, hydraulic conductivity, undisturbed aquifer thickness and lagoon radius as input variables. For an example of an aquifer of saturated depth of 40 m, undisturbed vadose zone thickness of 10 m, saturated hydraulic conductivity of 10 m/day, effective porosity of 0.2, a circular lagoon size of 90 m and water depth there of 1 m we found that the infiltration front propagates to the water table in less than 0.2 days; after 40 days of a continuous mound growth in a hypothetical piezometer located 400 m from the lagoon the water table drawup was in the range of 2.0–2.7 m if the aquifer hydraulic parameters and lagoon size are varied 20 % from the average value. This approach will permit to plan more detailed site investigation, facilitate management decisions on the rate of water release to the lagoon, and foresee waterlogged areas.

Influence of landscape position and transient water table on soil development and carbon distribution in a steep, headwater catchment

Upland headwater catchments, such as those in the Appalachian Mountain region, are typified by coarse textured soils, flashy hydrologic response, and low baseflow of streams, suggesting well drained soils and minimal groundwater storage. Model formulations of soil genesis, nutrient cycling, critical loads and rainfall/runoff response are typically based on vertical percolation, development of soil horizons parallel to the land surface, mineral weathering inputs limited to the rooting zone and drainage from lumped catchment reservoirs (e.g., the subsoil) as the dominant source of stream flow. However, detailed study of the hydrologic reference catchment at Hubbard Brook Experimental Forest, NH, USA shows striking spatial patterns of soil development that reflect the influence of transient water tables within the solum in nearly all landscape positions. Shallow bedrock and variably low hydraulic conductivity in the subsoil promote lateral flow and development of soil horizons along hillslope flowpaths rather than in vertical profiles. We distinguished several morphologic units based on the presence of diagnostic horizons indicative of differing patterns of podzolization and carbon storage. The distribution of soils appears to be highly dependent on local drainability and frequency and duration of transient saturation within the solum. As such, monitoring of hydropedologic groups and transient water table fluctuations may prove to be a sentinel for the effects of climate change on spatial distribution of soils and retention/release of solutes from upland catchments.

The role of hydrological transience in peatland pattern formation

Abstract. The sloping flanks of peatlands are commonly patterned with non-random, contour-parallel stripes of distinct micro-habitats such as hummocks, lawns and hollows. Patterning seems to be governed by feedbacks among peatland hydrological processes, plant micro-succession, plant litter production and peat decomposition. An improved understanding of peatland patterning may provide important insights into broader aspects of the long-term development of peatlands and their likely response to future climate change. We recreated a cellular simulation model from the literature, as well as three subtle variants of the model, to explore the controls on peatland patterning. Our models each consist of three submodels, which simulate: peatland water tables in a gridded landscape, micro-habitat dynamics in response to water-table depths, and changes in peat hydraulic properties. We found that the strength and nature of simulated patterning was highly dependent on the degree to which water tables had reached a steady state in response to hydrological inputs. Contrary to previous studies, we found that under a true steady state the models predict largely unpatterned landscapes that cycle rapidly between contrasting dry and wet states, dominated by hummocks and hollows, respectively. Realistic patterning only developed when simulated water tables were still transient. Literal interpretation of the degree of hydrological transience required for patterning suggests that the model should be discarded; however, the transient water tables appear to have inadvertently replicated an ecological memory effect that may be important to peatland patterning. Recently buried peat layers may remain hydrologically active despite no longer reflecting current vegetation patterns, thereby highlighting the potential importance of three-dimensional structural complexity in peatlands to understanding the two-dimensional surface-patterning phenomenon. The models were highly sensitive to the assumed values of peat hydraulic properties, which we take to indicate that the models are missing an important negative feedback between peat decomposition and changes in peat hydraulic properties. Understanding peatland patterning likely requires the unification of cellular landscape models such as ours with cohort-based models of long-term peatland development.

Analytical studies on transient groundwater flow induced by land reclamation using different fill materials

AbstractLand reclamation may have a significant influence on groundwater regimes. Analytical solutions have been developed in the past to study the impact of land reclamation on a steady‐state groundwater flow and transient flow in fill materials, assuming that the reclamation site consists of a single zone of uniform hydraulic parameters. In this paper, we derive analytical solutions to describe the transient water table change in response to multi‐stage land reclamation where the fill material is uniform in each stage but the hydraulic conductivity of the fill material varies from stage to stage. By introducing the method of separation of variables, we develop a transient analytical solution to study the impact of land reclamation consisting of fill material with different hydraulic properties on groundwater dynamics. The results show that the water table first increases significantly into the reclaimed zone following the fill material deposition, and then the increase gradually propagates into the original aquifer. The change of water table in the original aquifer mainly depends on the value of hydraulic conductivity of the fill materials. Examples in this paper illustrate how the aquifer system experiences a long time unsteady‐state flow as a result of the reclamation, and it takes at least tens of years for the system to approach a new equilibrium. It is suggested that for a large‐scale reclamation project, the response of the groundwater regime to reclamation should be carefully studied. Copyright © 2013 John Wiley & Sons, Ltd.

Identifying a parameterisation of the soil water retention curve from on-ground GPR measurements

Abstract. We show the potential of on-ground Ground-Penetrating Radar (GPR) to identify the parameterisation of the soil water retention curve, i.e. its functional form, with a semi-quantitative analysis based on numerical simulations of the radar signal. An imbibition and drainage experiment has been conducted at the ASSESS-GPR site to establish a fluctuating water table, while an on-ground GPR antenna recorded traces over time at a fixed location. These measurements allow to identify and track the capillary fringe in the soil. The typical dynamics of soil water content with a transient water table can be deduced from the recorded radargrams. The characteristic reflections from the capillary fringes in model soils that are described by commonly used hydraulic parameterisations are investigated by numerical simulations. The parameterisations used are (i) full van Genuchten, (ii) simplified van Genuchten with m = 1 − 1/n and (iii) Brooks–Corey. All three yield characteristically different reflections, which allows the identification of an appropriate parameterisation by comparing to the measured signals. We show that for the sand used here, these signals are not consistent with the commonly used simplified van Genuchten parameterisation with m = 1 − 1/n.

Analysis of groundwater-surface water interaction in horizontal unconfined aquifer bounded by a stream and groundwater divide using a non-linear numerical model

In this study, a non-linear numerical model was developed to analyse the interaction between the drainage water moving from an unconfined aquifer and a surface water body. This interaction can occur during or after a rainfall event, which can cause the groundwater table to change. At first, a laboratory-scale aquifer model with stream and groundwater divide boundaries was established to examine horizontal unconfined aquifer condition. Both steady-state and transient experiments were performed by setting the water table at a steady-state using a constant recharge rate, and then allowing the aquifer to drain. The transient water table data showed highly non-linear variations. Then the experimental data were recreated using a non-linear numerical model, which used non-linear one dimensional Boussinesq equation and Picard iteration for solving non-linear term of the Boussinesq equation. The numerical model was able to predict the observed groundwater table data. The proposed numerical model is a useful and cost effective tool for managing groundwater-surface water management problems.

Groundwater Fluctuations in Sloping Aquifers Induced by Time-Varying Replenishment and Seepage from a Uniformly Rising Stream

This paper analyses the classical problem of transient surface–groundwater interaction in a stream–aquifer system under rather realistic conditions. The downward sloping unconfined aquifer is in contact with a constant water level at one end, and a fully penetrating stream at the other end whose water level is rising at a uniform speed. Furthermore, the aquifer is replenished by a vertical time-varying recharge. Closed form analytical expressions for hydraulic head and flow rate in the aquifer are obtained by solving the linearized Boussinesq equation using Laplace transform method. Effects of aquifer parameters on transient water table and flow rate are illustrated with a numerical example. To assess the efficiency of the linearization method, analytical solutions are compared with numerical solutions of the corresponding non-linear equation.

Habitat preferences of the encroacher shrub, Seriphium plumosum

The ecology of Seriphium plumosum is not yet fully understood and therefore requires in-depth research to assist with the control of this encroacher plant. The habitat preferences of the plant were quantified for terrain morphological units in terms of soil fertility, saline/sodic conditions and soil–water relationships. The influence of S. plumosum density on grass production was also quantified. It is clear that the distribution (both occurrence and density) of S. plumosum is associated with soil characteristics. The study indicated the sensitivity of S. plumosum towards wet conditions, perennial and transient water tables, overly high soil pH, clay content, high soil fertility (organic matter and phosphate) and sodium chloride. The deeper the soil profile, the more favourable the habitat for S. plumosum, owing to a decrease in waterlogged conditions. The habitat prevalence of S. plumosum is possibly a reflection of not only climatologically wetness but also of topographic wetness. There was no clear general trend for the individual cations along the terrain units. A significant relationship was established between aboveground phytomass production loss of the grass sward and S. plumosum density. These results confirm the threat posted by S. plumosum in semiarid grassland areas, following changes in environmental characteristics.

Field Test of the Variable Source Area Interpretation of the Curve Number Rainfall-Runoff Equation

The Soil Conservation Service Curve Number (SCS-CN) method is a widely used empirical rainfall-runoff equation. Although the physical basis of the method has been debated, several researchers have suggested that it can be used to predict the watershed fraction that is saturated and generating runoff by saturation excess from variable source areas (VSAs). In this paper, we compare saturated runoff-contributing areas predicted with the VSA interpretation of the SCS-CN method with field-measured VSAs in a 0.5 ha hillslope in central New York State. We installed a trench below a VSA and simultaneously recorded water flux from different soil layers at the trench face and water table dynamics upslope of the trench. This setup allowed us to monitor runoff initiation and saturation-excess overland flow in response to rainfall and different water table depths in the hillslope during 16 storm events. We found that the SCS-CN method accurately predicted the observed VSA and showed best agreement if the VSA was defined as the area where the water table was within 10 cm of the soil surface. These results not only demonstrate that the VSA interpretation of the SCS-CN method accurately predicts VSA extents in small watersheds but also that the transient water table does not necessarily need to intersect the land surface to cause a storm runoff response.

Spatial variation in transient water table responses: differences between an upper and lower hillslope zone

AbstractTo better understand storage‐runoff dynamics, transient groundwater responses were examined in one of the steep watersheds in British Columbia's coastal mountains. Streamflow and piezometric data were collected for 1 year to determine the spatial and temporal relations between transient groundwater levels and discharge. Correlations between piezometer responses and lag‐time analysis were used to identify and better understand runoff generation mechanisms in this watershed. Results showed a large spatial and temporal variation in transient water table dynamics and indicated that two distinct zones existed: a lower hillslope zone and an upslope zone. Each zone was characterized by very different water table responses. The upper hillslope was disconnected from the stream for the majority of time, suggesting that during most events, it does not directly contribute to streamflow. Piezometers in the lower hillslope zone showed hydrologically limited responses, suggesting rapid subsurface flow, likely through the many macropores and soil pipes. The lag time between peak streamflow and peak groundwater level decreased with increasing antecedent moisture conditions and was more variable for piezometers further away from the stream than for piezometers close to the stream. The study results indicate that a single storage‐runoff model is not appropriate for this steep watershed and that a two‐ or three‐compartment model would be more suitable. Copyright © 2011 John Wiley & Sons, Ltd.