Water Table Height Research Articles

The hydrological basis for conservation of floodplain wetlands: implications of work at Narborough Bog, UK

1. Increasing concern for the environmental consequences of wetland loss has yet to be reflected in a wider application of hydrological techniques for their preservation. This paper looks at the specific example of floodplain wetlands by first considering how local hydrogeomorphological conditions may explain varied wetland occurrence. The effects of variations in individual hydrological fluxes are identified and the ability of standard hydrological monitoring programmes to represent these processes is considered. 2. These concepts are illustrated with reference to work on Narborough Bog, a small floodplain wetland in the UK. The characteristics of the wetland are outlined to illustrate how this determined the design of a monitoring programme investigating wetland hydrology and the practicalities of conservation. Measurements of water-table height were completed using a network of 30 piezometers located in the wetland and with additional measurements of river stage and meteorological variables. 3. The results of this data collection programme over the period 1991–1993 are described. The value of the data in characterizing the hydrology of the wetland is assessed and the need for sustained monitoring of variations in wetland water storage and flux is emphasized. The implications of the results for future wetland monitoring programmes are identified. 4. Hydrological data collected in this way can be used to develop a wetland mathematical model. This may be achieved by aggregating wetland sedimentological characteristics and by modelling specific periods of precipitation and evapotranspiration to determine the validity of the modelling process. Current shortcomings in the modelling process are discussed; however, wider application of this technique might help direct informed conservation of floodplain wetland landforms through improved understanding of the balance between internal and external water fluxes and their interrelationships. Similarly, models enable evaluation of the impacts of future catchment changes on wetlands. © 1997 John Wiley & Sons, Ltd.

Methane flux in a boreal fen: Season‐long measurement by eddy correlation

Eddy correlation measurements of methane flux were made at a fen in central Saskatchewan, as part of the Boreal Ecosystem Atmosphere Study (BOREAS) in 1994. Data were collected from mid‐May to early October. The water table was above the average peat surface throughout the measurement period. Detailed (near continuous) measurements allowed examination of temporal variability at hourly and daily time‐scales. As compared with the average nighttime flux, the average daytime methane flux was 25–45% higher in July and in August and 5–15% higher earlier and later in the season. Distribution of midday (1130–1430 LT) methane emission showed varying trends in different parts of the season. From mid‐May to early July, methane flux gradually increased from near zero to 4.1 mg m−2 h−1. The water table height (above an average hollow surface) varied from 0.09 to 0.18 m, but the trend in methane flux followed peat temperature (at 0.1‐m depth) more closely. The peat warmed from 3.4° to 16.3°C during this time period. Methane flux was negligible until peat temperature (at 0.1‐m depth) was above 12°C. From early July to early August there was a fivefold increase in methane flux from 4.1 to its seasonal peak of 19.5 mg m−2 h−1 on August 1. The water table ranged from 0.23 m to a brief seasonal plateau of 0.30 m on July 20–23. Sharp increases in the water table were followed by increasing trends in methane flux by approximately 12 days. Peat temperature reached its seasonal maximum (19.3°C) the same time when the methane flux peaked. After early August the methane flux declined steadily and reached a value of 2.4 mg m−2 h−1 in early October. The water table and peat temperature showed comparable trends and decreased steadily to 0.06 m and 5.7°C, respectively. The seasonally integrated methane emission (mid‐May to early October) was estimated at 16.3 g C m−2. Nonlinear regression analysis of methane flux against water table (lagged by 12 days) and peat temperature was performed for different periods of the season. Except for a brief period of very high water table (when many hummocks were inundated) the regression using water table and peat temperature explained between 68 and 94% of the variability in methane flux. The sensitivity of methane flux to water table (or the slope of the log CH4 flux/water table relationship) obtained from our daily flux values ranged from 3.4 × 10−4 to 5.0 × 10−4 m−1.

Theories of Ditch Drainage in Layered Anisotropic Soil

Approximate theoretical formulas are derived for predicting the water table position for steady rainfall or steady irrigation recharge into soil bounded by equally spaced ditch drains dug up to an impervious layer in single-, two-, and three-layered anisotropic soils. The ditch drainage problem is first solved for the single-layered case and subsequently, solutions are obtained to the twoand three-layered cases of the problem. All the solutions are based on exact mathematical procedures but utilize a physical assumption that the head loss in the arch-shaped region of the water table is negligible compared with the head loss for the remainder of the region. A correction, however, is provided to account for this head loss for all the solutions and the corrected formulas are found to be on the “safe side” for drainage design since the water table heights predicted by these formulas are higher than the actual ones. The accuracy of the proposed solution to the single-layered problem is tested by comparing it w...

Unsteady state drainage in a vertically heterogeneous soil

An analytical model is presented for prediction of transient water table heights between two parallel drains for a vertically heterogeneous soil. The drains lie on a horizontal impervious layer. The model was applied to field conditions for prediction of water table fluctuations in a clay soil whose hydraulic conductivity decreased exponentially with depth. The predicted results compared reasonably correctly with field observations for a given long-term rainfall input.

Ditch Drainage Theories for Homogeneous Anisotropic Soil

The problem of seepage of steady rainfall or irrigation recharge into a homogeneous and anisotropic soil medium, drained by equally spaced ditch drains overlying a barrier, is solved. Two general cases of the problem are considered: (1) ditch drains dug down to an impermeable barrier; and (2) ditch drains dug down to a gravel substratum. The proposed solutions are exact in the sense that they are based on potential theory and account for the surface of seepage. The validity of the proposed solution for the first case is tested by comparing the midway water-table heights obtained from it for some specific drainage situations with the corresponding upper and lower theoretical bounds of Youngs. The accuracy of the proposed analytical model for the first case is further tested by comparing the midway water-table height obtained from it for a specific drainage situation with the corresponding height obtained from other numerical work.

Effect of Recharge Duration on Water-Table Response

The problem of the effect of recharge duration on water-table response in areas with tile drains is studied using Green's function approach to the linearized Boussinesq's equation for one-dimensional, saturated flow with appropriate boundary conditions. Pulse and decaying sinusoidal type recharge excitations with given periods of drainage are fitted by Fourier series and used as forcing functions. The resulting response equation explicitly predicts water-table buildup over time and the oscillations at dynamic equilibrium, providing a method for analyzing the effect of recharge duration on water-table rise and flow toward the drains. The approach presented herein with this feature, is seen as an improvement on the drainage spacing approach of Maasland and McWhorter for annually repeated recharge events. However, it is seen that the duration of recharge, for constant recharge depth has negligible effect on the maximum water-table rise and flow toward drains at dynamic equilibrium in cases similar to those studied by McWhorter. Peak values of water-table height predicted by both the methods were close as parameters of the flow were changed but with different patterns. The method proposed in this paper results in drainage flow pattern different from the result of McWhorter and will be a useful tool for those cases where more realistic water-table heights and flow toward drains are important.

Water-table variation in a sloping aquifer due to random recharge

Variation in the level of the water table is closely linked with recharge. Therefore, any uncertainty associated with the recharge rate is bound to affect the nature of the water-table fluctuation. In this note, a ditch-drainage problem of a sloping aquifer is considered to investigate the effect of uncertainty in the recharge rate on water-table fluctuation. The rate of recharge is taken as an exponentially decaying function with its decay constant as a Gaussian random variable. Expressions for the first two moments of the water-table height, i.e. mean and standard deviation, are presented. By using these expressions, the effect of uncertainty in the recharge rate on the water-table fluctuation has been analyzed with the help of a numerical example.

Mungbean response to surface drainage when grown as a pre-rice crop on waterlog-prone ricelands

Large areas of the world's bunded rainfed lowland ricelands could be planted to a pre-rice crop if waterlogging damage during the early wet season is prevented. To build understanding necessary to develop effective field drainage practices for pre-rice crops, pot and field studies were undertaken on a Typic Tropaquept lowland rice soil in the Cagayan Valley, Philippines. The objective of the studies was to quantify effects of excessive moisture on mungbeans ( Vigna radiata (L.) Wilczek) encountering variable regimes of duration and elevation of water table height in the root zone during a shortterm waterlogging event. Small differences in level and duration of the root zone water table markedly affected plant performance. Yields were reduced by 40–100% when the water table level reached the soil surface for 6 days compared with the unstressed treatment, but were reduced by only 12–17% when the water level was 5 cm below the surface for the same time period. Regression analysis revealed a 4% reduction in yield per centimetre increase in water table level between 5 cm below to 5 cm above the soil surface during the vegetative stage, and a 6.5% reduction per centimetre during the reproductive stage. Field experiments evaluated two prospective surface drainage techniques that farmers could employ to elevate the crop above the zone of saturation during waterlogging events. Planting in furrows, and subsequently hilling up (HU) to create ridges was unsuccessful in improving plant performance (as the base of the plant was not elevated). Planting on 25 cm high ridges formed by a plow dramatically improved growth and yield of mungbean ( ≥ 360% advantage compared with the other treatments) when subjected to a range of waterlogging stress events. Standing water occurred for 5–7 days on the soil surface of HU, broadcast seeded (B), and drilled (D) treatments, but was 56 6 cbelow the base of the plants in the ridge treatment during the two flooding events. The ridging method was observed to be effective for farm-scale use in cultivating pre-rice mungbeans with either animal or tractor power.

An Analytical Solution of the Boussinesq Equation for Subsurface Drainage in the Presence of Evapotranspiration

A linearized form of the Boussinesq equation was solved analytically to predict water table fluctuations in subsurface-drained farmlands in the presence of evapotranspiration. It was assumed that the evapotranspiration rate decreases linearly with a decrease in water table height above the drains. The analytical solution was verified with the existing analytical and numerical solutions. There was a close match between the solutions. A few numerical examples are given to highlight the characteristics of the various parameters used in the analytical solution and to justify the inclusion of evapotranspiration into drainage design procedures for semi-arid regions. It is shown that if field parameters, representative of a semi-arid region, are used, the drain spacing can be increased by 18 to 25% when evapotranspiration is taken into account.

Drain pipe performance determined using a non-Darcy finite element model

The saturated flow towards drain pipes wrapped with voluminous envelope was investigated by means of a two-dimensional finite element model for non-Darcy flow. special attention was given to the flow in the immediate vicinity of the drain pipe which may cause the movement of soil particles and, as a result, clogging of the envelope. Non-linear elements were used for a better representation of the geometry around the drain and for more accurate prediction of hydraulic heads owing to nonlinear interpolation. Results showed the influence of non-Darcy flow on the head losses near the drain and on the water table heights for variable envelope permeability.

Plant transport and methane production as controls on methane flux from arctic wet meadow tundra

The roles of plant transport and CH4 production in controlling CH4 flux from wet meadow tundra communities were investigated. Plant transport was the dominant pathway of CH4 flux from this ecosystem. Most CH4 production (measured within situ anaerobic incubations) occurred well below the water table, and C supply (estimated by anaerobic CO2 production) was the best single predictor of CH4 production rates. Plant transport of CH4 was controlled both by CH4 supply and the plant species.Eriophorum angustifolium transported substantially more CH4 than didCarex aquatilis, due to differences in size and structure of the two species. The composition of the plant community was a greater control on CH4 flux from the site than either water table height (which varied only slightly) or CH4 production rates, indicating the importance of species-specific plant dynamics in controlling CH4 flux from arctic wetlands.

The effect of standing water above drains on the water table height midway between drains

Wesseling (1964) stated that standing water above drains as a result of submerged outlets creates a radial flow in the vicinity of the drains which promotes flow conditions so that a smaller rise of the water table height midway between drains results. Wesseling (1979) concluded the same for standing water above drains as the result of too high entrance resistance. Standing water above drains may also be due to overpressure in the drains as a result of too small pipe diameter or to irregular drain slopes. With the exception of submerged outlets the resulting water table rise midway between drains is however in the same order of the water table rise above the drain as can be derived from theoretical analysis. This conclusion was confirmed by measurements at an experimental field where the standing water above drains, as a result of overpressure, and the water table midway between drains were monitored in a field located at the northwest of the Nile Delta. In spite of the low discharge rates, overpressure was observed in the drains. It was mainly attributed to irregular drain slopes. The analysis of field data showed that the water table midway between drains rises at least the same as the water table height above the drains. Since overpressure in drains causes a decrease of the dewatering zone, a careful and accurate installation is of utmost importance for the proper functioning of a drainage system.

Climatic change and the release of immobilized nutrients from Welsh riparian wetland soils

The reduction in wetland water table height that could be anticipated from current climate change models was simulated within the laboratory using cores of peat-soil from a riparian wetland. The manipulation increased the rate of release of many solutes including nitrate (1250%), sulphate (116%), dissolved organic carbon (37%), sodium (66%), chloride (65%), iron (168%) and magnesium (16%). Calcium was the only solute to show a lower rate of release following the simulation (— 26%). These changes have major implications for the use of constructed wetlands in ameliorating water quality. The study suggests that without suitable design safeguards, wetlands may only represent a temporary solution to water quality problems. In the future, climatic change could reverse their beneficial effects.

Application of DRAINMOD under semi-arid conditions

The water table management model, DRAINMOD, was evaluated for possible applications under semi-arid climatic conditions by comparing the predicted midspan water table heights with the measured ones in four subsurface-drained test plots in Kota, India under the RAJAD (RAJasthan Agricultural Drainage) Research project. Though the measurements were also made on the subsurface drain outflows, however, they could not be considered in this study due to the submerged outlets during most part of this study. In general, a good agreement was found between the measured data and the predicted values. The average absolute deviations between the measured and predicted midspan water table heights ranged from 14 to 24 cm for the four test plots. The corresponding standard errors of estimate ranged from 15 to 30 cm. The above range of values compares favourably with the other validation studies that have been conducted to evaluate DRAINMOD and other mathematical models for the humid regions of North America. Based on our investigations, it appears that DRAINMOD can be used to design or evaluate subsurface drainage systems under the semi-arid climatic conditions. However, further evaluations are warranted before any concrete conclusions could be drawn.

Analytical solutions of unsteady drainage problems for soils subjected to variable recharge from a semi-confined aquifer

In drainage of agricultural lands, the upward vertical recharge from a semi-confined aquifer depends on the difference of the piezometric heads on the two sides of the semi-impermeable layer through which this recharge takes place. This means that the recharge through the semi-impermeable base depends on the unknown height of the unsteady water table. In the nonhomogeneous Boussinesq equation, which describes the drainage problems, the downward uniform rate of the recharge from rain or irrigation and the recharge from the semiconfined aquifer are expressed by two terms. By solving the Boussinesq equation expressions for the nondimensional height of the water table and the nondimensional discharge of the drains per unit drained area are obtained for three different initial conditions. Some known solutions are shown as special cases of the present solutions. Variation of nondimensional water table heights at half distance of the drain spacing and the nondimensional discharge of the drains with nondimensional time have been graphically illustrated with the help of synthetic examples.

Fluxes of CO2, CH4 and N2O from a Welsh peatland following simulation of water table draw-down: Potential feedback to climatic change

A potential effect of climatic change was simulated by manipulating the water table height within intact peat monoliths. The treatment decreased methane flux (maximum −80%) and increased both carbon dioxide flux (maximum 146%) and nitrous oxide flux maximum 936%). Returning the water table height to its original level caused both nitrous oxide and carbon dioxide flux to rapidly return to control levels. However, methane flux remained at its experimentally induced low levels.

Impacts of Climatic Change On Peatland Hydrochemistry; A Laboratory-Based Experiment

Abstract A laboratory simulation of a reduction in water table height that could be anticipated from current climate change models, resulted in a change in the efficiency with which a valley-bottom wetland acted as a sink/source of nutrients. Effects were confined to the upper 10 cm of the profile, but since this depth has the greatest hydraulic conductivity, it was noted that the effects of hydrochemical changes therein would be readily transferred to receiving waterbodies. Marked changes in leachate chemistry were observed, including increases in nitrate and sulphate concentrations, while concentrations of phenolics, dissolved organic carbon, potassium, iron and ammonium decreased. These changes have implications for the quality and productivity of waters draining wetlands.

Leakage of dense, nonaqueous phase liquids from waste impoundments constructed in fractured rock and clay: theory and case history

This paper examines the behaviour of dense, nonaqueous phase liquids (DNAPLs) in fractured media, with an emphasis on waste-disposal ponds constructed in fractured clay and rock. Calculations are presented to estimate the height of DNAPL that may accumulate at the base of a disposal pond prior to initial entry into a water-saturated fracture. This height is found to be a function of the fluid densities, the DNAPL–water interfacial tension, the fracture aperture, and the position of the water table. A numerical model is applied to estimate the steady-state rate of DNAPL leakage from a disposal pond underlain by vertical fractures. This rate of leakage is found to be a function of the spacing of fractures, the fracture aperture, the DNAPL density, and the height of the water table in the formation outside of the impoundment. It is demonstrated that a wide range of leakage rates can occur over a relatively narrow range of parameters. A conceptual analysis of two-phase flow examines the conditions that lead to both uniform and sparse DNAPL migration pathways beneath a disposal pond in fracture networks. A case history is presented as field evidence of the concepts discussed. In particular, the case history demonstrates that sparse DNAPL migration pathways can occur in fractured rock, and that relatively low dissolved phase concentrations can exist in the immediate vicinity of DNAPL source zones. Key words : nonaqueous phase liquids, ground water, fractures, disposal ponds.

Modelling water-table movement in drained soils with depth-dependent hydraulic conductivity

A model is presented for the prediction of moving water-table heights in drained soils in which the hydraulic conductivity decreases with depth. The model assumes that the non-steady state water tables approximate to a succession of steady state ones that can be calculated from the known hydraulic conductivity variation, the specific yield and rainfall data. Predicted water tables were in agreement with dipwell measurements on an experimental site on clay soil.

Effect of radial soil heterogeneity around a subsurface drain on the water table height computed using a finite element model

The problem of two-dimensional saturated flow towards a subsurface drain wrapped with a voluminous envelope, was solved by a finite element approach. Special attention was given to water flow in the vicinity of the pipe and to the permeability pattern of the envelope and the surrounding soil. Therefore nonlinear elements were used for a better representation of the geometry around the drain, and for more accurate predictions of hydraulic heads due to nonlinear interpolation. Soil heterogeneity around the drain detected and quantified using data from X-ray computed tomography (CT) images and textures analyses, was expressed as gradually varying hydraulic conductivity in the radial direction. Results showed the influence of the heterogeneous soil zone as well as clogging of the envelopes on the water table height at the midpoint between the drains. The hydraulic conductivity of the envelope was found to be the most significant factor with respect to the functioning of the drain.