Drainage of Sloping Lands with Variable Recharge Model Validation and Applications

Semianalytical transient equations describing the behavior of water tables in subsurface drained soils when drains rest on a sloping impervious barrier have been derived previously and represent the bases of the SIDRA model. To validate (SIDRA), water table elevations and drainflow rates have been monitored for six years in the French Alps on fields with a slope of 8%. The predicted drain flow rates and water table shapes compare reasonably well with data of a drainage experiment site but the improvement provided by taking the slope into consideration was limited. Running the model with different slopes confirmed that high water table elevations and peak flows were not significantly changed with a slope of 8%. As could be predicted from the analysis in steady state, low water table elevations were the most affected by slope. With the soil parameters of the field experiment and from an analysis in tail recession conditions, it was shown that there is a clear threshold of 12% slope below which the slope has no significant influence and can be neglected in drainage design.     

Potential Impact of Climate Change on Subsurface Drainage in Iowa’s Subsurface Drained Landscapes

The study presents hydrologic simulations assessing the potential impact of climate change on subsurface drainage and its pattern in Iowa’s subsurface drained landscapes. The contemporary (representing the decade of 1990s) and future (representing the decade of 2040s) climatic scenarios were generated by downscaling the projections of global climatic model HadCM through two regional climatic models RegCM2 and HIRHAM to a regional grid box of 52–55?km2, which contains Perry, IA. These climatic scenarios were used to drive the field scale deterministic hydrologic model DRAINMOD to simulate subsurface drainage from one of Iowa’s predominant hydric soils, WEBSter, cultivated with Continuous Corn (WEBS_CC), and equipped with a conventional drainage system (30-m drain spacing at 1.2-m drain depth). The simulation results consistently indicate an increase in subsurface drainage from WEBS_CC under future climatic scenario as compared to contemporary climatic scenario. This increase in subsurface drainage would be more in the winter months (from December to March) and early spring months (from April to May) than summer and fall months. Since subsurface drainage is a primary carrier of nitrate-nitrogen (NO3–N) from the agricultural lands, the extrapolation of this study simulations suggest that there would be a potential for increased NO3–N loss from Iowa’s subsurface drained landscapes under future (in the decade of 2040s) climatic conditions.     

Simultaneous Prediction of Saturated Hydraulic Conductivity and Drainable Porosity Using the Inverse Problem Technique

The saturated hydraulic conductivity K and the effective porosity f are two important input parameters needed for lateral drain spacing design, as well as some other applications. The technical and economic justification, of most drainage projects, is mainly connected to these two parameters. The current design procedure is based upon calculation of the lateral spacing, using some average values of K and f within the drainage area. The objectives of this study were to introduce a new method for simultaneous estimation of K and f parameters using the inverse problem technique, and to evaluate five different unsteady drainage analytical models of the Boussinesq equation, suggested by different researchers for simultaneous prediction of the parameters. Consequently, five different analytical models for predicting water table profiles were solved, using the inverse problem technique. Each model was then evaluated. A physical drainage model of 2.2?m length, 0.3?m width, and 0.5?m height was established in the laboratory and carefully packed with a sandy loam soil. A perforated drainage pipe of 4.5?cm in diameter was installed at the bottom end of the model. Many piezometers were inserted in the soil for spatial and temporal water table monitoring. Different data sets from the experiments and literature were used for model calibration. The newly proposed approach that is based upon measuring water table profiles, at different times, was then evaluated with both constant and variable f. The predicted values of the proposed approach indicated reasonable agreement with the measured data. With variable effective porosity, the method was even more accurate to predict the water table profiles. Using the inverse problem technique, all the analytical models provided good agreement with the measured data. Among these, however, the Topp and Moody model predicted more accurate results than other models.     

Drainage of Sloping Lands with Variable Recharge: Analytical Formulas and Model Development

Semianalytical transient equations for shallow subsurface transverse drainage systems installed in sloping lands are developed. They provide a general relationship between drain flow rates, water table elevations, and recharge rates. This relationship demonstrates that, depending on the recharge intensity, several drain flow rates can be observed at a given water table elevation. The recharge contribution is shown to depend on a water table shape factor and to decrease when the water table is low or the slope is steep. For very steep slopes, the recharge intensity no longer influences the drain flow rate. These equations can be used to confirm previous results obtained in steady-state conditions and to determine precisely under which conditions slope needs to be considered in drainage design. They have been incorporated into the field drainage model SIDRA, which simulates hourly values of water table elevations and drain flow rates. The model predictions are compared with the predictions of a steady-state equation and a numerical model, which solves the Boussinesq equation (SLOP model).     

Design and Management of Subsurface Horizontal Drainage to Reduce Salt Loads

Many irrigated areas have shallow water tables creating waterlogging and salinization problems. This has often been controlled by installation of subsurface horizontal pipe drainage; however, these systems export large amounts of salt off farm in the drainage effluent. Improved design and management of subsurface drainage systems to reduce drainage salt loads were tested in a replicated field experiment. Deep, widely spaced drains allowed to flow without control were compared to drains with management to reduce drain flow. These were also compared with shallow, closely spaced drains that protected the root zone only and an undrained control. The deep drains flowed continuously during the two irrigation seasons with an electrical conductivity of around 11 dS∕m resulting in a drainage salt load of 5,867 kg∕ha. The management measures reduced drainage volume and salinity resulting in a 50% reduction in salt load. The shallow drains only flowed directly after an irrigation or rainfall event with low salinity, around 2 dS∕m, resulting in a 95% reduction in salt load. This showed that by management there is great potential for reducing salt mobilization in existing drainage systems, and for new systems shallower drains will minimize salt loads.     

2. Numerical Simulations of Water Flow and Solute Transport Applied to Acid Sulfate Soils

Field investigations of Rassam et al. in 2001 have highlighted the effects of infiltration, drainage, and evapotranspiration on the dynamics of water flow and solute transport in acid sulfate (AS) soils. In this work, HYDRUS-2D is adopted as the modeling tool to elucidate the trends observed in that field experiment. Hypothetical simulations have shown that the relative contribution of drains to lowering the water table is significant only when closely spaced drains are installed in coarse textured soils, evapotranspiration being the main driving force in all other cases. AS soils reaction products that are close to a drain are readily transportable during infiltration and early drainage, but those produced farther away from it near the midpoint between drains are only slowly transported during a prolonged drainage process. Simulating the field trial of Rassam et al. has shown that drain depth and evapotranspiration significantly affect solute fluxes exported to the ecosystem. Managing AS soils should target minimal drain depth and density. Partial or full lining of the drains should be considered as a management option for ameliorating the environmental hazards of AS soils.     

Functional Relationship to Describe Drains with Entrance Resistance

Numerical flow models usually represent drains as a system dependent boundary condition. If soil is saturated, drains act as the Dirichlet boundary condition with pressure head set equal to zero, and if soil is unsaturated, drains act as the Neumann boundary condition with flow set equal to zero. The underlying assumption is that drains exhibit ideal behavior. In reality, however, this is generally not so, and the flow encounters additional resistances due to pipe slotting and clogging of the envelope material around the drains. To account for the resulting resistance, a Hooghoudt-type boundary condition was developed that prescribes drain flow in relation to the groundwater level at a reference position. The measured drain discharge in an old drainage system was compared with calculated discharge assuming an ideal drain. It was found that the ideal drain assumption led to large errors in simulated discharge. With a correctly formulated and calibrated Hooghoudt boundary condition, however, more accurate drain discharges were obtained.     

自来水厂输配水管线工程降水施工技术

在管沟土方施工及管线安装工程中,经常遇到地下水位高或附近有水源的情况,此时需要先进行降水或排水,使水位降到沟槽底部以下,以满足管沟土方施工及管线安装工程的需要,常用的排水、降水方法明沟排水和轻型井点降水 .     

Water Table Fluctuation in the Presence of a Time-Varying Exponential Recharge and Depth-Dependent ET in a Two-Dimensional Aquifer System with an Inclined Base

An analytical solution is presented for water table fluctuation between ditch drains in presence of exponential recharge and depth-dependent evapotranspiration (ET) from groundwater table in a two-dimensional gently sloping aquifer. The groundwater head above the drain is small compared to the saturated thickness of the aquifer. A sound mathematical transformation is devised to transform the two-dimensional groundwater flow equation into a simple form, which makes possible to obtain an analytical solution. The transient midpoint water table variations from the proposed solution compare well with the already existing solutions for horizontal aquifer. A numerical example is used to illustrate the combined effect of depth-dependent ET coupled with a time-varying exponential recharge on the water table fluctuation. The inclusion of a depth-dependent ET in the solution results in water table decline at a faster rate as compared to the case when ET is not considered. With an increase in slope of the aquifer base, water table profiles become asymmetric and the water table divide shifts towards the lower drain. The height of the water table profiles increases on moving away from the boundary of the aquifer and the highest level of the ground water table is obtained in the central portion of the aquifer basin due to the presence of drainage ditches on the aquifer boundary. When the effect of ET is incorporated in combination with recharge, the analytical solution results in accurate and reliable estimates of water table fluctuations under situations subjected to a number of controlling factors. This study will be useful for alleviation of drainage problems of the aquifers receiving surface recharge and surrounded by streams.     

Economics of Nitrate Losses from Drained Agricultural Land

Some of the highest losses of nitrate to surface waters come from drained agricultural land. This research studied, for Belgian farming conditions, (i) the effect of subsurface drainage density on nitrate losses and (ii) the economics of nitrate losses, using the nitrogen version of the program DRAINMOD-N. DRAINMOD was used to simulate the performance of the drainage system of the Hooibeekhoeve experiment, situated in the sandy region of the Kempen (Belgium) for a 14-year (1985–1998) period. A continuous cropping with maize was assumed. Daily NO3-N losses were predicted for a range of drain spacings and depths, two drainage strategies (conventional and controlled), and three fertilizer application rates (225, 275, and 325 kg?N?ha?1). Losses of N in subsurface drainage were assumed to occur almost entirely in the NO3-N form. Losses of organic and inorganic N in the form of NO3-N in surface runoff are small and were neglected. Hydrologic results indicated that increasing drain spacing or decreasing drain depth reduces drainage discharge while it increases runoff. The use of controlled drainage reduces subsurface drainage and increases runoff. Results also revealed that increasing the drain spacing or decreasing the drain depth reduces nitrate-nitrogen (NO3-N) drainage losses and net mineralization, while increasing denitrification and runoff losses. Controlled drainage caused a predicted reduction in drainage losses and an increase in denitrification and runoff losses. The optimal combination of drain density and management is one that maximizes profits and minimizes environmental impacts. Simulated results indicated that NO3-N losses to the environment could be substantially reduced by reducing the drainage density below the level required for maximum profits based on grain sales. The study concluded that, if the environmental objective is of importance equal to or greater than profits, drainage systems can be designed and managed to reduce NO3-N losses while still providing an acceptable profit.     

Applying MODFLOW Model for Drainage Problem Solution: A Case Study from Jahir Irrigated Fields, Israel

High water table and soil salinization processes are common in irrigated fields in Israel. Subsurface drainage systems are a common technique to solve soil salinity problems. Subsurface drainage models can contribute to the efficiency of the drainage system as it can assist in the selection of the proper drainage system and its proper placement in the field. In this study we used the MODFLOW groundwater flow model to simulate groundwater levels in Jahir irrigated fields, the Jordan Valley, Israel. Using a three-layer groundwater flow model, the most efficient drainage system was found to be a combination of deep drains with relief wells and a pump placed in the area with soil salinity problem and upward hydraulic pressure. It was found that simulated drainage system can yield nearly 200,000?m3 of water per year. Given certain information, a spatially distributed groundwater flow model such as MODFLOW can provide more reliable information than different analytical solutions for planning of an effective subsurface drainage system.     

Modeling Soluble Phosphorus Desorption Kinetics in Tile Drainage

This paper describes a simple model for the desorption and transport of soluble reactive phosphorus (SRP) to subsurface drains. The model assumes first-order kinetically rate-limited desorption in a soil surface mixing layer over a soil profile layer that rests on an underlying, shallow restricting layer. Input data include precipitation, soil hydraulic properties, drain outflow, free water surface fluctuation, sorbed P concentrations for the mixing layer and profile, desorption rate and equilibrium soil-SRP partitioning. Model results are compared to data on flow and SRP concentrations in drain outflow collected during natural rainfall events under field conditions. The concentration time series simulated follow the sharp rise, peak, and gradual recession of the observed field data. Predicted event mass loads resulting from observed and simulated tile discharges differ from the observed load by +8.2% and ?9.7%, respectively. Sensitivity analysis indicate that equilibrium assumptions would not provide satisfactory results and that mass transfer limits SRP release to the tile drain.     

Water Management of Irrigated-Drained Fields in the Jordan Valley South of Lake Kinneret

The Jordan Valley is one of the primary regions for growing winter crops of fruit and vegetables in Israel and Jordan. Control of water management in these fields is obtained by solid-set irrigation systems and subsurface drainage. Detailed field observations were conducted at a location near the Jordan River, south of Lake Kinneret. Water table heights were measured by approximately 100?piezometers. An exiting wide spacing (160?m) subsurface drainage system was monitored and the total drainage discharge from this regional drainage system to Lake Kinneret was measured. Rainfall, irrigation, and evapotranspiration rates were measured and overall hydrological balance was conducted. The old irrigation method in the region was border irrigation with very high leaching fraction and poor irrigation efficiency. During the 1970s the irrigation method was changed to computer operated drip irrigation. The leaching fraction was reduced and irrigation efficiency increased. Reduction of the total drainage discharge to Lake Kinneret by a factor of about 10 was observed. Water table rise under hand moving sprinkler and soil-set drip irrigation methods were measured and compared for assessment of salinization of the root zone by upward movement of groundwater. The result indicates the strong effect of irrigation time interval on the extent of these rises. The effect of irrigation mode on the extent of water table rises was measured at the field by comparing that under hand moving sprinkler irrigation to that under water solid set drip method. This effect depends, among other variables, on the irrigation time interval, a fact which complicates prediction of water table rise under different irrigation practices. These field results support previous theoretical analysis by the writers and highlighted the interrelationship between irrigation practice and drainage design. The effect of water table drawdown towards the Jordan River was monitored and found to be about 4.6%. The strong influence of the Jordan River on water table height at the drained field is magnified by the existence of sandy layers in the soil profile. This observed gradient may be used for the estimation of lateral seepage flow from the irrigated agricultural field towards the adjacent Jordan River. This study provides a useful source of data for future studies in similar situations.     

Describing Near Surface, Transient Flow Processes in Unconfined Aquifers below Irrigated Lands: Model Application in the Western San Joaquin Valley, California

In order to rigorously examine near surface, field to field interactions between irrigation management regimes and a shallow fluctuating water table, an enhanced deforming finite element (DFE) model was recently developed. The enhanced DFE model, through a process of iteration within each time step, avoids making common assumptions regarding the changing geometry of an aquifer free surface. This paper demonstrates the usefulness and effectiveness of the model by employing it to an irrigated region in the western San Joaquin Valley, Calif., where shallow subsurface tile drains have been installed to control shallow water tables. By virtue of the problems created by the need to dispose off the drainage water, this region has been the focus of several important regional scale modeling exercises, which have evaluated the utility of management strategies, such as source control, groundwater pumping, and land retirement. By refining the focus of the analysis, the enhanced DFE model is found to be able to show that both sources control and managed pumping could be more effective drainage control strategies than predicted based on the results of regional models.     

Hydraulic Considerations in Geosynthetic and Aggregate Subsurface Drains

The hydraulic design of certain types of subsurface drains has recently been put on a more rational footing, and deficiencies in earlier design methods have been demonstrated. However, significant limitations remain in hydraulic design methods for geosynthetic and aggregate subsurface drains. It is important to decouple the groundwater hydrology from the internal hydraulics of the drain, and properly design subsurface drains for open-channel rather than pressurized conditions. Present design methods can inadvertently result in pressurized flow. The assumption of uniform flow (Manning’s equation alone) is also improperly made in some present design methods. Consequences can include unintended pressurized flow and attendant nonuniformity of inflow on the one hand and uneconomical design on the other. Current standard guidelines provide relatively little guidance for the design of geosynthetic and aggregate drains. A current ASTM standard, commonly referenced by geosynthetic manufacturers, has significant limitations. Deficiencies and qualifications are identified for present design methods. Guidance is given for the improved design of geosynthetic and aggregate subsurface drains based on sound hydraulic principles.     

Soil Hydraulic Properties Affecting Discharge Uniformity of Gravity-Fed Subsurface Drip Irrigation Systems

The use of subsurface drip irrigation (SDI) is increasing for many reasons, including its many agronomic advantages and the ability for safe application of wastewater to crops. In contrast to surface drip irrigation, soil hydraulic properties may affect SDI performance, particularly for new SDI systems designed to operate under low pressure (e.g., 2?m of head). This work introduces a new approach for solving problems of predicting discharge in SDI laterals. We accomplish this by coupling models of head loss in laterals and soil impacts on dripper discharge. The coupled model enables an evaluation of the performance of SDI laterals while changing inputs, such as the lateral diameter, length and slope, dripper nominal discharge and exponent, inlet pressure head, soil hydraulic properties, and soil spatial variability. This model is used to determine the coefficient of variation of discharge for two numerical comparisons.     

Analytical and Numerical Modeling of Consolidation by Vertical Drain beneath a Circular Embankment

In the analysis of axisymmetric problems, it is often imperative that aspects of geometry, material properties, and loading characteristics are either maintained as constants or represented by continuous functions in the circumferential direction. In the case of radial consolidation beneath a circular embankment by vertical drains (i.e., circular oil tanks or silos), the discrete system of vertical drains can be substituted by continuous concentric rings of equivalent drain walls. An equivalent value for the coefficient of permeability of the soil is obtained by matching the degree of consolidation of a unit cell model. A rigorous solution to the continuity equation of radial drainage towards cylindrical drain walls is presented and verified by comparing its results with the existing unit cell model. The proposed model is then adopted to analyze the consolidation process by vertical drains at the Sk?-Edeby circular test embankment (Area II). The calculated values of settlement, lateral displacement, and excess pore-water pressure indicate good agreement with the field measurements.     

Generalized Analytical Solutions for Groundwater Head in Inclined Aquifers in the Presence of Subsurface Drains

Analytical solutions for groundwater head in the presence of subsurface drains are important in assessing the effectiveness of an existing drainage system under a probable extreme variation in the rate of recharge and designing a new drainage system. Generalized analytical solutions for groundwater head in inclined aquifers in the presence of parallel subsurface drains are obtained considering the transient rate of recharge as a power series (polynomial) function and depth-dependent rate of evapotranspiration. An appropriate function, new to analytical drainage studies, is used for correctly representing the depth-dependent rate of evapotranspiration. The solutions are obtained considering the practical situation of drains placed at shallow depth in a considerable depth of aquifer. Two conditions of large and small saturated thicknesses in comparison to the increase in groundwater head are considered. A mathematical criterion is proposed to distinguish between large and small saturated thicknesses. The analytical equations for discharge to drains for different cases considered are also obtained. The discharge equations used by prior investigators are found inappropriate.     

Finite-Element Grid Configurations for Drains

Several different finite-element grid configurations were evaluated for use in the numerical approximation of steady and transient flow to a single drain. By comparing the numerically simulated drain flow rates and head distributions with analytic values, a nested configuration was found to be appropriate for an effective drain radius of 0.01 m, and a square configuration was suitable for an effective drain radius of 0.05 m. Using an analytic solution, a method was developed to determine the distance of influence of a drain as a function of its effective radius and the geometry of its flow domain. The distance of influence was found to be independent of material type. An appropriate between-drain grid spacing was selected for the numerical simulation of multiple drains by increasing the grid mesh spacing outside the distance of influence. The position of the water table and drain flow rate with time were used to evaluate the between-drain grid spacing for transient variably saturated flow. Grid Péclet and Courant numbers, together with the shape of the solute advance front, were used to evaluate the suitability of the selected single drain configuration and between-drain grid spacing for solute transport. The resulting finite-element grid configuration for single and multiple drains ensures a stable efficient numerical solution, and it has applicability to numerical modeling of multiple subsurface drains.