Water Table Management Systems Research Articles

ROLE OFWATER TABLE MANAGEMENT IN REDUCING METRIBUZIN POLLUTION

The role of water table management systems in reducing pollution from agricultural lands was investigated bymeasuring the mobility and degradation of metribuzin (4-amino-6(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one) in a three-year field lysimeter study. Nine large PVC lysimeters, 1 m long 0.45 m diameter, were packed with asandy soil. Three water table management treatments were used, consisting of two subirrigation treatments with constantwater table depths of 0.4 and 0.8 m and a free drainage treatment. Metribuzin was applied in the summer months of 1993to 1995. Soil and water samples were collected at different time intervals after each natural or simulated rainfall event,from the 0.45 and/or 0.85 m depth below the soil surface. The results from the three sampling seasons showed that themetribuzin concentration reduced with soil depth and time in all treatments. But, comparison of mass balance ofmetribuzin levels in the three treatments showed a significant reduction in the 0.4 m subirrigation treatment. Theshallower water table in the 0.4 m subirrigation treatment seems to have provided more favorable moisture conditions inthe crop root zone; thus enabling the soil biomass to degrade metribuzin faster.

Artificial Neural Network Model for Subsurface-Drained Farmlands

This paper describes the development of an artificial neural network (ANN) model to simulate fluctuations in midspan water-table depths and drain outflows, as influenced by daily rainfall and potential evapotranspiration rates. Unlike conventional models, ANN models do not require explicit relationships between inputs and outputs. Instead, ANNs map the implicit relationship between inputs and outputs through training by field observations. Compared with conventional models, the ANN model requires fewer input parameters since the inputs that remain constant are not considered by ANNs. Therefore ANNs can be executed quickly on a microcomputer. These benefits can be exploited in the real-time control of water-table management systems. The model was developed using field observations of water-table depths from 1991 to 1993 and drain outflows from 1991 to 1994 made at an agricultural field in Ottawa, Canada. The root mean squared errors and standard deviation of errors of simulated results were found to range from 46.5 to 161.1 mm and 46.6 to 139.2 mm, respectively, thus showing potential applications of ANNs in land drainage engineering.

APPLICATION OF PESTFADE TO SIMULATE SALT MOVEMENT IN SOILS

This paper describes the application of a new solute transport model called PESTFADE (PESTicide Fate And Dynamics in the Environment) for simulating salt accumulation and leaching in soils. PESTFADE is a one-dimensional, transient, numerical model which can simulate water and solute transport in unsaturated, homogeneous soil systems. Also, the model considers the effects of dispersion, adsorption and macropore flow on solute movement. The numerical solution was verified with an analytical solution for a simplified case, and the model was validated using data from a soil column study involving irrigation with different amounts of total dissolved salts. Results from both the verification/validation runs indicate that PESTFADE can simulate salt movement in soils with good accuracy. Since the model can be run for different salinity, irrigation water management and drainage scenarios, it can provide information on appropriate water table management systems which can minimize salinity problems in arid and semi-arid regions.

LINKFLOW, A WATER FLOW COMPUTER MODEL FOR WATER TABLE MANAGEMENT: PART 2. MODEL VERIFICATION

A computer simulation model, LINKFLOW, was developed to compute the movement of water during various water table management practices. The development of the model and validation with field data has been covered in related articles (Havard et al., 1995a,b). This article describes the use of the model to investigate the effects of anisotropy and clay lenses on water table elevations during water table management. The first application considers subirrigation and drainage in an anisotropic soil, and the second describes effect of clay lenses on subirrigation and drainage system performance. The results of simulations suggest that water table management system performance can be significantly affected by soil anisotropy. Case simulations for subirrigation and drainage showed 18% and 25% reductions in water movement into and out of the drains respectively when the anisotropy of the soil increased from one to five. Further simulations showed the effect of clay lenses in the soil profile and how backfilling during installation would benefit subirrigation and drainage system performance. Backfilling around the drains for a soil with low permeability lenses showed 70% increase in water flow after 21 days of subirrigation.

LINKFLOW, A WATER FLOW COMPUTER MODEL FOR WATER TABLE MANAGEMENT: PART 3. MODEL APPLICATION

A computer simulation model, LINKFLOW, was developed to compute the movement of water during variouswater table management practices. The development of the model and validation with field data has been covered inrelated articles (Havard et al., 1995a,b). This article describes the use of the model to investigate the effects of anisotropyand clay lenses on water table elevations during water table management. The first application considers subirrigationand drainage in an anisotropic soil, and the second describes effect of clay lenses on subirrigation and drainage systemperformance. The results of simulations suggest that water table management system performance can be significantlyaffected by soil anisotropy. Case simulations for subirrigation and drainage showed 18% and 25% reductions in watermovement into and out of the drains respectively when the anisotropy of the soil increased from one to five. Furthersimulations showed the effect of clay lenses in the soil profile and how backfilling during installation would benefitsubirrigation and drainage system performance. Backfilling around the drains for a soil with low permeability lensesshowed 70% increase in water flow after 21 days of subirrigation.

Economical design of water table management systems in humid areas

A PC-based computer software package was developed for designing water table management systems in humid regions. The package employs DRAINMOD, a computer simulation model, to estimate the average crop yields that would result from various water table management systems. Next, the package estimates the yearly cost of agricultural production with the help of a comprehensive cost calculation module. The net benefits that would result from different water table management systems are thus calculated. The package can, therefore, be used to optimize the design of water table management systems in humid regions. The operation of the package is described with the help of an example case study of a corn field near Montreal, Quebec, Canada.

Drainage and Irrigation Effects on Cotton Production

Excessively wet and dry soil conditions can occur during the same year in the southeastern Coastal Plain of the United States. Water management systems that provide both subsurface drainage during wet soil conditions and irrigation during dry soil conditions are desired. Several water table management alternatives, possibly with surface irrigation such as microirrigation, could satisfy these needs. Three water table management (WTM) systems and microirrigation were evaluated for three cotton cultivars on a southeastern Coastal Plain soil during 1987-1989. The WTM systems included controlled drainage-subirrigation (CDSI), controlled drainage (CD), and subsurface drainage (SSD). All WTM systems had both surface microirrigation and rainfed treatments. Cotton cultivars were Coker 315, DPL 50, and DPL 90. Seasonal rainfall, subirrigation, and microirrigation amounts varied considerably during the three-year period. Water requirements for subirrigation in the CDSI system were high (1477 to 2841 mm), but neither microirrigation nor subirrigation water requirements were closely related to seasonal rainfall amounts. Cotton lint yields among WTM systems were significantly different in two of three years; yields for the CDSI system were lowest (836 and 766 kg/ha) and yields for CD and SSD were highest (1022 and 942 kg/ha, respectively). Wetter-than-optimum soil conditions in all irrigated treatments, especially in combination with the CDSI system, probably caused the reduced yield. Microirrigation produced significantly greater lint yields than the rainfed treatments in the first two years of the study (1127 and 1116 kg/ha versus 492 and 801 kg/ha), but not in the last year (872 versus 874 kg/ha) when seasonal rainfall was least of the three years but was better distributed. There were significant yield differences among cotton cultivars in two years, but no cultivar consistently produced the greatest or least yield. Cotton yield increases obtained with these WTM system-microirrigation combinations suggest the need to control the water table closer to the soil surface in southeastern Coastal Plain soils when surface irrigation is not used. The CDSI could provide a profitable management alternative if a water table fluctuates near the soil surface much of the time, especially during the growing season. Where subsurface drainage is needed part of the year, it may be more profitable to use CD or SSD systems with surface irrigation, especially when maintaining the water table near the soil surface in CDSI systems requires a large water volume. However, the combined cost of the subsurface drainage and microirrigation systems would be very high and might not be profitable for crops such as cotton.

EVALUATION OF DRAINMOD FOR SOUTHERN ONTARIO CONDITIONS

DRAINMOD, a well-known water table management model, was evaluated with field measurements made in a subsurface-drained corn field in southern Ontario, Canada. Four test plots were monitored from July to October 1991 and from May to September 1992 for midspan water table levels and drain outflows under two water table management practices, i.e. subsurface drainage and subirrigation in 1991 with two replicates each and subsurface drainage and controlled drainage in 1992, also with two replicates each, under conventional (fall mouldboard plough) tillage treatment.The average absolute deviations between the average measured midspan water table depths and the predicted values ranged from 24 to 100 mm for conventional drainage, and 82 to 353 mm for controlled drainage/subirrigation. The corresponding standard errors ranged from 32 to 118 mm for conventionally drained and 98 to 435 mm for controlled drained/subirrigated plots. The average absolute deviation between the measured and predicted drain outflows and the corresponding standard errors ranged from 0.28 to 0.36 mm/d and 0.76 to 0.90 mm/d, respectively, for the conventionally drained plots.The above values for the statistical parameters are comparable to the values obtained elsewhere in similar studies. From the two years of evaluations, therefore, it appears that DRAINMOD holds a lot of promise for designing and/or evaluating water table management systems in southern Ontario. However, the model should be evaluated at other sites in southern Ontario before any concrete conclusions are drawn.

Evaluation of the Hydrologic Component of the ADAPT Water Table Management Model

A subsurface, water table management model ADAPT (Agricultural Drainage and Pesticide Transport) has been developed by combining drainage and subirrigation algorithms from DRAINMOD with the GLEAMS model. In addition, the model incorporates improved snow melt and runoff algorithms, macropore flow due to cracking, and deep seepage. Theory for the hydrologic components of the model is presented together with an evaluation of the model using data from a long-term field experiment at Castalia in North Central Ohio. Predicted surface runoff, subsurface drainage, and combined surface and subsurface drainage are compared with the field observations. In general, the model predictions are within the range of the variations of the observed replications. Sensitivity analysis shows that surface runoff estimates are sensitive to changes in curve number, while subsurface drainage flows are sensitive to deep seepage estimates. Model input requirements are not excessive and the model gives reasonable estimates of the hydrologic component of water table management systems. ADAPT can be used in designing water table management systems and does not require extensive calibration. The pesticide component of ADAPT is currently being evaluated and development of a nutrient component has been initiated.

Water Table Management Practice Effects on Water Quality

Impacts of water table management (WTM) practices on water quality were modeled using a linked version of CREAMS and DRAINMOD (Parsons and Skaggs, 1988). The CREAMS denitrification component and the linked DRAINMOD-CREAMS model were modified to simulate daily hydrology (runoff, infiltration, evaporation, and soil moisture content), erosion, and nutrient processes for different WTM conditions. Measured data from Baton Rouge, Louisiana, were used to validate the linked model, and then controlled drainage-subirrigation (CD-SI) was simulated to investigate the effects of different WTM systems on runoff, erosion, and nitrogen losses.

Hydraulic Capacity of Control Devices for Water Table Management Systems

In the design of water table management systems, the hydraulic capacity of the drain tubes and the outlet are usually assumed to be adequate to carry drainage and runoff from the fields and, thus, outlet limitations are not addressed. Unfortunately, some of the devices developed for closed control systems may sacrifice a portion of the outflow pipe area in the control mechanism and restrict the outflow rate. Three commercial outflow control devices were evaluated for outflow rate and were compared to a hypothetical flashboard riser system. Each of the devices restricts the flow by about 70% as compared to the flashboard riser when the head is about 0.15 m (0.5 ft). However, at an area of about 0.45 ha (1.11 ac), the flow capacity of the 15 cm (6 in.) tubing will begin to limit the mainline outflow rates for the site and system conditions in the analysis. With proper design, currently-available, commercial flow-control devices will be of sufficient capacity and will not be a limiting factor in determining drainage system performance.

Design Guidelines for Water Table Management Systems on Coastal Plain Soils

Design Guidelines for Water Table Management Systems on Coastal Plain Soils

Sump-Controlled Water Table Management Predicted with DRAINMOD

ABSTRACT Fluctuations in depth of the water table midway between subsurface-drainage/subirrigation conduits in a sump-controlled water table management system were predicted within an average deviation of 8 cm in DRAINMOD simulations conducted for a Commerce silt loam soil in the Mississippi Delta. The average daily sump water level from a field experiment was used as a model input to establish changes in the drainage outlet water level boundary conditions.

Weather Forecasts as a Control Input for Water Table Management in Coastal Areas

ABSTRACT FOUR water table management methods were compared in computer simulations of a controlled-drainage and subirrigation system using soils and climate conditions of the Mississippi Delta region. No significant difference in the predicted relative corn yield was obtained among the four management methods when the operation for each method was adjusted to maximize yield. However, stopping subirrigation pumping whenever the rainfall probability index (RPI) exceeded 55% reduced pumping requirements for three of the methods 12 to 21% compared to the continuous subirrigation method. The optimal management of soil-water for agricultural cropland in humid, coastal areas of the U.S. often involves complex daily control of the water table because of the erratic spatial and temporal distribution of rainfall. Periods of excess and deficit soil-water conditions are common within the same growing season. Thus, water management requires both controllable drainage and irrigation facilities. A water table management system popular in the humid region, especially in the southern coastal plains, uses a subsurface draintube system for both controlled-drainage and subirrigation. Controlled-drainage permits storage of rainfall and irrigation water in the soil profile below the depth of an overflow pipe at the drain outlet. Free'' drainage to the full depth of the drainline is often needed during periods of extended or heavy rainfall to reduce the duration of excess water in root zone. During subirrigation, the water level at the drain outlet is maintained just below the overflow pipe by pumping from an external source. A sump-type structure for controlling the water level at the drain outlet for a water table management system is illustrated in Fig. 1. The check-valve, activated by a double-float, allows fast subsurface drainage during peak flow events, functioning like the two-stage weir described by Fouss, et al. (1987b). It is often necessary to pump from a sump in level and low lying topography of the coastal plains, where subsurface drainage by gravity flow is not possible. Water table management with this type of system has a high potential for achieving maximum crop production and water use efficiency if properly controlled to compensate for changes in weather conditions. The timing of changes needed in controlled-drainage and subirrigation to manage optimally the water table depth is a major problem for farmers, especially in coastal areas with fine textured soils. In the Mississippi Delta frequent occurrences of rainfall can cause large variations in water table depth because of the small, 3 to 8%, drainable soil porosity. Results from computer simulation of four management methods for water table control are presented for soils and climate conditions of the Mississippi Delta. The primary research objective was to determine the best operational parameters that efficiently utilized rainfall during the growing season without creating periods of severe excess soil-water conditions, and which minimized the need for pumping drainage and subirrigation water. Special attention was given to the potential use of rainfall probability information from the daily weather forecast as a system control input to minimize subirrigation pumping.