Predicting effects of drainage water management in Iowa's subsurface drained landscapes

Long-term hydrologic simulations are presented predicting the effects of drainage water management on subsurface drainage, surface runoff and crop production in Iowa's subsurface drained landscapes. The deterministic hydrologic model, DRAINMOD was used to simulate Webster (fine-loamy, mixed, superactive, mesic) soil in a Continuous Corn rotation (WEBS_CC) with different drain depths from 0.75 to 1.20 m and drain spacing from 10 to 50 m in a combination of free and controlled drainage over a weather record of 60 (1945-2004) years. Shallow drainage is defined as drains installed at a drain depth of 0.75 m, and controlled drainage with a drain depth of 1.20 m restricts flow at the drain outlet to maintain a water table at 0.60 m below surface level during the winter (November-March) and summer (June-August) months. These drainage design and management modifications were evaluated against conventional drainage system installed at a drain depth of 1.20 m with free drainage at the drain outlet. The simulation results indicate the potential of a tradeoff between subsurface drainage and surface runoff as a pathway to remove excess water from the system. While a reduction of subsurface drainage may occur through the use of shallow and controlled drainage, these practices may increase surface runoff in Iowa's subsurface drained landscapes. The simulations also indicate that shallow and controlled drainage might increase the excess water stress on crop production, and thereby result in slightly lower relative yields. Field experiments are needed to examine the pathways of water movement, total water balance, and crop production under shallow and controlled drainage in Iowa's subsurface drained landscapes.     

Verification of drainage design criteria in the Nile Delta,Egypt

A monitoring programme to verify the design criteria of subsurface drainage systems was conducted in a pilot area in the Nile Delta in Egypt. The programme, which covered a 9-year period, included the monitoring of the cropping pattern, crop yield, soil salinity, watertable, discharge and salinity of the drainage water and overpressure in the subsurface drainage system. The results showed that the yield of all crops (wheat, berseem, maize, rice and cotton) increased significantly after the installation of the subsurface drainage system. Optimum growing conditions for the combination of crops that are cultivated in rotation in the area required that the watertable midway between the drains had a average depth of 0.80 m. A corresponding drain discharge of 0.4 mm/d was sufficient to cope with the prevailing percolation losses of irrigation water and to maintain favourable soil-salinity levels. The additional natural drainage rate in the area was estimated at 0.5 mm/d. The most effective way to attain these favourable drainage conditions is to install drains at a depth between 1.20 to 1.40 m. For drain-pipe capacity the Manning equation can be used with a design rate of 1.2 mm/d, for collector drains this rate should be increased to 1.8 mm/d to compensate for the higher discharge rates from rice fields. These rates should be used in combination with a roughness coefficient (n) of 0.028 to take sedimentation and irregularities in the alignment into account. When this value of the roughness coefficient is used, no additional safety has to be incorporated in the other design factors (e.g. the design rate).     

Subsurface drainage to combat waterlogging and salinity in irrigated lands in India: Lessons learned in farmers’ fields

The introduction of irrigated agriculture in the arid and semi-arid regions of India has resulted in the development of the twin problem of waterlogging and soil salinization. It is estimated that nearly 8.4 million ha is affected by soil salinity and alkalinity, of which about 5.5 million ha is also waterlogged. Subsurface drainage is an effective tool to combat this twin problem of waterlogging and salinity and thus to protect capital investment in irrigated agriculture and increase its sustainability. In India, however, subsurface drainage has not been implemented on a large scale, in spite of numerous research activities that proved its potential. To develop strategies to implement subsurface drainage, applied research studies were set-up in five different agro-climatic sub-regions of India. Subsurface drainage systems, consisting of open and pipe drains with drain spacing varying between 45 and 150 m and drain depth between 0.90 and 1.20 m, were installed in farmers’ fields. The agro-climatic and soil conditions determine the most appropriate combination of drain depth and spacing, but the drain depths are considerably shallower than the 1.75 m traditionally recommended for the prevailing conditions in India. Crop yields in the drained fields increased significantly, e.g. rice with 69%, cotton with 64%, sugarcane with 54% and wheat with 136%. These increases were obtained because water table and soil salinity levels were, respectively, 25% and 50% lower than in the non-drained fields. An economic analysis shows that the subsurface drainage systems are highly cost-effective: cost-benefit ratios range from 1.2 to 3.2, internal rates of return from 20 to 58%, and the pay-back periods from 3 to 9 years. Despite these positive results, major challenges remain to introduce subsurface drainage at a larger scale. First of all, farmers, although they clearly see the benefits of drainage, are too poor to pay the full cost of drainage. Next, water users’ organisations, not only for drainage but also for irrigation, are not well established. Subsurface drainage in irrigated areas is a collective activity, thus appropriate institutional arrangements for farmers’ participation and organisation are needed. Thus, to assure that drainage gets the attention it deserves, policies have to be reformulated.     

Analysis of groundwater behaviour in a farmer controlled subsurface tile drainage system in Mardan, Pakistan

Extensive subsurface drainage system was installed in districtMardan in the North West Frontier Provinceof Pakistan in 1987 to control increasingwater logging and salinity problems due tocanal irrigation. Several recentlycompleted fields studies have indicatedthat subsurface drainage system hasenormously lowered watertable in certainareas due to extensive drainage network. Therefore, a study of controlled subsurfacedrainage technique was initiated in MardanScarp area to observe the temporal andspatial variations in water table depths ofthis specific case under various modes ofcanal irrigation and monsoon rains. Twoartificially drained areas, consisting of40 ha and 160 ha respectively, werecontrolled and selected for extensivemonitoring. A total of 98 observationswells (7.6 cm dia. and 4.1 m depth) wereinstalled in between lateral drains toobserve water table fluctuation. Theresults of this study are very interesting.Each of the two areas monitored in thestudy behaved differently. It was observedthat in one of the areas design water tabledepth at 1.1 m was maintained with properfunctioning of the controlled techniqueapplied to the subsurface drainage system. The results from this area showed that 25to 55% of the time throughout the yearachieved this objective whereas in thesecond area desired water table could notbe maintained and water table depth in thisarea remained between 2.0 to 2.7 m causingunnecessary water stress to plants. Alsoit was observed that watertable in theformer area is mostly controlled by thefunctional behavior of the irrigationcanal. In addition, the proper functioningof controlled techniques in subsurfacedrainage system supplemented veryefficiently to retain the groundwater levelto the optimal limits in dry season and tothe design ones in the others for timelyneeds of the crops. Also rainfalls havesignificant impact on the spatial andtemporal behaviors of water table depths inboth the areas during the monsoon season.     

Subsurface drainage of a three layered soil with slowly permeable top layer

Heavy textured soils are known for the difficulties imposed to subsurface drainage. Field studies on heavy textured soils in the irrigation commands of India have shown that such soils have relatively more pervious soil at depths greater than 1 m from the ground surface. Considering this fact, a performance study of subsurface drainage in heavy textured layered soils of Mahi Right Bank Canal Command of Gujarat (India) was taken up by modifying the drainage equation of de Zeeuw and Hellinga [de Zeeuw, J.W., Hellinga, F., 1958. Neerslag en Afvoer, Land bocwkundig Tidschrift 70, 405–421] for layered soils, which predicted the water table fluctuations and drain discharge corresponding to irrigation or rainfall inputs taking into account the stratification of the soil profile. The equation was tested on the field data obtained from a pilot project of the study area. The study showed that the watertable head gets influenced by the location of interface between the soil layers. The predicted results conformed with the field data.     

Optimal sampling density of hydraulic conductivity for subsurface drainage in the Nile delta

The density of hydraulic conductivity measurements is of primary importance for the design of large-scale subsurface drainage projects. This study was conducted to investigate the effect of sampling density on hydraulic conductivity estimation, and to determine the optimal sampling density using results of 3488 hydraulic conductivity tests from a 33 500 ha subsurface drainage project in the Nile Delta of Egypt. Kriging was used to obtain hydraulic conductivity point and block estimates based on the original 300 m sampling grid and on reduced sampling grids with spacings of 600, 900, and 1200 m. Kriged estimates from the complete grid were compared to those from reduced grids. The correlation coefficient between kriged estimates of block hydraulic conductivity based of the 300 m versus the 600 m measurement grid was larger than 0.90, which indicates that the optimal sampling grid spacing is in the order of 600 m. Slopes of the estimation standard deviation versus sampling density showed that the optimal sampling grid spacing was between 400 and 600 m, and could be determined from sampling grids with a spacing of 900 m or less. It appears that, for the Nile Delta, the optimal sampling density of hydraulic conductivity can be determined from a preliminary survey with a grid spacing of 900 m.     

暗管排水条件下微咸水灌溉对土壤盐分动态及夏玉米生长的影响

进行暗管排水条件下微咸水灌溉田间试验,设置3种暗管埋深,分别为80 cm(D1)、120 cm(D2)以及无暗管排水(D0),3种微咸水浓度,其电导率分别为0.78 dS/m(S1),3.75 dS/m(S2)和6.25 dS/m(S3),共9个处理,每个处理3组重复.试验结果表明:暗管排水措施可以有效排除微咸水灌溉过程中土壤中累积的盐分;在玉米全生育期内,暗管埋深D1条件下,3种浓度微咸水S1,S2和S3灌溉时根系土壤电导率分别下降了39.00%,31.56%和29.43%,暗管埋深D2条件下,根系土壤电导率则分别下降了31.91%,18.08%和7.44%;夏玉米干物质累积量、穗棒累积量和穗棒质量分配率及最终产量均随着微咸水浓度的升高而降低;在相同微咸水浓度下,不同暗管埋设条件下的夏玉米最终产量从大到小依次为D1,D2,D0;3种暗管埋设条件下的作物需水量从大到小依次为D0,D2,D1的规律;暗管埋深80 cm的处理(D1)下夏玉米水分利用效率最高,而未埋设暗管的处理(D0)水分利用效率最低;当暗管埋设条件一定时,夏玉米水分利用效率随微咸水浓度的升高呈逐渐降低的趋势.     

Determination of the variation of hydraulic conductivity with depth in drained lands and the design of drainage installations

For lands drained by ditches dug to a horizontal impermeable floor, the variation of the soil's hydraulic conductivity with depth may be obtained from the relationship between water-table height and drain-outflow rate. Some relationships obtained on an experimental plot on a clay soil, drained by tile drains with gravel backfill, and on another in the same field which was mole-drained, were analysed to give the variation of hydraulic conductivity with depth by assuming that their performances approximated to that of ditches. For the tile-drained plot, the hydraulic conductivity value increased by three orders of magnitude near the bottom of the plough layer; this was reduced in a subsequent year when the field was uncultivated under grass with consequent higher water tables. The mole-drained soil was more permeable than the tile-drained soil at a lower depth, and its hydraulic conductivity at this lower depth did not change in the subsequent year when the field was uncultivated. An assumed uniform hydraulic conductivity value, calculated using drainage theory and matching at one water-table height, gave relationships between water-table height and drain outflow which did not agree with observations.A general hydraulic approach to drainage design is suggested whereby the drainage from an investigational area may be used to measure the hydraulic conductivity variation with depth and to design the correct drainage scheme for a predicted stress period of rainfall. Even if the drainage rate from an area is not measured, the water-table recession alone in an area drained by ditches may give sufficient information to design a drainage system on a rational physical basis.     

Subsurface drainage performance study using SALTMOD and ANN models

Relative performance of artificial neural networks (ANNs) and the conceptual model SALTMOD was studied in simulating subsurface drainage effluent and root zone soil salinity in the coastal rice fields of Andhra Pradesh, India. Three ANN models viz. Back Propagation Neural Network (BPNN), General Regression Neural Network (GRNN) and Radial Basis Function Neural Network (RBFNN) were developed for this purpose. Both the ANNs and the SALTMOD were calibrated and validated using the field data of 1998–2001 for 35 and 55 m drain spacing areas. Data on irrigation depth, evapotranspiration, drain discharges, water table depths, mean monthly rainfall and temperature and drainage effluent salinity were used for ANN model training, testing and validation. It was observed that the BPNN model with feed forward learning rule with 6 processing elements in input layer and 1 hidden layer with 12 processing elements performed better than the other ANN models in predicting the root zone soil salinity and drainage effluent salinity. Considering coefficient of determination, model efficiency and variation between the observed and predicted salinity values as the evaluation parameters, the SALTMOD performed better in predicting root zone soil salinity and the BPNN performed better in predicting the drainage effluent salinity. Therefore, it was concluded that the BPNN with feed forward learning algorithm was a better model than SALTMOD in predicting salinity of drainage effluent from salt affected subsurface drained rice fields.     

农田不同排水措施地下排水效果模拟试验研究

在地下水位较高、地表易于形成积水的中国南方地区,通过农田排水措施可以及时排除多余地表积水,快速降低地下水位,以达到排涝降渍、协同调控的目的.文中基于室内砂槽试验,揭示暗管排水、明沟排水、不同反滤体高度的反滤体排水及改进暗管排水等措施的地下排水规律及效果.结果表明:将暗管周围土体置换为高渗透性土体介质的改进暗管排水可明显提高排水流量,当土体置换高度达2 cm时(对应于田间条件40 cm),其排水流量均高于相同埋深条件下的其他排水措施,达暗管排水的1.59~1.66倍;改进暗排在地表积水消失时仍保持较大的排水流量,可达相同埋深暗管流量的2倍以上,在积水层消失后,能迅速降低农田土壤水的渍害胁迫,将地下水位降低至暗管埋设高度;各种排水措施,在地表积水即将消失时,出现了流量与水头变化幅度较大的现象.相对于各种地下排水措施,改进暗管排水在除涝降渍中存在明显优势.研究结果可为涝渍灾害易发地区高效除涝降渍减灾工程设计和建设提供参考.     

Nitrogen loss through subsurface drainage effluent in coastal rice field from India

Experiments were conducted to estimate nitrogen loss through drainage effluent in subsurface drained farmers’ field at a coastal site near Machilipatnam, Andhra Pradesh, India. The concentration of three forms of nitrogen, namely, NH₄–N, NO₂–N and NO₃–N in the subsurface drainage effluent from 15, 35 and 55 m drain spacing areas were measured in 1999 and 2000. The area with 15 m spacing was already reclaimed during 1986–1998 by the subsurface drainage system. The soil salinity of the root zone was brought down from an initial high of 35 to 4 dS m⁻¹. The subsurface drainage system with 35 and 55 m drain spacing was laid in the adjoining area and commissioned in 1998. Earlier raising of any crop in the area with 35 and 55 m spacings was not possible due to very high salinity, sodicity and poor drainage conditions. The nitrate-nitrogen loss dominated in reclaimed land with 15 m spacing whereas ammonium-nitrogen loss dominated in the land that was highly saline and in the initial stage of reclamation by the subsurface drainage technology with 35 and 55 m drain spacing. The total nitrogen loss of 3.75 kg per ha per year in 15 m drain spacing area was minimum and 23.53 kg per ha per year in 35 m drain spacing area was maximum. The nitrate-nitrogen loss contributed the maximum of 82% and ammonium- and nitrite-nitrogen contributed 11 and 7%, respectively, in 15 m drain spacing area whereas the ammonium losses contributed 93 and 82% in 35 and 55 m drain spacing areas, respectively. The losses in the form of nitrite and nitrate remained negligible in 35 m drain spacing area, but the losses to the tune of 8 and 15% in the form of nitrite and nitrate, respectively, occurred in 55 m drain spacing area.     

Watercourse Pollution due to Surface Runoff following Slurry Spreading. Part I: Calibration of the Soil Water Simulation Model SOIL for Fields Prone to Surface Runoff

The Swedish soil water model SOIL has been calibrated for several drained fields in Scotland and Ireland. Drainage efficiency in these fields varies, with inefficient drainage systems leading to saturated profiles and large surface runoff flows. The model has been modified to represent drainflow in typical Scottish and Irish fields in which permeable backfill extending to the surface is present directly above plot drains. When the conductivity of the backfill material is low, surface runoff is shown to be enhanced in specific soil types. Overall, the predictions of the modified model are in reasonably good agreement (as shown by the efficiency factor values) with measured water table levels, drain and surface runoff flows in these fields. These calibrated fields are to be used in subsequent work on pollution from surface runoff following slurry spreading. A useful indicator of potential runoff risk in such systems is the total saturated hydraulic conductivity of the profile, defined here as arising from the combination of the saturated soil and drain conductivities. Fields are classified into high risk if the conductivity of the profile is lower than 6 mm/d, low risk if the conductivity is greater than 18 mm/d, and moderate risk for intermediate conductivities. A sensitivity analysis of the model with regard to drain and surface runoff flows, varying the drain spacing, a backfill resistance term, the soil matrix and macropore saturated hydraulic conductivities, soil porosity and the pore size distribution index, is also presented. This analysis shows that in order of increasing importance, backfill resistance, macropore saturated hydraulic conductivity and drain spacing, have the largest effect on the generation of surface runoff.     

Evaluating Drainage Design Parameters for the Fourth Drainage Project, Pakistan by using SWAP Model: Part II – Modeling Results

This paper presents the results of modelsimulations to evaluate drainage designparameters for the Fourth Drainage Project(FDP), Punjab, Pakistan. The SWAP model wasapplied to compute the effects of landdrainage (12 combinations of drain depthand spacing) on soil moisture conditions inthe root zone and their effect on cropyield and soil salinization. For theconditions considered, the selection ofdrain depth is found to be more criticalthan that of drain spacing. Deeper drainsperform technically better in relation tocrop growth and soil salinization. Theoptimum drain depth for the multiplecropping system of the FDP-area was foundto be 2.2 m. This drain depth will producereasonably good crop yields at rather lowdrainage intensity while keeping the rootzone salinity within acceptable limits.This drainage design also maintained thegroundwater table depth below the root zonethroughout the growing season. The outcomeof this study reveals that the drainagedesign criteria applied for the FDP israther conservative with high drainageintensity. The FDP-area can effectively bedrained with a 25 percent lower drainageintensity (q _drain/h)provided no operational or maintenanceconstraints are present. However, the finaldecision on the optimum combination ofdrain depth and drain spacing would requirea thorough economical analysis. Thenon-steady state approach proved successfulin analyzing the complex interactionsbetween irrigation and drainage components.It is a valuable tool to optimize thedesign of drainage systems against cropyields and soil salinization.     

Controlled drainage — effects on drain outflow and water quality

A field experimental project was set up in southern Sweden to assess the effects of controlled drainage on hydrology and environment. Controlled drainage makes it possible to vary the drainage intensity with the variation in drainage requirement during season by controlling the height of a riser in the drain outlet and thus to a certain degree control the amount of outflow of solutes via the drainage system. During periods with low drainage demand, the riser in the drain outlet can be raised and the groundwater level in field will rise up to the level of the riser before the discharge takes place. Three plots, each with an area of 0.2 ha (40 m×50 m) were installed on a loamy sand. One plot was drained by conventional subsurface drainage (CD) and two plots were drained by controlled drainage (CWT). The plots contained four lateral drain tubes, at 10 m spacing and placed at 1 m depth. Each plot was isolated by a double layer of plastic sheeting placed in the back-filled trenches to a depth of 1.6 m to prevent lateral leakage and subsurface interactions. Measurements of precipitation, drain outflow and soil and air temperatures were carried out hourly. Groundwater levels were measured and samples of drain outflow were collected twice a month for nitrogen and phosphorous analyses. Mineral nitrogen contents in soil were measured three times a year.Controlled drainage had a significant hydrological and environmental effect during the 2 years of measurement (1996–1998). Compared with CD, the total drain outflow from CWT was 79% less in Year 1 and 94% in Year 2. The total reduction in nitrate losses with CWT corresponded to the reduced outflow rates. Compared with CD, the total amounts of nitrate in drain outflow were 78% less in Year 1 and 94% in Year 2. The highest concentrations of nitrate were measured at the time of the largest outflow rates. The phosphorous losses were 58% less for CWT as compared to the CD values in Year 1 and 85% less in Year 2. The reduction in nitrogen content in the soil profile during the winter season was 60–70% less in CWT than in CD.     

Entrance resistance of enveloped drainage pipes

The entrance resistance and the effective radius of corrugated PVC drain pipes without envelope and with six different prewrapped envelopes were evaluated in a sand tank experiment.By applying the theory of resistances, the entrance resistance of the naked pipe was found to be 0.0136 days/m. With envelopes, the values were 0.0024 to 0.0067 days/m, depending on the types of envelope.The effective radius of the naked pipe was found to be 0.47 cm for a drain pipe with an actual radius of 3.0 cm. This value increased to between 1.20 and 2.50 cm when envelope material was used. Then the values of the calculated entrance resistances were substituted in the steady state drainage equations under normal field conditions to evaluate the effect on drain spacing. In all equations, a tendency towards increasing the drain spacing was observed when envelope material was used.     

Using the DRAINMOD-N model to study effects of drainage system design and management on crop productivity,profitability and NO3–N losses in drainage water

The environmental impacts of agricultural drainage have become a critical issue. There is a need to design and manage drainage and related water table control systems to satisfy both crop production and water quality objectives. The model DRAINMOD-N was used to study long-term effects of drainage system design and management on crop production, profitability, and nitrogen losses in two poorly drained soils typical of eastern North Carolina (NC), USA. Simulations were conducted for a 20-yr period (1971–1990) of continuous corn production at Plymouth, NC. The design scenarios evaluated consisted of three drain depths (0.75, 1.0, and 1.25 m), ten drain spacings (10, 15, 20, 25, 30, 40, 50, 60, 80, and 100 m), and two surface conditions (0.5 and 2.5 cm depressional storage). The management treatments included conventional drainage, controlled drainage during the summer season and controlled drainage during both the summer and winter seasons. Maximum profits for both soils were predicted for a 1.25 m drain depth and poor surface drainage (2.5 cm depressional storage). The optimum spacings were 40 and 20 m for the Portsmouth and Tomotley soils, respectively. These systems however would not be optimum from the water quality perspective. If the water quality objective is of equal importance to the productivity objective, the drainage systems need to be designed and managed to reduce NO₃–N losses while still providing an acceptable profit from the crop. Simulated results showed NO₃–N losses can be substantially reduced by decreasing drain depth, improving surface drainage, and using controlled drainage. Within this context, NO₃–N losses can be reduced by providing only the minimum subsurface drainage intensity required for production, by designing drainage systems to fit soil properties, and by using controlled drainage during periods when maximum drainage is not needed for production. The simulation results have demonstrated the applicability of DRAINMOD-N for quantifying effects of drainage design and management combinations on profits from agricultural crops and on losses of NO₃–N to the environment for specific crop, soil and climatic conditions. Thus, the model can be used to guide design and management decisions for satisfying both productivity and environmental objectives and assessing the costs and benefits of alternative choices to each set of objectives.     

Dynamics and modeling of soil water under subsurface drip irrigated onion

Subsurface drip irrigation provides water to the plants around the root zone while maintaining a dry soil surface. A problem associated with the subsurface drip irrigation is the formation of cavity at the soil surface above the water emission points. This can be resolved through matching dripper flow rates to the soil hydraulic properties. Such a matching can be obtained either by the field experiments supplemented by modeling. Simulation model (Hydrus-2D) was used and tested in onion crop (Allium cepa L.) irrigated through subsurface drip system during 2002-2003, 2003-2004 and 2004-2005. Onion was transplanted at a plant to plant and row to row spacing of 10 cm × 15 cm with 3 irrigation levels and 6 depths of placement of drip lateral. The specific objective of this study was to assess the effect of depth of placement of drip laterals on crop yield and application of Hydrus-2D model for the simulation of soil water. In sandy loam soils, it was observed that operating pressures of up to 1.0 kg cm⁻² did not lead to the formation of cavity above the subsurface dripper having drippers of 2.0 l h⁻¹ discharge at depths up to 30 cm. Wetted soil area of 60 cm wide and up to a depth of 30 cm had more than 18% soil water content, which was conducive for good growth of crop resulting in higher onion yields when drip laterals were placed either on soil surface or placed up to depths of 15 cm. In deeper placement of drip lateral (20 and 30 cm below surface), adequate soil water was found at 30, 45 and 60 cm soil depth. Maximum drainage occurred when drip lateral was placed at 30 cm depth. Maximum onion yield was recorded at 10 cm depth of drip lateral (25.7 t ha⁻¹). The application of Hydrus-2D confirmed the movement of soil water at 20 and 30 cm depth of placement of drip laterals. The model performance in simulating soil water was evaluated by comparing the measured and predicted values using three parameters namely, AE, RMSE and model efficiency. Distribution of soil water under field experiment and by model simulation at different growth stages agreed closely and the differences were statistically insignificant. The use of Hydrus-2D enabled corroborating the conclusions derived from the field experimentation made on soil water distribution at different depths of placement of drip laterals. This model helped in designing the subsurface drip system for efficient use of water with minimum drainage.     

Application of SALTMOD in Coastal Clay Soil in India

SALTMOD is a simulation model whichpredicts root zone soil salinity, drainagewater quality and water table depth inagricultural land under differentgeo-hydrological conditions and varyingwater management scenarios. The model wasapplied to the data from coastal AndhraPradesh of India where subsurface drainagesystem is laid out at several drainspacings at the experimental site. Fielddata for 1999, 2000 and 2001 were collectedfrom 35 and 55 m drain spacing plots forSALTMOD application. Modelling was doneconsidering two simulation approaches. Thefirst approach (Simulation-I) used the sameinitial values for the entire simulationperiod. In the second approach(Simulation-II), the computations wereperformed year-by-year, giving each yearthe current input values obtained from thesimulation results of the previous year.Results of these two approaches weredifferent from each other. Simulation-IIgave better predictions than that ofSimulation-I in terms of closeness to theobserved values. Simulation results ofsoil salinity in the root zone, drainagewater quality and quantity and depth towater table revealed that the salinity ofroot zone was predicted more accuratelythan that of drainage water quality anddepth to water table. Also throughsimulation, it was found that the salinityof drainage water was relativelyindependent of the root zone soil salinity. Model application study suggests thatSALTMOD can be used with confidence toevaluate various drain spacings of asubsurface drainage system and facilitatereasonable prediction of the reclamationperiod.     

Modeling the impact of alternative drainage practices in the northern Corn-belt with DRAINMOD-NII

The hydrologic and water quality impacts of subsurface drainage design and management practices are being investigated through field and simulation studies throughout the northern Corn-belt. Six years of data from an ongoing field study in south central Minnesota (Sands et al., 2008) were used to support a modeling effort with DRAINMOD-NII to investigate: (1) the performance of the model in a region where soils are subject to seasonal freeze-thaw and (2) the long-term hydrologic and water quality characteristics of conventional and alternative subsurface drainage practices. Post-calibration model prediction and efficiency were deemed satisfactory using standard model performance criteria. Prediction errors were primarily associated with early spring snowmelt hydrology and were attributed to the methods used for simulating snow accumulation and melting processes, in addition to potential sublimation effects on ET estimates. Long-term simulations with DRAINMOD-NII indicated that drainage design and/or management practices proposed as alternatives to conventional design may offer opportunities to reduce nitrate (NO₃)-nitrogen losses without significantly decreasing (and in some cases, increasing) crop yields for a Webster silty clay loam soil at Waseca, Minnesota. The simulation study indicated that both shallow drainage and controlled drainage may reduce annual drainage discharge and NO₃-nitrogen losses by 20-30%, while impacting crop yields from −3% (yield decrease) to 2%, depending on lateral drain spacing. The practice of increasing drainage intensity (decreasing drain spacing) beyond recommended values appears to not significantly affect crop yield but may substantially increase drainage discharge and nitrate-nitrogen losses to surface waters.     

Evaluating a multi-level subsurface drainage system for improved drainage water quality

This paper describes a multi-level drainage system, designed to improve drainage water quality. Results are presented from a field scale land reclamation experiment implemented in the Murrumbidgee Irrigation Area of New South Wales, Australia. A traditional single level drainage system and a multi-level drainage system were compared in the experiment in an irrigated field setting. The single level drainage system consisted of 1.8 m deep drains at 20 m spacing. This configuration is typical of subsurface drainage system design used in the area. The multi-level drainage system consisted of shallow closely spaced drains (3.3 m spacing at 0.75 m depth) underlain by deeper widely spaced drains (20 m spacing at 1.8 m depth). Data on drainage flows and salinity, water table regime and soil salinity were collected over a 2-year period.