Subsurface Irrigation Research Articles

Some physical and chemical properties of hardsetting Alfisols can be affected by trickle irrigation

The effect of replacing furrow irrigation with subsurface trickle irrigation on some soil physical and chemical properties of hardsetting red Alfisols when sown to cotton were evaluated on two farms (Bellevue and Brae Park) in the Macquarie valley of New South Wales, Australia, during 1998 and 1999. The soil at Bellevue had a heavier soil texture than that at Brae Park. Soil was sampled from depths of 0–0.15 m, 0.15–0.30 m, 0.30–0.45 m and 0.45–0.60 m in "paired" fields sown to either trickle- or furrow-irrigated cotton. The soil properties evaluated were soil structural stability as dispersion index, hardsetting characteristics, salinity as electrical conductivity (EC) of a 1:5 soil:water suspension, and sodicity as EC/exchangeable Na ratio.

Enhanced Subsurface Irrigation Hydrology Model

This paper provides a theoretical basis for new features added to the SWMS_3D model of Simunek et. al., including spatially varied land use, dynamic root distributions and water uptake, and various fluxes at the soil-atmosphere interface due to different irrigation practices. Spatially varied land use is incorporated into the model by separating \IET\dp\N into \IE\dp\N and \IT\dp\N. Distribution of roots is made a function of time by introducing a root growth function and root length density distribution with crop specific parameters. Various fluxes at the soil-atmosphere interface are accommodated in node-specific boundary conditions for each ground surface node by switching between specified head and specified flux boundary conditions. Ponding on the surface is allowed to account for different irrigation practices. After incorporating new features, the model is tested with analytical solutions or experimental data available in the literature. Comparison of analytical/experimental and model generated values shows that the model can produce satisfactory results.

Simulation of infiltration from porous clay pipe in subsurface irrigation

Advances in the traditional method of subsurface porous clay pipe irrigation rely on knowledge of the distribution of water in the soil. Knowing the relationships among the hydraulic and physical parameters in the system is important for both the design and management of the system. To simulate the infiltration from the porous clay pipe and predict the wetted zone geometry in the soil, a computer model is developed herein. Laboratory experiments were conducted on soil samples representing two different soil textures in a specially designed bin to understand the flow phenomenon and to validate the developed model. In a given soil texture, the installation depth of the pipe and the volume of water applied in the soil are the major factors affecting the wetted zone. The relationships among various parameters, namely lateral spacing, installation depth, irrigation run time, hydraulic conductivity of the body of the pipe, and hydraulic head in the system, were established using the developed model.

Effect of a single irrigation at different reproductive growth stages on soybean planted in early and late June

When subsurface irrigation sources are lacking, storage of precipitation in farm ponds and lakes may provide a source for irrigation, but the amount of water available may be limited. Deficit irrigation at selected growth stages may help avoid crop stress at critical times. The objective of this study was to determine the effect of timing of a single irrigation on soybean cultivars planted at two dates. The experiment was conducted from 1987 to 1990 on a Parsons silt loam (fine, mixed, thermic Mollic Albaqualf) near Parsons, Kansas. The experiment was designed as a split-plot with the eight whole plots being a factorial arrangement of irrigation treatments and planting dates and subplots were soybean [Glycine max (L.) Merr.] cultivars. The four irrigation treatments were: (1) no irrigation, (2) 2.5 cm at growth stage R1 to R2 (beginning to full bloom), (3) 2.5 cm at R4 (full pod), and (4) 2.5 cm at R6 (full seed). The two planting dates were early and late June. The three, maturity group IV, soybean cultivars were Crawford, Douglas, and Sparks. In the less stressful years, 1987 and 1989, yield was unaffected by irrigation when soybean cultivars were planted in early June, but irrigation increased yield of late-June-planted soybean by 0.4 Mg ha–1 or more. In 1988 and 1990, climatic conditions were dry and hot. This resulted in low yields, but regardless of planting date, soybean responded to irrigation applied at any growth stage in 1988 and at R6 in 1990. Yield increases from irrigation appear to be more related to increasing seeds per plant than seed weight. Selection of the timing of a single irrigation between growth stages R1, R4, and R6 may not result in large yield differences because all increased the number of seeds per plant. Irrigation at R6 also increased average seed weight. With typical weather conditions in the mid-latitude section of the eastern Great Plains, soybean planted in late June are more likely to benefit from a single irrigation than those planted in early June.

Numerical Simulations of Soil Water Balance in Support of Revegetation of Damaged Military Lands in Arid Regions

Large areas of soil cover across the desert southwestern United States have been highly impacted by military training and require revegetation. Revegetation efforts commonly optimize growing conditions by increasing plant-available water through irrigation. Simulations of soil water balance were conducted using the Simultaneous Heat and Water (SHAW) model to examine how key environmental factors, including precipitation, soil properties, and irrigation strategies, control soil water balance and plant-available water. Simulations include surface and subsurface irrigation of 7 mm (water) applied monthly and 21 and 28mm applied three times a year. Results indicate that any augmentation of soil water provides for an increase in transpiration relative to precipitation between about 50 to 250%. Subsurface irrigation provided a slightly greater increase in transpiration relative to surface irrigation, except for surface irrigation at monthly intervals, which provided the lowest increase in simulated transpiration, because most of the applied water is lost to evaporation. Results indicate that surface and subsurface irrigation three times annually and during the spring and summer months provides the greatest effective benefit. Simulations also indicate that use of bark chips as a surface mulch will decrease plant-available water, whereas surface mulch of a light colored gravel will enhance moisture retention. Construction of catchments will also increase soil water where surface runoff occurs. Application of numerical models for evaluating soil water balance provide a cost-effective method for assessing techniques designed to enhance the availability of plant water required for revegetation efforts.

Runoff and drainage losses of atrazine, metribuzin, and metolachlor in three water management systems.

Rainfall can transport herbicides from agricultural land to surface waters, where they become an environmental concern. Tile drainage can benefit crop production by removing excess soil water but tile drainage may also aggravate herbicide and nutrient movement into surface waters. Water management of tile drains after planting may reduce tile drainage and thereby reduce herbicide losses to surface water. To test this hypothesis we calculated the loss of three herbicides from a field with three water management systems: free drainage (D), controlled drainage (CD), and controlled drainage with subsurface irrigation (CDS). The effect of water management systems on the dissipation of atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine), metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazine-5(4H)-one), and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] in soil was also monitored. Less herbicide was lost by surface runoff from the D and CD treatments than from CDS. The CDS treatment increased surface runoff, which transported more herbicide than that from D or CD treatments. In one year, the time for metribuzin residue to dissipate to half its initial value was shorter for CDS (33 d) than for D (43 d) and CD (46 d). The half-life of atrazine and metolachlor were not affected by water management. Controlled drainage with subsurface irrigation may increase herbicide loss through increased surface runoff when excessive rain is received soon after herbicide application. However, increasing soil water content in CDS may decrease herbicide persistence, resulting in less residual herbicide available for aqueous transport.

Runoff and Drainage Losses of Atrazine, Metribuzin, and Metolachlor in Three Water Management Systems

Rainfall can transport herbicides from agricultural land to surface waters, where they become an environmental concern. Tile drainage can benefit crop production by removing excess soil water but tile drainage may also aggravate herbicide and nutrient movement into surface waters. Water management of tile drains after planting may reduce tile drainage and thereby reduce herbicide losses to surface water. To test this hypothesis we calculated the loss of three herbicides from a field with three water management systems: free drainage (D), controlled drainage (CD), and controlled drainage with subsurface irrigation (CDS). The effect of water management systems on the dissipation of atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine), metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazine-5(4H)-one), and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] in soil was also monitored. Less herbicide was lost by surface runoff from the D and CD treatments than from CDS. The CDS treatment increased surface runoff, which transported more herbicide than that from D or CD treatments. In one year, the time for metribuzin residue to dissipate to half its initial value was shorter for CDS (33 d) than for D (43 d) and CD (46 d). The half-life of atrazine and metolachlor were not affected by water management. Controlled drainage with subsurface irrigation may increase herbicide loss through increased surface runoff when excessive rain is received soon after herbicide application. However, increasing soil water content in CDS may decrease herbicide persistence, resulting in less residual herbicide available for aqueous transport.

FOLIAR AND SOIL APPLICATIONS OF INSECTICIDES FOR CONTROLLING SILVERLEAF WHITEFLY ON FRESH MARKET TOMATOES, FALL 2000

On 21 Sep, transplants were set 18 inches apart on raised beds of EauGallie fine sand soil covered with white polyethylene mulch. Plots were three-21-ft long rows on 5-ft centers and were irrigated by seepage, subsurface irrigation system. Treatments were replicated four times in an RCB design. Admire and Platinum were applied on 21 Sep to each plant in 4 oz of water. Applaud and Knack were applied once on 15 Nov as the controlling effects of Admire diminished and whitefly densities approached or reached 3 5 whitefly sessile nymphs and/or pupae/10 leaflets. Foliar applications were made with a high-clearance, self-propelled sprayer operated at 200 psi and 3.4 mph. It was fitted with eight Albuz orange nozzles per row and delivered 60 (four nozzles open), 90 (six nozzles open) or 120 (eight nozzles open) gpa, depending upon plant height. Because of a moderate population of southern armyworm larvae, Mattch (2 qt/acre), Javelin (2 lb/acre) or SpinTor (6 oz/acre) were applied weekly. The number of plants in each plot with definite symptoms of whitefly-vectored geminivirus, primarily tomato yellow leaf curl virus, were recorded weekly. The numbers of sessile nymphs (2nd and 3rd instars) and pupae (4th instar or red eye nymphs) of the silverleaf whitefly were counted weekly on the terminal leaflet from the 7th to 8th leaf counting from the top of each of 10 plants in the center row of each plot.

A modified ceramic sampler and lysimeter design for improved monitoring of soil leachates

This paper describes the design and use of modified solution samplers and non-weighing lysimeters in a field experiment examining the response of a turf–soil system to landfill leachate irrigation over a 2-year period. The two designs were shown to offer significant performance advantages, were cost effective and overcame many of the reported problems commonly associated with each technique. The quantities of leached chloride detected in the 20%, 50% and undiluted leachate irrigated plots by the modified solution samplers (1677, 4319 and 8021 kg ha −1, respectively) and microlysimeters (1759, 4512 and 8160 kg ha −1, respectively) were significantly higher than the conventional sampler design (1407, 3767 and 7052 kg ha −1, respectively). Additionally, the microlysimeter design functioned reliably throughout the experiment, achieving solution flow rates analogous to the unconfined plots. Therefore, it was concluded that both the designs appear to be suitable for monitoring changes in soil solution composition associated with sub-surface wastewater irrigation.

Elevated atmospheric CO 2 alleviates drought stress in wheat

Rising levels of atmospheric CO 2 affect transpiration and water absorption processes that influence total leaf water potential ( Ψ W) of wheat ( Triticum aestivum L.). The objective of this case study was to determine the effect of a rise in atmospheric CO 2 concentration on predawn, midday, and sunset Ψ W as soil matric potential ( Ψ M) ranged from −0.03 to −1.50 MPa. ‘Yecora Rojo’ was sown on 15 December 1992 (130 plants m −2) and 7–8 December 1993 (180 plants m −2) in an open field at Maricopa, AZ (33.1°N, 112.0°W), in air enriched for 24 h per day to a atmospheric CO 2 concentration of ∼200 μmol mol −1 (550 μmol mol −1) above ambient (370 μmol mol −1) using a free-air CO 2 enrichment (FACE) apparatus (main-plot) in four replicates. A sub-surface drip-tape irrigation system provided two soil–water content treatments: 50% (dry) and 100% (wet) replacement of potential evapotranspiration (split-plot). Treatments were control-dry (CD), FACE-dry (FD), control-wet (CW) and FACE-wet (FW). A pressure chamber was used to measure Ψ W on 3–5 upper-canopy fully-expanded sunlit leaves at predawn, midday, and sunset between tillering and hard dough during 1993 and 1994. The Ψ W for the dry plots was regressed on Ψ M from field capacity (−0.03 MPa) to the permanent wilting point (−1.50 MPa). Slopes (standard error) were derived for CD ( Ψ W[CD] (MPa) Ψ M[CD] (MPa) −1) compared with FD ( Ψ W[FD] (MPa) Ψ M[FD] (MPa) −1) at predawn (5.1 (1.2) compared with 1.1 (0.2)), midday (4.3 (1.8) compared with 0.7 (0.3)), and sunset (5.9 (1.5) compared with 1.0 (0.9)). Regardless of time of day, more negative Ψ W for a given Ψ M occurred in control compared with FACE ( P<0.01). As Ψ M ranged from −0.03 to −1.50 MPa, a ∼200 μmol mol −1 rise in atmospheric CO 2 concentration resulted in a curvilinearly increase in drought stress alleviation from 0 to 77% at predawn, 0 to 67% at midday, and 0 to 79% at sunset. Hence, as the CO 2 concentration of the atmosphere rises, both drought avoidance and tolerance mechanisms will be enhanced resulting in improved water relations in wheat.

Phytoremediation of aged petroleum sludge: effect of irrigation techniques and scheduling.

The use of higher plants to accelerate the remediation of petroleum contaminants in soil is limited by, among other factors, rooting depth and the delivery of nutrients to the microsites at which remediation occurs. The objective of this study was to test methods of enhancing root growth and remediation in the subsurface of a contaminated petroleum sludge. The phytoremediation of highly contaminated petroleum sludge (total petroleum hydrocarbons >35 g kg(-1) was tested in the greenhouse as a function of the frequency and the depth of irrigation and fertilization. Water and dissolved plant nutrients were added to the soil surface or at a depth of 30 cm, either daily or weekly. Equivalent quantities of water and nutrients were added in all cases. Daily irrigation at a depth of 30 cm invoked greater root growth and enhanced contaminant degradation relative to all other treatments. In the absence of plants, residual concentrations of petroleum hydrocarbons after 7 mo were higher than with plants. The presence of plant roots clearly improved the physical structure of the soil and increased microbial populations. Thus, the plant roots in conjunction with daily additions of soluble N and P appeared to enhance oxygen transport to greater depths in the soil, stimulate petroleum-degrading microorganisms, and provide microbial access to soil micropores. Subsurface irrigation with frequent, small amounts of water and nutrients could significantly accelerate phytoremediation of field soils contaminated with petroleum hydrocarbons.

Biofiltration of Residual Fertilizer Nitrate and Atrazine by Rhizobium meliloti in Saturated and Unsaturated Sterile Soil Columns

This study was undertaken to investigate whether microbial bioaugmentation of subsurface soil with subsurface irrigation could be used as a biofiltration/biocontrol technology for agricultural pollutants. Nine Plexiglas columns, 458 mm long x 139 mm in diameter, were packed with a sterilized sandy loam soil. Subsurface irrigation, through a controlled water table management system, was used to deliver bacteria, Rhizobium meliloti A-025, to the soil and to maintain aerobic (unsaturated) or anaerobic (saturated) conditions in the columns. Nitrate and atrazine, a fertilizer and a corn herbicide, were applied to the soil surface, and leaching was affected by simulated rainfall events. The soil and drainage waters were analyzed for nitrate and atrazine residues after each rainfall simulation throughout the experimental period during which the soil was kept saturated for a total of 80 days and unsaturated for a total of 70 days. The monitoring of transport and survival of the implanted bacterial strain (A-025) showed that subsurface irrigation was successful in introducing and transporting the bacteria throughout the soil columns. During the saturated period, significantly more (95% probability) nitrate-N leached into the drainage waters from the control columns than from the bioaugmented columns; the increase being 450% or more for the abiotic control columns. The amount of atrazine that leached into the drainage waters during the unsaturated period was also significantly more from control columns as opposed to bioaugmented columns, with the increase being 262%.

The impact of irrigation return flow on aspects of the water quality of the upper Gamtoos Estuary, South Africa

Groundwater quality in the Loerie Flats, and subsurface irrigation return flow to the upper Gamtoos estuary was monitored from November 1992 to April 1994. The nutrient loading of groundwater was highly variable both spatially and temporally, with elevated nitrate-N levels reaching 163 mg·l -1 . The elevated nutrient concentrations in the groundwater were directly related to the timing of land-surface application of fertilisers. The nutrient loading to the estuary, via subsurface irrigation return flow was calculated using Darcy's equation together with the mean concentration of nutrients found in groundwater in the Loerie Flats area. Results indicate that an average of 0.52 t nitrate-N, 0.02 t nitrite-N and 0.10 t of total phosphorus enter the Gamtoos estuary from the 5.5 km 2 Loerie Flats area on an annual basis. Such nutrient loads emanate from less than 20% of the total agricultural area bordering the estuary. Despite the elevated nutrient input to the estuary via groundwater discharge, the nutrient content of the estuary was generally low, but at times elevated concentrations of nutrients were recorded in the upper estuary. It was found that estuarine mixing processes and dilution, with adequate flushing have prevented the periods of low quality water from persisting in the Gamtoos estuary. WaterSA Vol.27(3) 2001: 367-372

Multidimensional linearized nonsteady infiltration toward a shallow water table

The Green's function method is used to derive a general analytical model pertaining to multidimensional nonsteady infiltration toward a shallow water table with arbitrary initial conditions, boundary conditions, and root uptake forcing functions and for various simple source geometries. The general Green's function solution allows the derivation of particular analytical solutions pertaining to cases of surface and subsurface irrigation, evaporation, root uptake, and moisture redistribution. The model assumes that the hydraulic conductivity function is an exponential function of the water pressure and a linear function of the moisture content. One‐dimensional solutions are obtained for time‐dependent distributions of the flux at the land surface and of root uptake in the subsurface. Two‐ and three‐dimensional solutions are also derived for point and line sources in the domain, for a strip, a rectangle, and a disc source at the land surface, and for two‐ and three‐dimensional moisture redistribution and root uptake. Most of the solutions are a product of two terms; one term is a function of depth while the other term is a function of time and the lateral coordinates. Numerical results show the effect of the water table and soil type on the two‐dimensional moisture movement from a strip source.

Multidimensional quasi‐linear steady infiltration toward a shallow water table

The present work explores further a class of analytical solutions of moisture movement in unsaturated porous media, characterized by an exponential dependence of the hydraulic conductivity on water pressure. The Green's function method is used to derive a general analytical model pertaining to multidimensional steady infiltration to a shallow water table with arbitrary boundary conditions and arbitrary root‐uptake forcing functions and for various simple source geometry. The general solution is expressed in integral form from which particular analytical solutions pertaining to cases of surface and subsurface irrigation, evaporation, and root‐uptake in the presence of a shallow water table can be easily deduced from the general analytical model. New solutions include the one‐dimensional infiltration subject to discrete extraction functions, the two‐dimensional infiltration from a strip source with a uniform exponential and a periodic flux distribution, and the three‐dimensional infiltration from a rectangular source at the land surface. Particular solutions have also been obtained for infiltration from a periodic series of point, line, and strip sources. Algebraic equations for the parameter estimation of the hydraulic conductivity function were also derived from the simple one‐dimensional solution. The proposed model offers the analyst significant flexibility in deriving results and analyzing infiltration phenomena of practical interest and provides a basis upon which parameter estimation can be effected.

Irrigated Agriculture and Wildlife Conservation: Conflict on a Global Scale.

/ The demand for water to support irrigated agriculture has led to the demise of wetlands and their associated wildlife for decades. This thirst for water is so pervasive that many wetlands considered to be hemispheric reserves for waterbirds have been heavily affected; for example, the California and Nevada wetlands in North America, the Macquarie Marshes in Australia, and the Aral Sea in central Asia. These and other major wetlands have lost most of their historic supplies of water and some have also experienced serious impacts from contaminated subsurface irrigation drainage. Now mere shadows of what they once were in terms of biodiversity and wildlife production, many of the so-called "wetlands of international importance" are no longer the key conservation strongholds they were in the past. The conflict between irrigated agriculture and wildlife conservation has reached a critical point on a global scale. Not only has local wildlife suffered, including the extinction of highly insular species, but a ripple effect has impacted migratory birds worldwide. Human societies reliant on wetlands for their livelihoods are also bearing the cost. Ironically, most of the degradation of these key wetlands occurred during a period of time when public environmental awareness and scientific assertion of the need for wildlife conservation was at an all-time high. However, designation of certain wetlands as "reserves for wildlife" by international review boards has not slowed their continued degradation. To reverse this trend, land and water managers and policy makers must assess the true economic costs of wetland loss and, depending on the outcome of the assessment, use the information as a basis for establishing legally enforceable water rights that protect wetlands from agricultural development.

The effects of free‐air CO2 enrichment and soil water availability on spatial and seasonal patterns of wheat root growth

Abstract Spring wheat [ Triticum aestivum (L). cv. Yecora Rojo] was grown from December 1992 to May 1993 under two atmospheric CO2 concentrations, 550 μmol mol–1 for high‐CO2 plots, and 370 μmol mol–1 for control plots, using a Free‐Air CO2 Enrichment (FACE) apparatus. In addition to the two levels of atmospheric CO2, there were ample and limiting levels of water supply through a subsurface trip irrigation system in a strip, split‐plot design. In order to examine the temporal and spatial root distribution, root cores were extracted at six growth stages during the season at in‐row and inter‐row positions using a soil core device (86 mm ID, 1.0 m length). Such information would help determine whether and to what extent root morphology is changed by alteration of two important factors, atmospheric CO2 and soil water, in this agricultural ecosystem.Wheat root growth increased under elevated CO2 conditions during all observed developmental stages. A maximum of 37% increase in total root dry mass in the FACE vs. Control plots was observed during the period of stem elongation. Greater root growth rates were calculated due to CO2 enhancement until anthesis. During the early vegetative growth, root dry mass of the inter‐row space was significantly higher for FACE than for Control treatments suggesting that elevated CO2 promoted the production of first‐order lateral roots per main axis. Then, during the reproductive period of growth, more branching of lateral roots in the FACE treatment occurred due to water stress. Significant higher root dry mass was measured in the inter‐row space of the FACE plots where soil water supply was limiting. These sequential responses in root growth and morphology to elevated CO2 and reduced soil water supports the hypothesis that plants grown in a high‐CO2 environment may better compensate soil‐water‐stress conditions.

Sustainable and effective irrigation through a new subsoil irrigation system (SIS)

Since scarcity of water is a major problem in semi-arid and arid areas of Spain and many other Mediterranean regions, water consumption in irrigated agriculture has to be reduced to a sustainable level that is also adapted to the environment. This goal can be reached by applying the highly effective and resource preserving techniques of micro-irrigation. In order to improve these techniques, a new subsurface irrigation system was developed by modification of conventional subsurface systems through the following innovative elements: a new design of the lateral hoses which prevents the penetration of roots into the external water outlets and the block up by soil particles; an impermeable polyethylene foil placed below the lateral pipes to prevent water loss through deep percolation, especially in sandy substrates; a special installation equipment consisting of a V-shaped device which releases foil and pipe simultaneously into the soil without disturbing the natural soil profile. After the experience of several years of operation, these elements have proved to be highly effective. Compared to other irrigation methods, the high irrigation efficiency achieved by this system was outstanding. Minimum maintenance requirement and a long life span are additional positive characteristics of the system. The beneficial outcomes of this system give reason for an optimistic appraisal of the strategies involved towards sustainable irrigated agriculture.

Multidimensional linearized nonsteady infiltration with prescribed boundary conditions at the soil surface

The present work explores a class of analytical solutions of moisture movement in unsaturated porous media characterized by an exponential dependence of the hydraulic conductivity and the moisture content on water pressure. The Green's function method is used to derive a general analytical model pertaining to multidimensional nonsteady infiltration in a semi‐infinite flow domain with arbitrary initial conditions, boundary conditions, and root‐uptake forcing functions and for various simple source geometry. The general solution is expressed in integral form from which particular analytical solutions pertaining to cases of surface and subsurface irrigation, evaporation, root uptake, and moisture redistribution can be easily deduced from the general analytical model. The model offers the analyst significant flexibility in deriving results and analyzing infiltration phenomena of practical interest. New explicit solutions have been obtained for one‐dimensional infiltration under various prescribed time‐dependent flux boundary conditions and for two‐ and three‐dimensional moisture redistribution. For constant initial or boundary conditions, the multidimensional solution is essentially the product of two or three time‐dependent terms with each term being a function of only one space variable.

EVALUATING DRAINAGE DESIGN PARAMETERS FOR WASTEWATER IRRIGATION APPLICATIONS TO MINIMIZE IMPACT ON SURFACE WATERS

DRAINMOD, an agricultural water management computer model, was used to evaluate the hydrology for anirrigation scenario for an Ohio community using treated wastewater as the water supply. This model was used to simulatethe runoff potential of the proposed site under drainage and wastewater irrigation, to evaluate subsurface drainagesystem design parameters and irrigation interval, and to evaluate the effect of wastewater irrigation on potential cropyield. Constituent concentrations of the wastewater were within normal recommendations for agricultural irrigation, andwere not the focus of the study. A 40-year climatic record and USDA Soils5 data were used as inputs to the model toevaluate hydrologic response over a range of input values of irrigation rate and interval, and drain spacing and depth.Simulation results indicate that an irrigation interval of 1 or 2 days could meet the annual irrigation application criteriaof 67 cm (26 in.). Frequency analysis of runoff over the 40-year period for the 2-day irrigation interval revealed thatapproximately 99% of all days produced zero runoff. A response surface of average annual irrigation depth for the 2-dayirrigation interval indicated that the target depth of 67 cm could be attained with a range of drain spacing (7.5 to 12.5 m;25 to 50 ft) and depth (85 to 150 cm; 33 to 60 in.) combinations. Within the range of drain spacing and depths evaluatedwith wastewater irrigation at the 2-day irrigation interval, relative crop yield increased when an increase in drain depthwas coupled with a decrease in drain spacing.