Water Distribution in Laterals and Units of Subsurface Drip Irrigation. II: Field Evaluation

The performance of drip irrigation and subsurface drip irrigation (SDI) laterals has been compared. Two emitter models (one compensating and the other noncompensating) were assessed. Field tests were carried out with a pair of laterals working at the same inlet pressure. A procedure was developed that recorded head pressures at both lateral extremes and inlet flow during irrigation. Both models showed similar behavior and soil properties affected their discharge. On the other hand, the performance of a field SDI unit of compensating emitters was characterized by measuring pressures at different points and inlet flow. Finally, the distribution of water and soil pressure in the laterals and the unit were predicted and irrigation uniformity and soil pressure variability were also determined. Predictions agreed reasonably well with the experimental observations. Thus, the methodology proposed could be used to support the decision making for the design and management of SDI systems.     

SDI Dripline Spacing Effect on Corn and Soybean Yield in a Piedmont Clay Soil

A subsurface drip irrigation (SDI) system was installed in the Piedmont of North Carolina in a clay soil in the fall of 2001 to test the effect of dripline spacing on corn and soybean yield. The system was zoned into three sections; each section was cropped to either corn (Zea mays L.), full-season soybean [Glycine max (L.) Merr.], or winter wheat (Triticum aestivum) double cropped to soybean representing any year of a typical crop rotation in the region. Each section had four plots; two SDI plots with dripline spacing at either 1.52 or 2.28 m, an overhead sprinkler irrigated plot, and an unirrigated plot. There was no difference in average corn grain yield for 2002–2005 between dripline spacings or between either dripline spacing and sprinkler. Irrigation water use efficiency (IWUE) was greater for sprinkler irrigated corn than for either SDI treatment and there was no difference in IWUE in soybean. Water typically moved laterally from the driplines 0.38 to 0.50 m. SDI yield and IWUE increased relative to sprinkler yields and water use efficiency in the second and third year of the study. This may suggest that initial fracturing of the heavy clay soil during SDI system installation and subsequent settling of the soil affected water distribution.     

Water Distribution in Laterals and Units of Subsurface Drip Irrigation. I: Simulation

A complete methodology to predict water distribution in laterals and units of subsurface drip irrigation (SDI) is proposed. Two computer programs have been developed for the hydraulic characterization of SDI; one for laterals and the other for units. Emitter discharge was considered to depend on hydraulic variability, emitter’s manufacture and wear variation, and soil pressure variation. A new procedure to solve the hydraulic calculation of SDI looped network has been established. Moreover, spatial distribution of soil variability was estimated by a geostatistical modeling software that is coupled with the computer programs. Thus the evaluation and performance of laterals and units of SDI can be addressed by changing input variables such us: length and diameters of laterals; coefficients of emitter’s discharge equation; coefficient of variation of emitter’s manufacture and wear; local losses at the emitter insertion; inlet pressure; and soil hydraulic properties and its spatial variability. Finally, the methodology has been applied to different scenarios, and some recommendations are outlined for the selection of emitter discharge and inlet pressures.     

Comparison of Crop Contamination by Microorganisms during Subsurface Drip and Furrow Irrigation

This study was conducted to compare subsurface drip irrigation (SDI) with furrow irrigation (FI) in crop contamination with microbial-contaminated water irrigation. Escherichia coli, Clostridium perfringens, and coliphage PRD-1 were added to water used to irrigate cantaloupe, lettuce, and bell pepper. Samples of produce, surface, and subsurface (10?cm) soil for each irrigation system were collected on Days 1, 3, 5, 7, 10, and 14 after the application of the study microorganisms. Overall, greater contamination of produce occurred in FI plots than in SDI plots. The microorganisms were detected on the surfaces of cantaloupe and lettuce, but were never recovered on the bell peppers. The greatest amount of contamination occurred with PRD-1 on cantaloupe. The study microorganisms survived longer in the subsurface soil than the soil surface. PRD-1 showed greater persistence than E. coli in soil, while C. perfringens experienced little inactivation during the experiment periods. This study showed that subsurface drip irrigation has great potential to reduce health risks when microbial-contaminated water is used for irrigation water.     

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.     

Modeling of Evaporation Reduction in Drip Irrigation System

Surface drip irrigation is an efficient system for delivering water to crops; however, conditions at the soil surface affect evaporation rate and efficiency. A method is proposed, sand tube irrigation (STI), to increase the efficiency of drip irrigation systems. This method is specific to permanent tree crops where soil is not tilled or turned. The STI method consists of removing a soil core beneath the emitter and filling the void with coarse sand. The SWMS??2D model, implemented in a 3D axisymmetric form, was used to simulate infiltration, water redistribution, evaporation from the soil surface, and rise of water inside the sand tube. Model simulations were compared with laboratory measurements determined from a weighing lysimeter. The simulated values of water height inside the sand tube and temporal position of the wetting front in both lateral and upward directions closely matched the experimental measurements. The advancement of the wetting front in the downward direction and evaporation estimates was predicted with less accuracy. Experiments showed that relative to surface drip irrigation, the STI method reduced evaporation by approximately 26% over a 4-day period.     

Application of Conditioner Solution by Subsurface Emitters for Stabilizing the Surrounding Soil

Poor uniformity of water application by subsurface drip irrigation has been examined and some explanations are suggested in this paper. Use of soil conditioners for soil structure stabilization around subsurface drip irrigation pipes was suggested by the authors and tested in the laboratory. The conditioners preserve the structure of existing aggregated and may effectively reduce soil clogging. A silt loam soil was uniformly packed in a 1×0.8×0.15?m box. Two holes were dilled in the box wall through which two emitters were inserted, one for applying solution of soil conditioner and one as a control. Stabilization was achieved by applying two types of polymer solutions differing by their molecular weights through an emitter buried in a silt loam soil. A measured water volume was injected through the emitters into the soil and, after 48?h following irrigation, the box was dismantled. Gravimetric soil moisture content and aggregate water stability were measured in vertical and horizontal distances from the emitter. The highest stabilizing effectiveness was obtained with a volume of 1.5?L polymer solution at 5?g/L concentration, which was applied to the soil at an initial moisture content of 13%. The volume of stabilized soil increased with the volume of applied solution, but the volume ratio of stabilized soil to applied solution decreased with the increase in solution volume. A polymer of relatively low molecular weight was found less effective since a large portion of the solution was consumed by fast penetration into soil aggregates without improving the soil structure. The proposed method offers a simple and easy means for preparing a stabilized soil envelope around subsurface drip irrigation pipes, which may improve the long-term performance and uniformity of the application of these systems. Yet for some of these aspects, further field evaluation is needed, since the results of the study are from a laboratory experiment limited to one soil only.     

Wetting Pattern Models for Drip Irrigation: New Empirical Model

Reliable information about the wetted dimensions of soil under drip irrigation helps designers to determine optimal emitter flow rates and spacings to reduce system equipment costs and provide better soil water conditions for the most efficient and effective use of water. This study presents a new empirical formula that predicts soil wetted dimensions around a drip emitter. The coefficients were obtained by using regression analysis on the results of field experiments done on the Pardis Agricultural Farm of Tehran University in Karaj, Iran. These data were also used to evaluate the semiempirical model of Zur and Schwartzman, the empirical model of Amin and Ekhmaj, and the analytical model WetUp. Statistical comparisons (mean error, root mean square error, and model efficiency) are made of the simulated data with the observed data. To evaluate the models, published experimental data by Risse et?al. and Li et?al. were also used. The results demonstrate that the suggested equations can be used for a wide range of discharge rates and soil types. The best result was obtained from the new empirical model proposed in this investigation. The lowest mean error for the wetted radius and wetted depth was 8.21 and 8.62?cm, respectively.     

Procedures for Determining Maximum Emitter Discharge in Subsurface Drip Irrigation

One problem associated with subsurface drip irrigation (SDI) is the reduction in discharge resulting from soil-water back-pressure at the emitter outlets. An experimental setup was made to measure emitter discharge and pressure at the emitter outlet in different soils. Experiments were carried out with 2–24??L/h noncompensating and compensating emitters, operating at a constant lateral pressure of 10?m. Emitter discharge was reduced to a range of 2–10% for noncompensating models and to less than 1% for compensating models. Soil pressure ranged from 0.15–2.07?m. Laboratory conditions were simulated with HYDRUS-2D/3D. Experimental values of discharge and soil pressure showed good agreement with estimated values. Finally, maximum emitter discharge to limit the decrease of discharge was determined for an operating pressure of 10?m. For a 10% decrease, considering a constant radius of the spherical cavity in the soil, maximum emitter discharge was 2.35??L/h for loamy soil and 12.44??L/h for sandy soil for noncompensating emitters, and 10.73 and 54.51??L/h, respectively for compensating emitters. These values increased when considering a cavity radius variable with emitter discharge.     

Bacteriophages MS2 and PRD1 in Turfgrass by Subsurface Drip Irrigation

The contamination of turfgrass by bacteriophages MS-2 and PRD-1 was assessed in the field under sprinkler irrigation (SI) and subsurface drip irrigation (SDI). No contamination of turfgrass by MS-2 was observed using SDI in the summer or winter seasons. In the summer, PRD-1 was detected in low numbers in SDI turfgrass; however, at significantly lower numbers than in SI turfgrass (p<0.05). In contrast, SI turfgrass was readily contaminated with MS-2 and PRD-1 during both seasons. Column experiments showed that viral migration was greater in sandy soil than in clay soil. Descending viral transport was more pronounced than upward migration, but only significantly greater (p<0.05) in sandy soil. The survival in soil of MS-2 and PRD-1 was compared with that of poliovirus 1 and enteric adenovirus 40. MS-2 showed shorter survival in comparison to the other viruses (p<0.05). The results obtained in this study suggest that SDI used to irrigate turfgrass with wastewater may effectively reduce the risk of contamination by potential viral pathogens.     

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.     

Analytical Expressions for Hydraulic Calculation of Trapezoidal Drip Irrigation Units

Analytical expressions have been developed relating water distribution indexes in trapezoidal drip irrigation units to design variables which define these units: lengths and diameters of pipes, emitter and lateral spacing, slopes, emitter flow equation parameters, and equivalent lengths characterizing local losses. The proposed expressions are founded in classical hydraulics. They are more accurate than predictions in irrigation practice and are easier to handle than the simulation models frequently proposed to irrigation technicians. Unit design and irrigation decision making and evaluation can thus be furthered. An example for the application of the proposed expressions is presented. First, lateral and submain diameters are determined for different shapes of irrigation units to achieve a given water application uniformity. The irrigation time to supply the desired irrigation depth is then calculated. Results are finally compared with values obtained by simulations that take into account hydraulic and manufacture variations in the unit.     

New Computational Fluid Dynamic Procedure to Estimate Friction and Local Losses in Coextruded Drip Laterals

The design of trickle irrigation systems is crucial to optimize profitability and to warrant high values for the emission uniformity (EU) coefficient. EU depends on variation of the pressure head due to head losses along the lines and elevation changes, as well as the water temperature, and other parameters related to the emitters (manufacturer’s coefficient of variation, number of emitters per plants, emitter spacing). Trickle irrigation plants are usually designed using small diameter plastic pipes (polyethylene or polyvinyl chloride). The design problem, therefore, needs to consider head losses along the lines as well as emitter discharge variations due to the manufacturer’s variability. Variations in the hydraulic head are a consequence of both friction losses along the pipe and local losses due to the emitters’ connections, whose importance has been recently emphasized. Since each local loss depends on the emitter type (in-line or on-line) as well as on its shape and dimensions, the morphological variability of the commercially available emitter requires experimental investigations to determine local losses in drip laterals. On the other hand, local losses can be estimated by the mean of computational fluid dynamics (CFD) models, allowing analysis of velocity profiles and the turbulence caused by the emitters’ connections. FLUENT software can be considered a powerful tool to evaluate friction and local losses in drip irrigation laterals, after the necessary validation has been carried out by means of experimental data. The main objective of this study was to assess a CFD technique to evaluate friction and local losses in laterals with in-line coextruded emitters. The model was initially used to choose the turbulence model allowing the most accurate estimation of friction losses in small diameter polyethylene pipes, characterized by low Reynolds number. Second, the possibility of using CFD to predict local losses in drip irrigation laterals with a commercially available coextruded emitter was investigated. Simulated local losses were obtained as difference of the total and friction losses along a trunk of pipe, where one single emitter was installed, not considering the emitter outflow. The proposed procedure allows to evaluate local losses for other different emitter models, avoiding tedious and time-consuming experiments.     

Determining Minor Head Losses in Drip Irrigation Laterals. I: Methodology

Minor head losses at emitter insertions along drip laterals were predicted by a derivation of Bélanger’s theorem and analyzed by the classic formula that includes a friction coefficient K multiplied by a kinetic energy term. A relationship was established for K as a function of some emitter geometric characteristics. These take into account the flow expansion behind the reduction of the cross-sectional area of the pipe due to obstruction by the emitter. Flow constrictions at emitter insertions were estimated by analogy with contraction produced by water jets discharging through orifices. An experimental procedure was also developed to determine minor losses in situ, in the laboratory or in the field. An approach is suggested to calculate either K or the emitter equivalent length le as a function of lateral head losses, inlet head, and flow rate. Internal diameter and length of lateral, emitter spacing, emitter discharge equation, and water viscosity must be known. Approximate analytical relations to study flow in laterals were developed. They may be used to design and evaluate drip irrigation units. Analytical and experimental procedures are validated in the companion paper by Juana et al.     

Physically Based Modeling of Interacting Surface–Subsurface Flow During Furrow Irrigation Advance

Surface–subsurface flow during furrow irrigation is analyzed employing both a laboratory experiment and a surface–subsurface flow model. The proposed model overcomes the restrictions of traditional furrow irrigation models by rigorously describing the subsurface flow at computational nodes using the physically based two-dimensional (2D) model Hydrus-2D. Surface flow is portrayed by an analytical zero-inertia model. In order to couple both models efficiently, an iterative procedure was developed. Using a sensitivity analysis we investigated the space interval separating 2D infiltration computations. This variable showed little effect on model performance, thus permitting the selection of rather generous distances. Due to the similarity of the hydrographs at neighboring cross sections we investigated transferring the results of Hydrus-2D computations to the next downstream location. This was performed by interpolating cumulative infiltration using infiltration opportunity times. This procedure uncovered other dependencies, making the interpolation strategy unattractive. To validate the coupled surface–subsurface model, an irrigation furrow was set up in a 26.4 m long, 0.88 m wide, and 1.0 m deep tank, filled with 50 t of sandy loam soil and equipped with surface and subsurface measurement devices. Although the model results compared favorably with the observed data, the study also showed an important impact of surface cracking and preferential flow during the irrigation experiments.     

Analytical Relationships for Designing Rectangular Drip Irrigation Units

Approximate analytical expressions were obtained that relate uniformity indices of water distribution in rectangular drip irrigation units as a function of the variables that define that unit: lengths and diameters of laterals and submain, spacing of emitters and laterals, ground slopes, parameters of the emitter discharge equation, and equivalent lengths characterizing local losses. The proposed expressions offer greater precision than might be needed in irrigation practice. They do not require iterative calculations and improve the procedures normally used. They may be useful in the design of drip irrigation units and in their evaluation and management. An example of their application is offered. The proposed relationships simplify studies of the sensitivity of variables involved in optimum hydraulic design. Users are thus allowed a rational understanding of their influence, improving that which can be gained from computer programs. Graphs obtained with the mentioned expressions are also offered. They can be of interest, although their use is not treated specifically in this paper.     

Validation of a Predictive Form of Horton Infiltration for Simulating Furrow Irrigation

Due to spatially varying conditions the improvement of furrow irrigation efficiency should be sought not just for a limited number of furrows or for one specific irrigation event. A simplified predictive modeling approach of the averaged advance-infiltration process is proposed in this paper. Horton’s equation, derived from the asymptotic form of the Talsma-Parlange infiltration equation, allows us to use a predictive approach for the advance infiltration process by means of the exact solution of the Lewis and Milne water balance equation. The references to the works of White and Sully, for a surface point source, result in the use of parameters which characterize the hydraulic properties of the soil: Δθ (saturated water content minus initial water content); Ks (saturated conductivity); and λc (macroscopic capillary length). The physical meaning of parameters involved in the proposed modeling is attested using field experiments carried out in a loamy soil plot context. Assuming a same Δθ measured value before irrigation for the whole of a 30 furrow sample, the averaged values of λc and Ks obtained from calibration on the advance trajectory are comparable to those derived from local infiltration tests (disk permeameter and double ring methods). The applicability of the model is then extended to heavy clay soil where the parameters λc and Ks still agree with the values proposed in the literature. This paper can be considered as a contribution to the development of a tool for evaluating the impact of irrigation practices on the efficiency at the plot and cropping season scale.     

Comparison of HYDRUS-2D Simulations of Drip Irrigation with Experimental Observations

Realizing the full potential of drip irrigation technology requires optimizing the operational parameters that are available to irrigators, such as the frequency, rate, and duration of water application and the placement of drip tubing. Numerical simulation is a fast and inexpensive approach to studying optimal management practices. Unfortunately, little work has been done to investigate the accuracy of numerical simulations, leading some to question the usefulness of simulation as a research and design tool. In this study, we compare HYDRUS-2D simulations of drip irrigation with experimental data. A Hanford sandy loam soil was irrigated using thin-walled drip tubing installed at a depth of 6 cm. Three trials (20, 40, and 60 L?m?1 applied water) were carried out. At the end of each irrigation and approximately 24 h later, the water content distribution in the soil was determined by gravimetric sampling. The HYDRUS-2D predictions of the water content distribution are found to be in very good agreement with the data. The results support the use of HYDRUS-2D as a tool for investigating and designing drip irrigation management practices.     

Effects of Magnetized Water and Irrigation Water Salinity on Soil Moisture Distribution in Trickle Irrigation

Magnetized water is obtained by passing water through a strong permanent magnet installed in or on a feed pipeline. This study was performed at Gorgan Agricultural and Natural Resources Research Center, Gorgan province, Iran, to investigate soil moisture distribution under trickle irrigation. Two main treatments of magnetic and nonmagnetic water and three subtreatments of irrigation water salts, including well water as a control, 200-ppm calcium carbonate, and 400-ppm calcium carbonate were used. The experiment was laid out with a complete randomized block design with three replications. Soil moisture distribution around the emitters were measured 24?h after irrigation during the 3-month irrigation period. The results showed that the mean soil moisture contents at depths of 0–20, 20–40, and 40–60?cm below the emitter for the magnetized irrigation water treatment were more than the nonmagnetized irrigation water treatment, and the differences were significant at the 5% level. The irrigation with magnetic water as compared with the nonmagnetic water increased soil moisture up to 7.5%, and this increase was significant at the 1% level. The effect of irrigation water salinity on soil moisture was significant. The highest soil moisture content was from the 400-ppm calcium carbonate subtreatment. The use of magnetized water for irrigation is recommended to save irrigation water.