Journal of Irrigation and Drainage Engineering

A nonlinear model for the optimum design and operation of drip irrigation systems on flat terrain is presented in this paper. The analysis is based on dividing a field into subunits and evaluating various shift patterns and the corresponding pipe and pump sizes in order to identify a minimum cost solution. The decision variables are the lengths of two given pipe sizes for the laterals, the diameters of all other pipes, the size of pump, the area and dimensions of subunits, as well as the shift patterns and the irrigation time for each shift and set of subunits irrigated simultaneously. The optimization procedure involves a complete enumeration approach, which minimizes the sum of the capital cost of the system and the present value of operating costs. The model is applied to a case study of a level rectangular field supplied from a ground-water source at its center. The results obtained indicate that one shift operation is the most cost-effective provided the emitter flow rate is above a defined minimum level.

The problem of seepage of steady rainfall or irrigation recharge into a homogeneous and anisotropic soil medium, drained by equally spaced ditch drains overlying a barrier, is solved. Two general cases of the problem are considered: (1) ditch drains dug down to an impermeable barrier; and (2) ditch drains dug down to a gravel substratum. The proposed solutions are exact in the sense that they are based on potential theory and account for the surface of seepage. The validity of the proposed solution for the first case is tested by comparing the midway water-table heights obtained from it for some specific drainage situations with the corresponding upper and lower theoretical bounds of Youngs. The accuracy of the proposed analytical model for the first case is further tested by comparing the midway water-table height obtained from it for a specific drainage situation with the corresponding height obtained from other numerical work.

Migration of chemicals in soils to surface or ground water has become a serious threat in many agricultural regions, with nitrates affecting the broadest area due to intensive fertilization and nitrate solubility. This project evaluated electroreclamation utilizing subsurface horizontal drainage tubing and parallel electrodes to concentrate and remove nitrates from soil. Nitrate concentration in saturated soil tended to increase from anode to cathode after the application of a small direct-current electrical field as the nitrate solution moved toward the cathode. When soil moisture content dropped below the saturation level, the nitrate began to migrate toward the anode. Research is recommended to evaluate the effect of a broader range of electrical parameters and analyze the feasibility of commercial application.

Subsurface drip irrigation has become a common method for the irrigation of field crops, trees, and landscaping. When the predetermined discharge of the emitter is larger than the soil infiltration capacity, water pressure at the dropper outlet increases and can become positive. This pressure buildup in the soil decreases the pressure difference across the dripper and, subsequently, decreases the trickle discharge in a manner that depends on the dripper characteristic curve. A device was developed to measure simultaneously the pressure difference at the inlet and outlet of a subsurface emitter and the emitter discharge. In a preliminary study, discharge rate declines of 10–50% were measured for unplugged subsurface emitters. The extent of the discharge decrease due to back pressure depends on: (1) the soil type (the lower the hydraulic properties the larger the decrease); (2) the dripper discharge (larger decreases occur for higher nominal discharge); (3) possible cavities near the dripper outlet (a larger cavity decreases the back pressure); and (4) the drip system hydraulic properties. The increase in back pressure is rapid at the beginning and then approaches a final value after several minutes, which allows the use of an analytical approximation that assumes steady-state conditions.

Soil variability can affect the flow rate of water from subsurface trickle emitters. This is because of a buildup of pressure in the soil when water cannot be easily transmitted away from the emitter. In this paper, an analysis was developed showing the relationship between discharge versus the design discharge as a function of emitter characteristics and the soil hydraulic properties. When the design flow volume increases or the hydraulic conductivity of the soil decreases, the pressure head of the soil next to the emitter increases, which reduces the flow rate (other factors remaining equal). The subsequent effect on irrigation uniformity was examined using soil data from a field in the Arava Valley, Israel. The average of the calculated ratio of the actual discharge to the designed discharge was 0.905, 0.825, and 0.704 for designed discharges of 1, 2, and 4 L/h, respectively. Corresponding coefficients of variability were 0.072, 0.124, and 0.193, respectively; likewise Christiansen's uniformities were 0.95, 0.91, and 0.85.

Effective use of water supplies via irrigation requires good methods for determining crop water needs. To implement irrigation scheduling programs requires an accurate estimate of water use by the crop. This study was undertaken to compare different forms of the evapotranspiration (ET) equations that include Priestley-Taylor and Penman-Monteith for reference ET. To estimate actual ET, the Priestley-Taylor with an adjusted coefficient for available soil water and the Penman-Monteith with a variable surface resistance were compared to water use for grain sorghum, cotton, and grass forage at three locations: Davis, Calif.; Lubbock, Tex.; and Logan, Utah. Both models provided acceptable results; however, the Penman-Monteith model with daily meteorological data input provided more consistent results over the growing season. The Priestley-Taylor overestimated actual ET when crops were limited in soil water because the adjustment for available soil water was not sensitive to soil-water depletion. Irrigation scheduling using the Penman-Monteith model requires daily meteorological data, an estimate of the available soil water depletion, and a measurement of crop leaf area. This method would be useful for irrigation scheduling programs.

An irrigation module delivers nearly constant discharge irrespective of water-level fluctuations in the supply channel. Low discharge baffle-sluice modules have been designed for the field outlets considering the desirable features of a good module. The flow-regulating characteristics of three baffle-sluice modules with the option of reduced module width and baffle heights are discussed in detail. The modules are found to deliver discharges within ±5 of the design value in a modular range of 10 cm. Thus the concept of baffle-sluice modules already conceived and designed can contribute to the modernization of water management systems in supplying the requisite amounts of irrigation water depending on crop requirements.

Control of seawater intrusion through a series of injection-extraction wells is studied using a vertically integrated, two-dimensional, sharp interface model. The model is based on the Galerkin weighted-residual technique and has been tested against the existing analytical solutions. The model is then cast in terms of the nondimensional parameters, and a series of parametric studies are conducted to obtain the characteristic curves for the cases of a seawater extraction barrier alone and its combination with the freshwater injection barrier. These curves could be used to assess the effect of variations in the input parameters on the position of the seawater-freshwater interface toe position.