Journal of Irrigation and Drainage Engineering

A terminology for describing irrigation conveyance systems is presented. It is modeled after one introduced by the International Commission on Irrigation and Drainage (1978), and Bos (1979) and (1985); however, this work was significantly modified to provide a greater degree of comprehensiveness and to remove ambiguity. The terminology presented is based on a classification of irrigation conveyance subsystems identified by their relationship to the irrigation water demands they are intended to satisfy. The terminology ignores the management/ownership characterstics of conveyance systems and their relationship to water supply. These conveyance system attributes, as well as methods with which to identify and provide names for different segments of irrigation conveyance systems, are discussed in terms of the additional descriptive information function they provide. The terminology developed permits a concise relevant discussion of irrigation conveyance system design and operations.

The Mein‐Larson extension of the Green‐Ampt infiltration model is extended further to analyze the infiltration process under a variable rain on a covered natural soil. Such a condition represents the infiltration under center pivot irrigation when the vegetation cover is developed. A graphical procedure is developed to handle the calculations. A numerical example is used to illustrate the application of the model to determine potential runoff. The performance of the model is demonstrated by two numerical examples presented by Heermann and Duke (1983) and is compared with the performances of two existing models.

For flow past a two‐dimensional sharp‐crested weir located at the end of a horizontal rectangular channel, experimental results related to velocity and pressure distributions in the region of the nappe close to the weir crest section and the pressure distribution on the weir face were obtained. Based on experimental results and simplified theoretical considerations, a general relationship between the weir discharge coefficient $C_d$ and the parameter H/W (or W/H) is determined using the momentum principle. The present experimental results corroborate the proposed relationship for the weir discharge coefficient.

With rapid growth of agriculture coupled with increasing population and urbanization, the demand for water in the Kingdom of Saudi Arabia is continually on the increase, causing alarming drawdown in the deep aquifers, the traditional source of water. Sewage could be a valuable source to augment this dwindling water supply, at least the irrigation supplies, and should not continue to be wasted. However, between the two extremes of use of untreated sewage for irrigation as practiced in some countries and very stringent standards that necessitate an expensive tertiary treatment, there exists a middle‐of‐the‐road approach. This moderate alternative combines flexibility with a permit system on a case‐by‐case basis to make the best use of this valuable commodity. This viewpoint is strengthened by the experimental results obtained on a $50‐m\times 25‐m(160‐\mathrm{ft}\times 80‐\mathrm{ft})$ University of Petroleum & Minerals (UPM) farm, where secondary treated effluent with chlorination was used through a sprinkler system to grow alfalfa, summer squash, and onions based on the bacteriological analysis of the crops. Possible guidelines for establishing water criteria for the wastewater reuse for irrigation in the kingdom are suggested in light of the experiments and taking into account the practices in various countries.

A total of 14 methods to predict $K\left(h\right)$ from soil texture and/or other data were tested. Nine of the methods involved first predicting $K\left(h\right)$ from texture and/or other data, then using a method to predict $K\left(h\right)$ from $\theta \left(h\right)\text{.}$ Using each of these methods, $K\left(h\right)$ relationships of several soils were predicted. Some of the methods required matching the predicted $K\left(h\right)$ to the measured $K\left(h\right)$ at saturation. The predicted $K\left(h\right)$ was compared statistically and graphically to measured $K\left(h\right)$ data, which is given in the literature for these soils. Results showed that for the methods that did not match the predicted $K\left(h\right)$ to the measured $K\left(h\right)$ at saturation, a method by McCuen, et al., provided the best estimate of the measured $K\left(h\right)$ Additionally, matching the $K\left(h\right)$ at saturation improved the performance of the McCuen, et al., method. The resulting method provided the best estimate of the measured $K\left(h\right)$ of all the methods tested for the loamy and clayey soils. For the sandy soils, substitution of Ghosh's closedform equation for $\theta \left(h\right)$ into the Burdine equation for K resulted in the best approximation of $K\left(h\right)$ of all the methods tested.

A numerical model is developed for simulating the interaction of a surface water body with a variably saturated porous medium. A boundary condition simulating the soil‐water pressure head at the boundary between the porous medium and the surface water body, expressed as a function of the soil‐water flux at the boundary, is also integrated into the model. The interaction of the surface and subsurface flow system leads to a more accurate simulation of soil‐water flow velocity, which is of importance in problems dealing with the seepage of pollutants into the subsurface environment. The Galerkinfinite element method is used to solve the variably saturated flow equation and the Darcy equation, employing isoparametric elements that simulate the physical geometry of the porous medium. Solutions to the model give the transient position of the stage of the surface water body and the distribution of the phreatic surface in the porous medium, providing robust estimates of the rate of seepage into the porous medium. The capability of the model is illustrated by considering the interaction of a semipervious surface pond with a variably saturated stream‐aquifer system.

An analytical solution is derived using the Boussinesq equation to describe an unsteady rising water‐table profile in an aquifer lying over a mildly sloping impervious bed in response to constant replenishment. A Hele‐Shaw model is used for experimental verification of the analytical solution. Reasonably close agreement is found in the computed and observed phreatic surfaces. The analytical solution can be used with reasonable accuracy for the design of subsurface drainage in sloping phreatic aquifers.

A unique condition exists for the country of El Salvador: By knowing only the elevation, the type of crop suitable for the area and the crop water requirement can be evaluated. This paper shows how elevation can be used to estimate the maximum and minimum temperatures for crop selection and for estimating crop water requirements. The need of simple, reliable, and easy‐touse methods for scheduling irrigation is emphasized. Few irrigators are literate and only a very small percentage have received training and/or instruction relative to the amount and frequency of irrigation. Approval of a $25,200,000 water management project to promote the production of high value, labor‐intensive crops gives emphasis to the needs for improved extension programs and for farmer training in irrigation scheduling.

A model for the transient one‐dimensional water movement in the saturated‐unsaturated zone using a finite difference method with stochastic inputs is developed. Hysteresis in the soil water retention and hydraulic conductivity‐ versus‐moisture content relationship is incorporated. The model considers layered geologic formations with each layer having different parameters. Monte Carlo simulation, together with the nearest neighbor model, is used to incorporate the stochastic nature of hydraulic conductivity into the flow model. Outputs of the flow model include pressure head, water content, and the watertable elevation. Two Monte Carlo simulations of 100 realizations each are made for a 12‐day simulation period. Input standard deviations of the log saturated hydraulic conductivity are assumed to be 20% and 40% of the mean of log saturated conductivity for the two Monte Carlo runs, respectively. The standard deviations of the water‐table elevation and pressure head increase with an increase in the standard deviation of log saturated hydraulic conductivity. The model is applied to predict a long‐term water‐table fluctuation, and the predicted water table agrees well with the observed one.

The potential of canal side weirs for uniform water delivery to irrigation furrows and system flexibility is assessed. Weir discharge uniformity is affected by: (1) Canal velocity and weir entrance conditions; (2) variations in weir crest construction elevations; and (3) variations in water surface elevations along the distribution canal. For sharp‐entranced weirs, model studies show that the decrease in canal flow velocity down a typical distribution reach can result in an increase in weir discharge up to 15%. This velocity effect is significantly dampened by using a streamlined entrance to the weir. Field data on weir crest elevations have a standard deviation of about 3 mm, which translates into 6–8% weir discharge variation at normal discharge settings. A computer model of distribution canal flows indicates weir discharge uniformity is improved by shortening the length of the reach, reducing the required weir discharge setting, and increasing canal size. A design guideline is developed for the system.

Models for one‐, two‐, and three‐dimensional infiltration for irrigation are derived using an energy equation and Darcy's law. The models were verified with Fok and Chiang's (4) experimental data. A good agreement is found between the model‐computed and observed values of infiltration. Average relative error of cumulative infiltration values is less than 5%.