Journal of Irrigation and Drainage Engineering

Irrigation in a river basin generally implies that the drainage water from upstream schemes is reused in the downstream schemes. The processes of water and salt movement in the schemes (over the soil surface, through the soil profile, and through the substrate) are successfully modeled in other studies. In the model presented here (called RE‐USE), these processes are linked for a series of irrigation schemes along a river. The model can be run for variable conditions (agricultural, soil, hydrological, and climatological) and variable regimes (irrigation and drainage), although these conditions and regimes have to be similar for all considered schemes. Time‐dependent variables may be defined on a daily basis (e.g., for rainfall) or on a monthly basis (e.g., irrigation‐water requirements). The model is used to explore relationships between annual rainfall, irrigation efficiency, and irrigation‐induced soil salinization. The usefulness of the model in evaluating strategies for irrigation‐water management in a basin has also been demonstrated.

Using a linearized finite‐difference model of open‐channel flow, the canal operation problem was formulated as an optimal‐control problem, and an algorithm for gate opening in the presence of arbitrary external disturbances (changes in flow rates) was derived. An observer was designed to estimate the values for depths of flow and flow rates at the intermediate nodes based upon measured values of depth at two points in the pool. Control algorithms were derived for both a constant‐level control and a constant‐volume control. For the purpose of evaluation, the equations of the controller and the observer, derived with the linearized model, were included as modules in a nonlinear open‐channel flow model. In an example problem with an external disturbance of 20% of the initial flow rate in the pool, the performance of the control algorithm and the observer in maintaining a constant level at the downstream end of the pool was found to be acceptable.

A method for designing open drainage channels that incorporates the uncertainty associated with various design variables is presented. The design involves two random components: runoff (demand) and channel capacity (supply). The runoff is formulated considering the uncertainties of rainfall intensity, drainage area, and other watershed characteristics. The channel capacity is formulated considering the uncertainties of channel friction factor, longitudinal slope, channel width, side slope, and water‐flow depth. The probabilistic characteristics of the runoff and channel capacity are established based on the first‐order principle of probability theory. A reliability index that can be used to design an open channel to convey runoff for a given probability of failure is developed. This probabilistic approach should be valuable in the design of open drainage channels whose runoff and capacity variables are generally uncertain.

Farmer participation in the development of their irrigation systems promises greater farmer satisfaction with the system. It also relieves government of the administrative and cost burden associated with the operation and maintenance of irrigation systems. There are important differences between surface‐ and ground‐water irrigation systems, which are reflected in their operation and maintenance. These in turn impact on the organization and, more specifically, on the form that participation can take. Ground‐water irrigation systems using electrical or diesel pumps are much more cash‐intensive than surface‐water systems, which require mainly labor input from their members. Tube well systems are highly dependent upon a support system of workshops, mechanics, and electricians, and on the supply of diesel fuel or electricity, all of which require payment in cash. Surface‐water irrigation systems are less dependent on support from urban centers, and may be more self‐reliant. These factors affect the nature of farmer participation in tube well systems and the role of the agency promoting participative development.

It is surprising to note the lack of information usually given by manufacturers about their sprinklers, making it very difficult to correctly select a sprinkler and its working conditions when designing a sprinkler irrigation system or improving an existing one, using only the discharged volume and radius of throwing for every pressure‐nozzle combination. This paper shows the results of two studies made principally on three individual sprinklers: one measuring the single‐leg distribution curves and the other measuring the complete water distribution model of a sprinkler in windy conditions. Our purpose is therefore to establish a set of general rules valid for selecting a sprinkler. These rules must take into consideration the optimum working conditions that will maximize the possible uniformity of water distribution.

The purpose of this study is to develop function relationships for the velocity distribution of water flow in and above a model of branched grass. A trapezoidal cross‐section flume is constructed for this study, and branched plastic strips are used to simulate the grass in nature. A correct determination of flow characteristics for channels suffering from submerged weeds is desirable. Furthermore, knowledge of the changes of velocity distribution as a result of the presence of weeds and the changes in the shear stress at the bed will improve understanding of the different problems associated with this type of channel. Power‐ and logarithmic‐function relationships are developed to describe the velocity distribution of water inside and above branched flexible‐roughness elements (which represent the submerged weeds) in trapezoidal open channels. An experimental study was carried out in the hydraulic lab at Cairo University, Cairo, Egypt, to check the validity of the prediction equations that relate the velocity distribution to the depth of water in the channel cross section. More than 170 velocity profiles have been measured at different locations along the channel length and its cross sections.

The Irrigation Land Management (ILM) model was developed as a research tool to simulate the demand and response of a multiple‐field multicrop irrigation system in a variety of environments in order to assist managers in exploring strategies that can be used to improve system performance. It addresses the two‐fold problem of first determining the aggregate irrigation water requirement for a command area represented by multiple landowners, cropping patterns, and water‐management schemes; then, once water is delivered, simulating the command area's response to the water supply. It includes field‐ and weather‐generation components to assist in the exploration of alternative management strategies. The model was validated using field data from a large irrigation system in Utah and used to analyze its current management program.

A simple hydrologic balance model based on neutron‐probe measurements of soil moisture content was used to estimate evapotranspiration for alfalfa. The model results indicated large variations in the amount of estimated daily evapotranspiration for individual neutron‐probe access tubes, although field and crop conditions were the same. To identify the sources of variation in the depth of stored soil moisture that would affect an evapotranspiration estimate an analysis was performed. The variation in the amount of daily evapotranspiration was associated with a calibration component and an instrument component, both of which contributed to variance in probe readings at a given time and location. An experiment was conducted in the field to quantify the expected variation in local soil moisture measurements. From this experiment, procedures for reducing the variation due to local measurement errors were derived. The instrument component of the variance in the depth of stored water over the soil profile was linearly reduced by decreasing the soil layer thickness, e.g., decreasing the vertical distance between successive neutron probe readings. The instrument component of the standard error in the depth of stored water was reduced by increasing the number of neutron‐probe measurements per soil layer. A method was developed to quantify the minimum number of days between soil moisture measurements to achieve a specified error criterion.

Previous experimental works provide different values of the ratio between the end depth and the critical depth. Differences depend on the crosssection shape of the channel, the roughness of the channel, and the characteristics of the nappe (confined or free on the sides). In this paper, the characteristics of a free overfall in a rectangular channel, having different values of the width, are experimentally studied. The measurements show that for practical application, the relationship between the end depth and the critical depth is independent of channel width. For any rectangular channel of given width, a relationship to estimate the discharge by using the depth measured at the end section is proposed.

In this study, numerical investigations of flow over side weirs in triangular channels are reported. Numerical models are obtained from energy principles and solved by a finite difference method. The resutls are presented in graphical form. The study covers sub- and supercritical flow regimes. Derived expressions for the side-weir discharge and water-surface profiles for these regimes are compared with experimental rsults.

Four charts are presented for the selection of standard concrete trapezoidal canals. The charts cover Systeòme International (SI)– and English-dimensioned canals. If the channel invert slope and flow are known, a standard section can be chosen (bottom width and side slope) and the flow depth determined. Alternately, the invert slope and average cross-sectional velocity can be used as a basis of selection. Procedures are given for using the charts for channels of Manning roughnesses not included in the standards.

The evaporation rate from a class "A" evaporation pan located in a site surrounded by low-growing vegetation can be used to estimate potential evapotranspiration for a grass reference. The evaporation rate for pans and the evapotranspiration rate of grass differ depending on siting and weather conditions. Weather-based pan coefficients for the conversion of pan evaporation to evapotranspiration of a tall grass reference crop have been published and equations have been developed to estimate the published coefficients. However, the equations used to estimate pan coefficients are complex. A simpler equation to estimate potential evapotranspiration for a grass reference from evaporation pan data, upwind fetch of low-growing vegetation, mean daily wind run, and mean daily relative humidity is presented. The equation can be used to automate the computation of evapotranspiration from electronically recorded evaporation pans or to analyze historical evaporation pan data. The equation can be substituted for more complex equations or for tabular evaporation pan coefficients presented in Doorenbos and Pruitt's 1977 publication FAO Irrigation and Drainage Paper 24.

The hydraulic roughness of irrigation furrows is generally assumed to be uniform throughout a field. However, water flow, which varies along furrows, reduces roughness by breaking down soil aggregates and depositing sediments on the bed. A wide range of flow rates was applied with a recirculating infiltrometer to furrow sections constructed on a wide range of slopes to determine the effect of flow depth and velocity on hydraulic resistance. Although no depth effect was measured, final (steady-state) roughness varied inversely with average flow velocity. Since furrow flow velocity varies with flow rate and slope, roughness will also vary spatially with these factors. Since roughness affects wetted perimeter and thus infiltration and surface storage and, thus stream advance and recession, these interactions influence water distribution along irrigation furrows. Including this interaction in hydraulic models will improve their ability to describe the physical system.

Circular-crested weirs are used for flow measurement and have wide applications in irrigation systems, where they serve as overflow structures. For flow past such a weir, the discharge coefficient was obtained on the basis of momentum analysis. Tests were conducted over a wide range of geometric and hydrodynamic variables characterizing the weir flow. The pressure distributions at various vertical sections and the depth of flow at the crest section were obtained for the flow over the weir. The discharge coefficient determined on the basis of momentum analysis is compared with results obtained from direct measurement of the discharge.