Journal of Irrigation and Drainage Engineering

This paper presents a calibration curve using dimensionless parameters $[{\mathrm{Qm}}^{1.5}/(\sqrt{g}{B}^{2.5}\left)\right]$ and $[e^{5.5\left(\mathrm{SO}\right)}{\mathrm{mY}}_e/B]$ for the prediction of discharge $\left(Q\right)$ with the help of known end‐depth $\left(Y_e\right)$ and vice versa. The curve is based on published experimental data of different investigators for smooth trapezoidal free overfall in horizontal as well as sloping channels (both positive as well as negative slopes). The curve, best fitted to the data, shows an excellent agreement between the observed and predicted values of discharge with a correlation coefficient $\left(C_R\right)$ equal to 0.99753. The regression equation of the calibration curve has, also, been tested for unused data and an excellent agreement is obtained between the two. Furthermore, it is also observed that for horizontal channels, the theoretical value of the constant (i.e., slope) in the equation of the straight line $X_e=\left(\mathrm{constant}\right)\left(X_c\right)$ is greater than that obtained from the present investigation based on experimental data, where $X_e={\mathrm{mY}}_e/B$ and $X_c={\mathrm{mY}}_c/B$. The constant obtained in this work gives better results as compared to that obtained in previous research.

The basic unit in a free‐surface controlled water distribution network could be composed by a single reach with upstream and downstream gates. Different methods have been proposed in the literature to control water level, volume, or discharge in the reach. All of them use the same basic intrinsic assumption about the controlled reach: Every magnitude of intake and offtake of the reach has to be known. Consequently, unknown disturbances cannot be compensated. This paper proposes a new method with the purpose of manipulating the upstream gate to make the downstream discharge follow the demanded set point. It is formulated by means of a discrete‐time algorithm that computes, at each sampling instant, the control upstream gate opening using the measurement of the water levels at both sides of the gates as feedback information. The method allows the presence of unknown disturbances in the reach in the form of intake or offtake discharges, and it is able to drive the downstream discharge to the demanded set point and maintain it in spite of them.

The advantages of bentonite clay for sealing applications are well known. Bentonites have extremely low permeability, do not affect formation water chemistry, and have the ability to swell and deform in response to subsurface changes. The major limitation on the applicability of bentonite for water‐well annulus sealing is strength. Strength tests conducted in a physical model of a water well identified the expected magnitude of shear strength for several commercially available bentonite well‐sealing products. The dependence of bentonite strength in the annulus of a water well on both setting time and borehole geometry is discussed. Force balance calculations for seals constructed of both slurry grouts and granular products are presented. Seals constructed of bentonite slurry grouts do not have sufficient shear strength to resist hydrostatic forces in confined aquifer installations. Annulus seals constructed of granular bentonite products possess adequate shear strength to resist push‐out forces in limited instances. Designers of bentonite water‐well annulus seals in confined aquifer systems must be aware of the strength limitations of both granular‐ and slurry‐type bentonite‐clay well‐sealing products.

Irrigation water‐management characteristics within the command area of an arranged‐demand irrigation canal were examined, including on‐farm scheduling practices. Supply/demand ratios were used as the key indicators for the conceptual framework; however, spill ratio and the farmers' satisfaction with the system were also considered. To minimize the cost the methodology considered existing data sets and used secondary data where appropriate. Data sources included direct measurement, existing data sets, farmer questionnaires, and simulation of hypothetical demand. Total water use was not sufficient to meet the hypothetical demand of the crops, although some form of scheduling technique was used by more than half of the respondents. The farmers were satisfied with the level of service, although land loss due to seepage was a major concern. There were similarities between the hypothetical demand and actual supply hydrograph, although significant deviations over short time periods suggested that forecasting of actual demand at the canal level using evapotranspiration data would not be practical in this arranged demand canal.

The disposal of saline drainage wastewater in agricultural fields, mainly in arid and semiarid regions, is a problem that has plagued farmers, researchers, and decision‐makers. The problem has received increased attention not only because of saline wastewater, but also due to the environmental damage caused by the elevated concentration of some trace elements. A conceptual framework for on‐farm recycling of the saline wastewater is proposed as a means to diminish environmental damage and to reuse water efficiently. The support decision‐making model is based on data collected in the San Joaquin Valley, California, and is intended to predict the water flow rates of the various system components towards reuse. The model assumes that agricultural practices of surface water application will not change soon, and that drains have been installed in the fields. According to the case study, the cost of on‐farm reuse of saline wastewater is in the range of $0.07–$0.26 per m³.

The computation of wetted‐perimeter‐dependent intake in a furrow irrigation simulation model using an empirical infiltration formula was examined. Different formulations were applied under increasing and decreasing depth of flow (and wetted‐perimeter) conditions. Four computational methods were compared for the former condition and only one was tested for the latter. The merit of the procedures was evaluated in relation to the performance of a finite difference unsteady surface flow model. The hydraulic model was developed using fixed spatial increments and variable time steps and solved with a shooting technique. Differences in advance trajectory resulted from application of each formulation, and numerical accuracy was also different with each computational method. Convergence of the shooting solution was obtained with all infiltration formulas during the advance phase, but there were noticeable differences in convergence rate. Only one of the formulas provided consistent results during storage and recession calculations. Numerical instability resulted as well from the application of the procedures with limited convergence.

This paper analyzes the convergence of a numerical model for the quantification of lumped furrow infiltration parameters. The model minimizes the squared difference of observed and model‐predicted advance times as a function of distance. An alternative objective function was formulated in terms of velocity of the advancing wave. Predicted values were obtained with a hydrodynamic irrigation model with wetted‐perimeter‐dependent infiltration. Solutions were computed with the Marquardt method. The three parameters of the empirical extended Kostiakov equation were estimated when nonuniformity of infiltration was the only source of variability in the advance curve. Faster convergence and larger radius of convergence resulted from fitting velocities rather than advance times. Measurement errors and system perturbations impeded the simultaneous identification of three parameters, while computation of two coefficients from highly variable velocities required the application of weighted least squares. Similar parameters were obtained from few or many observations, but convergence improved with smaller data sets.

An analytical solution for regional flow in a converging or wedge‐shaped aquifer is given. Darcy's law and the continuity equation, expressed in cylindrical coordinates, were solved for flow in a vertical cross section of a two‐dimensional aquifer. The lower horizontal boundary and the vertical boundaries at $r=0$ and $r=a$ are no‐flow boundaries. The upper horizontal boundary is a prescribed head consisting of a linear increase with radius onto which is superimposed a sine wave. In plan view, the streamlines are radially convergent. The converging system shows a concentration of streamlines near the divide and a spreading of streamlines near the apex due to the prescribed potential on the upper boundary that limits the rate of water movement toward the apex. The effect of relief on local flow systems is limited, especially in the region near the valley bottom.

The sound development of irrigation and drainage enterprises in developing countries is a continuing need to reduce hunger, environmental degradation, and population growth, and to promote political stability and international trade. The disappointing results of many projects have been caused by a fragmentary rather than holistic approach to management. While advanced technologies can help, improved management, making use of already‐known technologies, is of far greater importance. A holistic approach should include application of management principles to improve interagency coordination; adopt performance incentives for staff and individual irrigators; use indigenous knowledge; and secure effective local participation for concurrent corollary activities including supply of inputs, agricultural extension, processing and marketing, farm credit, and operation and maintenance of the distribution system (mainly the lower portions serving individual farmers or blocks of farmers). International and bilateral agencies should collaborate more closely than heretofore and should encourage the use of holistic approaches including policy changes concerning water charges, crop supports, import and export taxes, and coordination of indigenous agencies.

Background, general instructions, and examples are given for determining runoff curve numbers (CN) from small watershed event rainfall and direct runoff data sets. The technique matches rank‐ordered (i.e., sorted) rainfall and runoffs, which preserves the return‐period matching between the rainfalls and the runoffs. Usually a secondary trend of CN with the storm rainfall itself emerges, and three different patterns are observed; complacent, standard, and violent. The standard and violent cases lead to a constant CN with increasing storm size, but the complacent case does not lead to a stable determination of curve number. Some measures of asymptotic attainment to the fitting equations and to the hydrologic definition of the watershed are also given.

A methodology is presented for optimal design of hydraulic structures in a branched irrigation canal network subject to parametric and objective uncertainty. Monte Carlo simulation of steady spatially varied flow is incorporated into the Hooke and Jeeves direct‐search method to determine a high‐performance, low‐cost solution. Fuzzy membership functions address subjectivity associated with interpreting expected values of performance measures in light of water delivery objectives. Solution of an example problem yields an optimal design for 10 pipe diversion structures in a lined canal network. Water supply level, potential crop evapotranspiration, canal hydraulic resistance, and irrigation application efficiency were modeled as random variables. Results indicated that predicted performance for a given design can be subject to significant variability. Coefficients of variation in system performance measures for adequacy, efficiency, dependability, and equity of water delivery were 0.13, 0.04, 0.17, and 0.20, respectively, at the optimal design. The method describes variability in system performance and relative cost for non‐optimal designs as well. Such information is valuable for assessing the anticipated impact of alternative designs on overall system performance.

Runoff and sediment yield for 30 runoff events on three experimental watersheds are calculated using the agricultural non‐point‐source pollution (AGNPS), areal non‐point‐source watershed environmental response simulation (ANSWERS), and chemicals runoff and erosion from agricultural management systems (CREAMS) runoff‐erosion models. The results are compared with measured runoff and sediment yield. The computed and measured runoffs show reasonable to poor agreement. The average ratios of computed to measured sediment yields for the various storms and watersheds show a large scatter. ANSWERS provides the most consistent results for estimates of runoff and sediment yield. All three models tend to underestimate sediment yield for large storms. For high intensity and low intensity storms on two small watersheds, the detachment models in ANSWERS and CREAMS have biases (ratio of calculated to measured sediment yields) that range between 0.9–1.0 and 0.4–1.6, respectively.

The study presented here consists of rainfall‐runoff events produced with a large rainfall simulator. Three test plots were 2.44 m wide, with lengths of 6.1 m, 13.72 m, and 24.38 m, and a slope of 2%. The soil on the plots was loamy sand. Rainfall intensities of 44 mm/h, 79 mm/h, and 160 mm/h were run for 1 h for each test. Samples of the runoff were analyzed for sediment particle‐size distribution, which led to several conclusions about the processes governing the erosion rates. Transport capacity tended to limit the erosion rate of larger particles, and supply limited the erosion rate of smaller particles. The data demonstrate that evaluating a single detachment or transport rate and assuming that it is constant for a season, or even a storm, may not always produce accurate results. The variations of these rates, due to armoring, must be taken into consideration.

Open-channel hydraulics is a subset of shallow-water theory that, in turn, is a subset of hydrodynamics. The distinguishing feature of shallow-water theory is the assumption of hydrostatic pressure. One-dimensional, open-channel hydraulics goes further in that it averages the velocity in both the vertical and the horizontal directions. The derivation of the open-channel equations from the general hydrodynamics relationships displays the approximations. The hydrostatic approximation is well documented in the literature, but some confusion persists in the averaging process, a confusion that leads to erroneous definitions of critical depth, Froude number, and even Bernoulli's equation. Critical depth must be defined so that it displays the singularities in the unsteady and steady equations of motion. Bernoulli's equation stems from conservation of momentum and should not carry an energy correction factor for nonuniform velocity distribution. In most circumstances, the error made by neglecting momentum and energy correction factors is tertiary, smaller than errors of erroneous friction and nonhydrostatic pressure.