Journal of Irrigation and Drainage Engineering

Examination of 15 World Bank—funded irrigation projects in sub‐Saharan Africa revealed no specific pattern of project performance or cost of development of irrigated agriculture in the area. Some of the projects have been successful while others have had questionable economic and social impact. The cost per hectare of irrigated agricultural development in sub‐Saharan Africa varies widely. The costs for some projects have been highly competitive compared to other parts of the world, while others are among the most expensive irrigation projects in the world. The success or failure of irrigation development in the region appears to be due to good or poor management from planning through operation and maintenance. With careful planning and management of the project, cost of irrigated agriculture development could be comparable to other parts of the world. In most sub‐Saharan African countries, major emphasis has been placed on large‐scale modern irrigation schemes. These countries must choose the right balance between large‐scale publicly managed schemes and small‐scale privately managed irrrigation systems. Priority should be given to development of small‐scale private‐sector irrigation development and improvement and modernization of traditional irrigation systems. Rehabilitation of existing projects should also receive priority. Potential of large‐scale irrigation development in selected areas should be explored and examined carefully. In general, the potential of large‐scale development has been overestimated because of the scarcity of information upon which the projects are planned. In sub‐Saharan Africa, potential of ground‐water irrigation has not yet been fully explored. Pilot projects in ground‐water irrigation should be conducted in selected areas to test economical and technical viability of ground‐water irrigation in sub‐Saharan Africa.

Backwater curves in circular sewers and drainage conduits are considered by an explicit approach. The new method is based on simple expressions for uniform and critical flows, and a transformation of the longitudinal coordinate by which the effect of relative uniform flow depth may be incorporated. As a result, the method presented allows the explicit determination of any backwater effect. This is of engineering concern, particularly in large sewer systems where the computational effort may now considerably be reduced. The degree of approximation is analyzed and it is found that the present solution may satisfy the demands in practice. Further, the solution is discussed and referred to the well‐known Bresse and Chow classifications. Considerations regarding basic lengths of backwater curves are presented, including the maximum extension of M‐ and S‐curves. Finally, a computational example illustrates the application of the results, to a grade break involving the transition from sub‐ to supercritical flow.

The assumption of considering the lower layer to be impervious, if its hydraulic conductivity is one‐fifth or less from that of the upper layer, leads to erroneous results. Many research workers have attempted solutions to the problem of subsurface drainage in layered soils with different boundary conditions. An alternate, simple, and explicit solution based on Girinsiky's potential has been proposed in this paper. Experimental data observed on a vertical Hele‐Shaw model have been used to verify the analytic results.

Considering the minimum cost as the objective, the furrow irrigation system design problem is formulated as an optimization problem. A combination of monotonicity analysis, constraint activity, and condensation are used to simplify the size of the mathematical model of the problem. This reduced model is used to analyze the sensitivity of the optimal design variables of a furrow irrigation system. The Soil Conservation Service (SCS) equations for the design of furrow irrigation along with several constraints are considered in the optimal design. Using an example problem and three different soil types, the system cost and design variables (inflow rate, length of furrow, and time of cutoff) are found to be sensitive to the supply flow rate, the design application depth, and the cost coefficients of water, labor, and ditch construction. Under the given set of conditions, the cost of a furrow irrigation system is very high in the case of a clay soil. Also, the application efficiency achieved was lower in the case of a clay soil than in light (sandy) and medium textured soils.

The general problem of solving for normal flow depth in open‐channel flow has a complication in that some types of channel cross sections do not always have a unique solution. This paper analyzes an alternative iterative procedure for quickly and accurately solving the implicit problem of determining the normal flow depth in complex channel sections. Conditions that guarantee a unique solution and guarantee that the iterative procedure will converge to the solution are developed. A computer program for quickly and accurately finding the unique solution, using the Chezy or Manning flow resistance equations, is available. Test runs for a rectangular, a triangular, a trapezoidal, and two complex channel cross sections are used to evaluate the effectiveness of the procedure. The test results show that the iterative procedure presented here meets the requirements of guaranteed convergence, computational efficiency (speed and accuracy), and the ability to handle both trapezoidal and complex channel cross sections.

An interactive computer program was developed for the design of inverted‐siphon conduits. In addition to the basic hydraulic processes involved in the design process, the program takes into consideration the head loss due to sediment transport, upstream and downstream protection, and seepage analysis. The program was written in FORTRAN 77 and run on a Macintosh computer. Application of the program was illustrated by means of a typical design example. The program is easily adaptable to other microcomputers.

The performance of subsurface drains in a clay soil overlying an artesian aquifer is evaluated at a site in Imperial Valley, California. Field studies are undertaken to examine water and salt movement through the soil profile and the response of subsurface drains under surface irrigation. Water penetrated to all depths soon after the passing of the irrigation advance front. Peak drain flow occurred shortly after irrigation had stopped. Soil and drain‐water salinity remain nearly constant throughout the year, and have changed little over the past several decades. The drains intercept only a small percentage of deep percolation water and do not contribute significantly to the overall drainage of the field.

Subsurface drains in clay soil overlying a shallow artesian aquifer intercept water from the aquifer and the deep percolation from the root zone. Flow down the backfilled drain‐line trench is significant immediately after irrigation. The zone of influence of such drains is very limited, and only a small percentage of deep percolation is intercepted by the drains. A mathematical model is constructed to simulate these different sources of flows contributing to total drain flow. The model was calibrated with field data described in the companion paper, and results from the model were used to develop empirical equations quantifying the contribution of flows from different sources to total drain flows.

Single‐sprinkler wind‐distorted distribution patterns are simulated utilizing drop trajectory computations, and compared with measured patterns. It is found that the exact formulation of the drag coefficient of single drops is not critical for applications focusing on water distribution. A drag correction factor, k is introduced to account for the effect of the incidence angle between drag force and orientation of a jet segment. The agreement between measured and computed patterns improves considerably when the proper value of k has been used. Three measures of similarity of computed to measured single patterns are put forth. The most comprehensive measure, denoted by σ, is a normalized sum of squares of differences between the measured and computed local application rates. The values of k that produce the best agreement in terms of σ seem to be an increasing function of the range of a sprinkler. Single patterns computed with the best values of k were used to calculate uniformity coefficients for a realistic spacing of the medium‐pressure sprinklers under consideration. The results suggest that the computational method can be used to forecast, with an error of only a few percent, the uniformity of application under wind speeds as high as 8 m/s. Considering the effort involved in field measurements, this seems a viable alternative.