Journal of Irrigation and Drainage Engineering

The dynamics of soil moisture in the root zone is of fundamental importance in agricultural water management. A simple, yet physically based model is presented which describes the processes of redistribution of the soil moisture profile, drainage, and evapotransportation (ET) from the root zone. Using the sharp wetting front approximation, relatively simple expressions are derived that are explicit in parameters describing the soil and climate/vegetation properties. Application of these models using parameter values selected a priori indicates good results. Comparison of the redistribution and drainage models with laboratory data shows good agreement, as does comparison of the ET model with field soil moisture measurements. Uncertainty in soil parameters, root zone depth, and timing of irrigation applications affect model performance.

Results of a field investigation on 10 selected deep tubewell irrigation systems in Bangladesh are presented. Development and application of a service area model to predict the impact of various improvement options are also discussed. The majority of the deep tubewells in the study are found to be severely underused in both the duration and efficiency of pump operation. The impact of each improvement alternative on service area expansion, as predicted by the model, varies among the tubewell systems. Increasing the pump operating hours is found to be the most effective measure for expanding the service areas of most study systems. Data used in the model application and economic analysis were obtained from field studies, technical papers, previous study reports, and judicious assumptions. Further verification of the model and updating of the conclusions may be needed. However, the model can be a useful tool in formulating successful service area development programs.

To clarify the nature of irrigation efficiency, economic efficiency is defined and its relation to technical efficiency examined. Yield efficiency in water demand and cost efficiency in water supply are both necessary for technical efficiency but are not sufficient conditions for maximizing profits. Maximizing the utility from income while accounting for risk is another form of economic efficiency and may be more descriptive of managers who are risk‐averse.

Canal cross‐section designs for uniform flows are contrasted and compared by using nondimensional shape parameters. The basic relations among the cross‐section shapes and design variables (the wetted perimeter, the water depth, the water surface width, the cross‐sectional area, the lining volume, the excavation volume, etc.) are exposed. These relations are used to uncover robust rules that can determine optimal canal designs for elementary problems, directly from flow information such as capacity, velocity, slope, and roughness. For problems involving complex limits and economics, the relations are combined with optimization methods to solve for the economically optimal cross sections. The possible cross sections are parameterized by at most two variables, so the calculations do not require the use of sophisticated optimization methods or large computers.

Variations in surface storage capacity over a watershed produce partial areas of saturation overland flow, which vary from storm to storm. A method is described by which the proportions of a watershed that contributes overland flow in different storms and at different times during the same storm can be determined by analysis of rainfall and runoff records. The method is demonstrated on two catchments in Queensland, Australia, with different hydrological characteristics. One watershed is 16.8 ha in area with only two to three events per year of overland flow (no baseflow) averaging 35 mm per annum runoff; the other is 7 km in area with substantial baseflow and an average of 920 mm per annum runoff. The calculated amounts and proportions of surface storage capacity are incorporated into water balance models of watershed runoff to show the accuracy of runoff estimation.

A complete algebraic model simulating furrow irrigation is presented. It can be used to analyze and predict the performance of sloping openended furrow irrigation systems, based upon the solution of a mass conservation equation. The model input variables are: length, slope, Manning roughness, infiltration parameters, furrow shape, inflow rate, and cutoff time. All the information necessary to estimate the distribution of water and water‐application efficiency for a particular irrigation event are obtained. Model performance is evaluated by comparison with field data and a more complex hydrodynamic model. Results show that the model gives reasonable results and, in most cases, more accurate results than more complicated models.

Nonlinear optimization models are developed to design and manage furrow and border irrigation methods. The objective function of the basic nonlinear model is constructed on the basis of a profit relationship, while that of the expanded nonlinear model is constructed in terms of benefit and costs. The models are solved with the MINOS 5.0 package. The values of the design variables yielded by the nonlinear models are similar to those given by an existing linear optimization model. The expanded nonlinear model gives objective function (profit plus other costs) values that are greater than those of the basic nonlinear and linear models.

Acre‐foot is an important water measurement term in the United States, particularly in the American west. Although less common, its derivative, acre‐inch, is widely used within the irrigation community. No meaningful equivalents for these terms exist in the current International System of Units (SI). The two metric equivalents proposed here, hectare‐meter for acre‐foot and hectare‐centimeter for acre‐inch, are presented as alternatives to those generally found in SI conversion tables. Each is conceptually similar to its counterpart in the U.S. Customary System, thus creating comparable mental images that, in turn, greatly aid comprehension. Official recognition within SI of these two proposed metric equivalents, deserves serious consideration. During the course of almost a century, acre‐foot and acre‐inch have evolved with the needs of irrigation and hydrology in the United States. As a result, they have acquired not only practical but cultural and historical significance, becoming part of a heritage that should not be lost. Measurement bilingualism is recommended as a new policy for the federal government, to replace that of voluntary metrication adopted in 1975. Such a policy would expand our communication skills while maintaining linguistic diversity.

A simple method called “upward infiltration method” is described for using common soil laboratory equipment to measure unsaturated hydraulic conductivity of soil samples in a short time. The method is based on measurement of water absorption rates under controlled tensions, which leads to an estimate of soil hydraulic conductivity during the wetting phase. The results are compared with those evaluated by the constant application rate method, the small increments of pressure change method, the one‐step method, and Jackson's theoretical model. The results of the upward infiltration method are in close agreement with the constant application rate method and are generally higher than the one‐step method and lower than Jackson's theoretical model. The measurement by this method is limited to about one meter of negative water pressure.

The discharge‐head relation of flows in circular and U‐shaped channels under circular thin plate gates is established. The coefficient of discharge is determined experimentally and it is found that only the effect of the relative gate opening is significant. Peculiarities of the free surface downstream of the gate section are described in detail.

A new approach to the design of flumes and weir for open channels is developed. The approach is based on matching the flume to approach channel conditions thus providing an appropriate Froude number in the approach channel for the passage of sediment and for producing a stable readable water surface at the head measurement station. The procedure is graphical and is applicable to approach sections and control sections of any shape.