Journal of Irrigation and Drainage Engineering

The results of a model study of an energy dissipator, called a hydraulic jump chamber, are given. This structure uses precast concrete blocks secured to their inside periphery. The results showed that three sets of blocks were required for the proposed design to function. Compared with other terminal structures that use hydraulic jumps to achieve velocity reductions, hydraulic jump chamber has a simpler design, which may lead to lower costs. Hydraulic specifications and design procedures are given that minimize material and labor costs. The diameter of this design is given by equations fitted to experimental data generated by model studies. These equations are functions of the depth and Froude number of the flow entering the chamber. Most other hydraulic design variables are related to the chamber's diameter by dimensionless parameters, such as the spacing of concrete blocks, their dimensions, and their position with regard to the inlet and outlet of the chamber.

An analytical model is developed to simulate all phases of furrow irrigation. The model transforms any real furrow cross section shape to a discharge‐equivalent semicircular shape, and can therefore be applied to any form of furrow cross section. Parabolic shapes are used to describe the surface and subsurface flow profiles and their coefficients are determined from the conditions in the gradually varied flow region, rather than in the region of rapidly varied flow at the advance front. Infiltration is simulated in three dimensions rather than in one or two dimensions. The recession phases are simulated by modifying Strelkoff's model such that the time‐varying rate of infiltration is taken into account. One observed data set is used to calibrate the model, and six observed data sets are used to verify it. The model is simple, accurate (less than 8% deviation for all phases), and easy to apply.

The Penman equation for estimating hourly evapotranspiration (ET) of an alfalfa reference crop was calibrated for both daytime and nighttime conditions. Hourly net radiation, sensible heat flux, and soil heat flux were measured in the field for 16 24‐hr periods, and evaporative flux was determined as the residual of the surface energy balance equation. The sensible heat flux measurements were made using a portable eddy correlation system that included a sonic anemometer and a fine‐wire thermocouple. Following calibration, the hourly Penman model was applied to independent weather data from two growing seasons. Since only conventional weather parameters were measured during these two seasons, empirical equations were first developed to relate net radiation to solar radiation, and soil heat flux to solar radiation and air temperature. The resulting hourly ET estimates were summed over 2–4 day intervals and compared to ET data obtained from soil water balance measurements. Good agreement was found between estimated and measured values.

Numerous methods for estimating effective rainfall have been proposed in the past, including: direct measurement techniques; empirical methods; and soil water balance methods. The best estimates of effective rainfall can be obtained by conducting soil water balance computations. A soil water balance model (SWBM) for estimating effective rainfall was used to test the accuracy of the United States Department of Agriculture Soil Conservation Service (USDA‐SCS) and the Hershfield effective rainfall estimation methods for a well‐drained soil and for a poorly drained soil. Estimates of mean annual monthly effective rainfall by the USDA‐SCS and estimates of mean annual growing season effective rainfall by the Hershfield method were found to compare closely with estimates from the SWBM for the well‐drained soil but not for the poorly drained soil. Effective rainfall estimates by these two methods for either soil condition did not compare well with the SWBM estimates for annual events with return periods higher than the mean annual event.

Warming of the global atmosphere by 2°‐5° C, and regional changes in precipitation by mid 21st century as greenhouse gases double, is expected. A composite model predicts the consequences on the geography of U.S. irrigation. Percentages of cultivated area irrigated are projected for each state. Predictions of actual area irrigated, which will depend on economic conditions and availability of new water supplies are not made. In all cases percentages irrigated rise, with the greatest impact of a warmer climate on the agricultural economy in the Western states. In the Western states this will occur because of decreases in total area cultivated due to a loss of rainfall agriculture. In the East, irrigation will increase, accompanied by some decrease in cultivated areas. Improved use of technologies could help meet increasing evapotranspiration needs, but large new surface supplies will generally be required to maintain or increase present levels of irrigation.

The automatic operation of Xiangtan Q‐type gate is based on hydraulic pressure only. As the upstream water rises above the top of gate, the gate opens automatically to safeguard the banks against flood. As the water level drops below the normal water level, the gate begins to close for retaining water. Hence it allows unmanned operation and requires neither lifter nor electric power supply. Q‐type gate is different from the conventional automatic hydraulic gates. There is no floating body as balancing weight. The gate arm is a simple rectangular plate turning along the surface of bearing that is also a simple elliptical plate made of common steel. Owing to simplicity in structure, the cost of work is lower and skillful maintenance is unnecessary. Q‐type gate can operate in free out flow or in submerged flow. Its discharge coefficient is analogous too, even larger than that of broad crest weir and its capacity to remove flood debris is better. Since 1980, a group of Q‐type gates have been installed on weirs and have operated safely for 5–8 years without any damage. In this paper the functions of Q‐type gate and the design of its key parts will be introduced.

Land grading is an important and necessary operation for successful and efficient surface irrigation. Currently, most lands are graded or reshaped to form a plane surface with uniform slopes in one or two directions. In this research, a new technique has been developed to perform land‐grading designs by selecting a best‐fit curved or plane surface. The profiles of such surfaces along either of the two major directions are assumed to be represented by a general power function. The main goal of this technique is to minimize the volumes of earth work required while obtaining a desirable smooth surface. Furthermore, the fitted surface can be subjected to a series of constraints: Limiting the slope at any point; choosing the desired surface shape, i.e., concave, convex, or plane; and limiting the elevation of the graded surface to allow gravity irrigation from a water source. Nonlinear programming has been used to perform the fitting procedure. The results obtained from the developed procedure have been compared with those obtained from previously developed methods. The new procedure proved efficient and very flexible in selecting a suitable graded surface for a given land.

Monitoring and evaluation of irrigation projects has been a neglected subject, but must play a more important role in the future if the irrigation management process is to be improved. The process is complex, since a large number of regular, specific tasks must be performed, both concurrently and sequentially, and coordinated by a variety of professionals within available time and resource constraints. The paper points out the need for monitoring and evaluation, outlines the main requirements for a functional system, and provides a realistic framework for carrying it out. For any evaluation to be used, it must be credible—objective, accurate, and fair. Reports should be clear, unambiguous, balanced in terms of strengths and weaknesses, and contain justifiable conclusions and recommendations. For monitoring and evaluation to succeed, irrigation managers need to develop a new evaluative mind‐set that enables them to appraise their projects' performance objectively, reflect on what has been learned for future use, and adjust policies on the basis of that knowledge whenever necessary.

The Lower Colorado River Authority (LCRA) of Texas provides an average of 530,000 acre‐ft annually for rice cultivation. To estimate water needs better, predictive equations were developed, using least‐squares regression, for the monthly and annual water diversions for the three largest rice irrigation systems served by LCRA. The equations provide statistically significant and physically meaningful relationships for predicting changes in water diversions with changes in rainfall, rice acreage planted, gross lake evaporation, and planting time of first crop rice. Depending on the particular irrigation system, the equations estimating annual diversions explain between 85% and 88% of the variation in the data, and those for monthly diversions explain between 47% and 81%, with the majority of these equations explaining better than 60%. The relationships developed in this study may be used for drought contingency planning, evaluation of the impact of water conservation practices on water diversions, and estimation of the impact of using local supply sources, such as ground water, on reducing river water diversion demands for irrigation.

A technique is demonstrated for characterizing water‐level fluctuations in irrigation canals using amplitude spectral estimates obtained with a fast Fourier transform. The technique, which is often used to analyze mechanical vibrations and electrical signals, is used with hourly water‐stage data to calculate dominant frequencies, amplitudes, and peaking times of water‐level fluctuations in eight irrigation canals in Utah and Washington. The results of this analysis indicate that water‐level fluctuations dominated by a daily cycle are statistically significant and that the time of peak water level occurred between 4 and 10 a.m. in all the canals studied. Amplitudes of water‐level fluctuations in six of the eight canals that were unlined and unregulated are found to depend on canal (reach) length. Low frequency fluctuations with periods ranging from three to 10 days existed in the canals, but are statistically significant only in the spring and fall. Observations are presented about the causes and possible measures for controlling water level fluctuations in the study canals.

The complex problem of irrigation management in a large heterogeneous basin is solved by using a multilevel optimization technique. The real problem consists of determining the optimal cropping patterns in various subareas of the basin, the optimal design capacities of irrigation facilities including both the surface and ground water resources, and the optimal water allocation policies for the conjunctive use. The objective of such a management is to obtain a high level of economic efficiency in the irrigation development and water use system within a hydrologically feasible policy domain. Various alternative activities, such as surface water diversion and pumpage, ground water withdrawal and recharge, and alternative future operational scenarios, have to be analyzed in an integrated way. The solution strategy is based on the physical decomposition of a large system into interconnected subsystems. A computationally efficient algorithm that can be implemented in a microcomputer is developed to solve the multilevel linear programming model by an iterative procedure. A case study is presented to illustrate the application in a realistic situation.

The Blasius equation which has been verified by many researchers and has been recently shown to work very well for small diameter plastic pipe can be combined with the Darcy-Weisbach equation to give a simple and accurate equation written in terms of flow rate, pipe length, and pipe diameter. It allows for correcting for viscosity changes, and is very accurate for flows resulting in Reynolds numbers less than 100,000. Unlike other commonly used empirical equations in a similar form, this equation is theoretically sound and dimensionally homogeneous.