Journal of Irrigation and Drainage Engineering

The characteristics and observed stability of fine and coarse soil cover layers placed on 16 selected canal reaches to resist water erosion and to protect asphalt, plastic membrane, and compacted earth linings are reported. The canal test locations, canal design characteristics, physical properties of the covers, and observations on the performance of the covers over periods up to 25 years are given. Based on the experience of the Bureau of Reclamation with the performance of the test reaches, a range of grading for granular covers to control erosion on canal linings with similar conditions is proposed. Soils which have generally performed well in coarse cover layers on canals with tractive forces less than $0.1\mathrm{lb}/{\mathrm{ft}}^2$ $(5N/m^2)$ have contained: (1) Maximum particle sizes between 3 and 6 in. (75 and 150 mm); (2) less than 50 percent passing a No. 4 (4.75‐mm) sieve, and less than 10 percent passing a No. 200 (75‐μm) sieve. To control sloughing of cover soils, consideration should be given to the grading of fine soil beneath coarse granular layers, and where economically justified, controlling moisture and density.

Fluid flow is analyzed through a two‐stage conical diffuser used to reduce the velocity of pipe flow. An analytical approach is described for determining the optimum diameter at the interface between the first stage, a conical diffuser, and the second stage, an abrupt enlargement, such that, for given approach and exit diameters and for a given diffuser length, the energy loss coefficient is the lowest possible. Over a practical range of diameter and length ratios (ratios of exit diameter and diffuser length, respectively, to approach diameter), the analysis demonstrates that the optimal internal planar angle varies from $5°47\prime$ to $17°11\prime$ (0.101 rad–0.300 rad), and the energy loss coefficient to be applied to the approach velocity head varies from 0.01–0.25. The energy loss coefficients for the optimally designed two‐stage conical diffuser are considerably less than for a single‐stage diffuser and for an abrupt enlargement. Equations are presented showing the interrelationship among coefficients used variously to express energy loss in a pipe‐diameter expansion as a function of approach and exit velocities.

When a linear flood routing approach is taken for furrow irrigation runoff, the problem of delay in system response is confronted. This was dealt with by a constant response‐variable delay linear model, and the corresponding input‐output relationship was derived. This relationship of Duhamel Integral type was discretized, and an unconstrained least squares minimization procedure was used for the system identification for a set of furrow irrigation field data. After identifying a suitable delay function, the developed unit hydrographs were used in the interfurrow and interirrigation modes.

The objective of the development work was to develop an algorithm for the computerized control of an open‐channel water distribution system that would allow the system to be operated on a demand basis rather than on a scheduled basis. The algorithm, which was of necessity empirical, is an adaption of proportional downstream control. The input to the algorithm is the water surface upstream of each control structure. The output from the algorithm is an adjustment in gate opening, implemented at the adjacent upstream control structure. The adequacy of the algorithm was verified by simulating the interaction between the algorithm and two different typical canal systems. These simulations were used to first establish viability of the algorithm in an automated control environment, and secondly to compare automated control to manual control. It was concluded that the control algorithm is capable of controlling an open‐channel water distribution system at a higher level of efficiency than manual control. Efficiency, under manual control, is measured in terms of the stability of the water surface elevations within the system.

Flow measurement devices can cause significant errors in inflowoutflow channel water loss measurements. The device effects, caused by the increased water surface elevation upstream of a device, must be subtracted from measured losses to correctly estimate conveyance water loss. Methods to calculate device effects are given. In channels with small slopes and in which loss rates increase sharply with water surface increases, inflow‐outflow loss measurements should be made in long channel sections with devices which create as little head loss as possible.

The upper limit lognormal (ULLN) distribution is a three‐parameter family of curves that offers two significant features to models of size frequency data: they have finite maxima and their shape ranges from symmetric to leftor right‐skewed to bimodal. Three estimation procedures for ULLN parameters are examined: sequential maximum likelihood, systemic maximum likelihood and the method of percentiles. These algorithms are compared numerically using randomly generated ULLN data, and conclude that sequential iteration through the maximum likelihood equations should not be used; percentile estimators are inaccurate, and often infeasible; systemic maximum likelihood yields feasible, accurate estimates, converges rapidly, and can be initialized readily with percentile estimates when the latter are feasible. Four drop‐size data sets are fitted with ULLN distributions that pass the one‐sample Kolmogorov‐Smirnov test, give good visual fits to the empirical data, and corroborate crude results from earlier studies, indicating that the ULLN model may be quite satisfactory for modelling the distribution of droplets emitted from irrigation sprinklers.

A review of evaporation reduction studies indicated that shades suspended above a water surface and floating covers on a water surface gave best results. Of the two, floating covers were most practical. Field tests using floating covers of foamed wax blocks, continuous wax, and foamed rubber reduced evaporation losses 36%–84% over an 8 yr period. Based on the amount of the water saved and the cost of materials used, the continuous wax covers provided the lowest cost water at $0.08 to $0.13/1,000 L/yr, followed by foamed rubber and foamed wax blocks at $0.17 and $0.18 per 1,000 L/yr, respectively. However, evaporation reduction efficiency of the continuous wax covers began to decrease after a few years of exposure, while the foamed rubber and foamed wax blocks showed no signs of deterioration. Long‐term durability, efficiency, and climatic range of usability should be considered along with costs when determining the most practical cover to use.

A model for land consolidation and the layout of irrigation systems was developed in Morocco about 20 yr ago and has been used widely there ever since. Known as the “rational layout” scheme, this approach facilitates modern farming activities and rotational water distribution in areas with diversified agriculture and small land parcels. Rational layout represents a fusion of two other approaches to irrigation layout which were tested individually with less success: (1) An agriculturist's approach emphasizing modern farming principles, particularly collective plowing; and (2) an engineering approach which provides a permanent consolidated on‐farm irrigation network, including irrigation ditches, drainage channels and farm roads. The rational layout scheme is unique in dealing with the various factors that must be considered in regrouping separate small plots into consolidated areas for irrigation. These include irregular plot size, surface or sprinkler irrigation systems, crop diversification, crop rotation, and agricultural mechanization. Areas using this system have shown increased irrigation efficiency, higher yields and cropping intensity, and increased use of modern agricultural practice.

Preparation of guidelines for cloud seeding was initiated by the ASCE Weather Modification Committee and is authored by a group of six experts in the field of weather modification. It is designed to give guidance to water resources managers who might become involved in the decision‐making process for implementing a weather modification project. The sections cover a variety of aspects of weather modification including the social, legal, environmental and economic issues as well as the scientific basis. The technical details of various devices for producing artificial nuclei, and the various methods for distributing the artificial nuclei through cloud masses are described in detail. Finally, there is a section that outlines the steps that must be taken to implement a cloud seeding project.