Journal of Irrigation and Drainage Engineering

Measurements of velocity profiles of a free surface flow over a permeable gravel bed indicate that the logarithmic velocity distribution can be preserved if the reference datum is located a small distance equal to about one third of the median diameter of the bed particles below the surface. The observed value of 0.28 for the Karman constant is significantly reduced below the commonly expected value of 0.4 for impervious boundaries which indicates that the boundary resistance of the permeable bed is higher than that of the impermeable bed having identical rugosity. It is also observed that the friction factor increases with the increase in Reynolds number. A method is proposed to predict the amount of seepage flow through the permeable bed by measuring the hydraulic gradient and the velocity profile above the bed and the bed material properties such as grain size and permeability. Although the measured bed flow showed considerable deviations from the predictions for the present experiments, the proposed method can be usefully applied in practical problems.

A procedure to select an optimum irrigation method for specified field conditions is presented. The procedure is developed on the basis of indices that show the acceptability of the irrigation methods to the selection parameters (crop density, type of sowing or planting, slope of the field, infiltration rate of the soil, etc.), cost, and financial feasibility criteria.

Using existing antecedent moisture condition (AMC) Curve Number relationships, a general expression of rainfall‐runoff is developed for AMC I and AMC III. The probability distribution of event runoff exceeding zero is found to be lognormal. Relative storm size is then proposed to be defined on the ratio P/S, where a “large” storm has $P/S>0.46\text{,}$ when 90% of all rainstorms will create runoff. Consequent problems arising in the definition of CN from rainfall‐runoff data are discussed.

Three lysimeters were established containing different sized Scotch pines (P. sylvestris), and the consumptive water use of each tree was monitored during the 1982 and 1983 growing seasons near Logan, UT. Weather data including maximum, mean, and minimum daily temperatures, solar radiation, and daily precipitation were collected. Consumptive use data of the first season were of limited use due to the transplanting stress experienced by the trees. The results of the second season were consistent with the usual water use of irrigated crops. Mean monthly crop coefficients were calculated based on the modified Blaney‐Criddle and the Jensen‐Haise methods, assuming water was extracted from only the crown projection area of the tree. A seasonal Blaney‐ Criddle crop coefficient was estimated to be 1.22. The growing season was long, and the crop coefficient (Blaney‐Criddle) during the winter at this site may be as high as 0.85. An equation was developed to find the composite crop coefficient for conifer tree farms relating tree size, tree spacing, and type of ground cover.

A mixed‐integer programming model was used to obtain least‐cost system rehabilitation plans for a 6,900‐ha (17,000‐acre) irrigation project in southeastern Idaho. Three types of gravity conveyance system components (existing unlined canal, concrete lined canal, and gravity pipe) were considered, along with five types of irrigation application systems (two gravity and three sprinkler application systems). The mixed‐integer programming model that complied with the constraints specified was flexible and effective. The specified constraints used in this study are water charges and water and land availabilities. The quantitative effects of different constraints were easily evaluated. The same modeling procedure can also be used in developing scenarios of alternative system configurations for a new irrigation project development for leastcost system planning. The model gives descriptive scenarios that can assist planners, irrigators, and other interested parties in making multiple‐objective planning decisions for developing or rehabilitating irrigation projects.

The Thornthwaite method for calculating evapotranspiration (ET) as used in the water management model, DRAINMOD, is modified for semi‐arid conditions since for such conditions the existing method had underestimated the average monthly ET rate by about 50%. The following three modifications are proposed: (1) The constant in the Thornthwaite general equation is increased depending upon the min‐max range of the annual mean air temperature wave; (2) the daylight hours wave is shifted by 45 days and brought parallel to the mean air temperature wave. This shift is made in order to account for the heat energy of the atmosphere surrounding the plants; and (3) actual evapotranspiration (AET) is not equated to zero during irrigation. The AET was measured in a drip irrigated banana field with lysimeters. The simulated monthly adjusted values of AET give satisfactory estimates with an average error of within ±10% when compared to that of the lysimeter data.

The equations governing the flow of water in irrigation furrows are put in nondimensional form by expressing each variable therein in ratio to an appropriate reference variable. Two systems of reference variables are considered, one based on normal depth in the given furrow geometry at the given inflow, the other on the given inflow and cutoff time. The first is especially valuable because of the clear physical significance of normal depth, but it is useless in horizontal furrows. The second system is pertinent to furrows set on any slope. The matter of choosing a system of reference variables is viewed in some generality to allow application to other systems, suitable for special applications, in particular, design applications. A set of formulas is presented for translation from one system to the other. The necessary input for a completely nondimensional treatment of furrow irrigation is outlined, and the concept of hypothetical dimensioned furrows is introduced. This is designed to provide a physical significance to variables entered in dimensionless form as input to a mathematical model of furrow irrigation.

Small drop structures employed in irrigation canals are often not ventilated. In this paper the importance of the residual pressure at the brink of a rectangular overfall is quantified by means of the 1‐D momentum equation. The pressure acting below the nappe under nonaerated conditions is studied experimentally and correlated to the drop height in nondimensional terms. Comparison is made with relevant results for ventilated drop structures and criterion for defining a low drop is proposed. The results of pressure estimates are then used for a theoretical evaluation of the brink depth ratio as a function of the nondimensional drop height. The theoretical predictions are found to be in good agreement with the experimental results.