Journal of Irrigation and Drainage Engineering

Wastewaters that have undergone treatment and are stored in open reservoirs frequently cause clogging problems while being applied through dripirrigation systems. This study is aimed at defining the clogging factors and mechanisms of blockage within three types of drippers as a basis for developing technical measures to overcome the problem. The relevant effluent constituents are defined and physical and chemical properties of the deposits in hundreds of emitters are examined, using both field and laboratory experiments. A gradual clogging mechanism is proposed: The sediment buildup begins with the deposition of amorphous slimes, to which other particles adhere. The clogging rate is more affected by particle size than by particle‐number density. The chemical composition of the deposits in the dripper changes with the season. Filtration prevents immediate clogging by large particles. Clogging potential may be decreased by modifying the emitter structure and by chemical pretreatment.

Transitions form a vital link in the canals constructed for irrigation and power‐generation purposes. As expansive transitions involve considerable head loss, any reduction of head loss will mean more power generation at the downstream end for power channels and increase in command area for irrigation canals. Furthermore, the minimum head loss design will increase the life of the transition structure. The design methods in vogue are arbitrary with no attention paid to minimizing the energy loss. Based on optimal control theory, a methodology has been presented for optimal design of a rectangular subcritical expansive transition. Analyzing a large number of optimal profiles, an equation for the design of rectangular transitions is presented.

A numerical model of two‐dimensional transient saturated flow to ditches is presented. The model is based on a finite differences solution of the differential equation governing two‐dimensional transient flow in a heterogeneous and anisotropic porous medium. Thus, the model can account for the partial penetration of ditches, flow above the initial drain level, vertical anisotropy, vertical flows, and associated head losses. The model solutions are found to compare well with one‐dimensional Donnan and Kraijenhoff solutions when the relative resistance to vertical flow is negligible (i.e., $\mathrm{Kz}/\mathrm{Kx}\gg 1$). These analytical solutions are, however, found to underestimate the water‐table rise when $\mathrm{Kz}/\mathrm{Kx}<1\text{.}$ Design curves have been prepared for a few general cases of a partially penetrating ditch system.

A design methodology is developed to obtain the least‐cost design of irrigation canals. The proposed methodology incorporates elements of the water section and the above‐water section, and is applicable to both lined and unlined canals. Every reach of the canal is described by four basic design variables; bed slope, bed width, upstream bed level, and upstream berm level. A systematic procedure is used to generate design alternatives covering the solution domain. Only those alternatives satisfying a group of preset functional, hydraulic, operational, maintenance, and construction constraints are considered feasible, and are screened to find the least cost. The total cost included the cost of earthwork, lining and trimming (if any), control structures, land acquisition, operation, and maintenance. A detailed cost model is used to estimate the earthwork cost taking into account excavation, deposition, haulage, and soil import or export. The usefulness of the proposed methodology is demonstrated through its application to a reach of El‐Nasr Canal, a recently constructed main lined carrier in Egypt.

The slope of the soil surface has a significant effect on water distribution from sprinklers. Field tests were conducted to quantify the effects. A ballistic trajectory model was developed to simulate precipitation data for a sprinkler working on different ground slopes and at different sprinkler riser angles. The model uses precipitation data from a single sprinkler working on a zero slope plane under no wind conditions. The model is verified against measured data for a sprinkler operating on a slope. A study is made of the influence of sprinkler riser angle, nozzle angle, and soil surface slope on the uniformity of water distribution. Two different precipitation patterns, similar to Christiansen's sprinkler patterns B and E, are studied in detail. The results show that the sprinkler risers should be kept perpendicular to the soil surface to maximize the uniformity of water application and to minimize the erosion risk. Higher nozzle angles are preferable to lower nozzle angles when working on steep slopes, but higher nozzle angles increase sensitivity to wind. With higher nozzle angles, surface slope is less critical.

Flow to subsurface drain tubes was investigated using numerical solutions to the Richards equation for combined unsaturated and saturated flow. Solutions to three sets of problems involving steady infiltration and transient drainage of initially curved and flat water tables were obtained for two soils and various profile depths. The numerical solutions were used to illustrate various aspects of the drainage process and to evaluate simple analytic expressions for calculating drain flow rates and water table elevations. An approximate method was developed for predicting transient drainage events by combining the Hooghoudt equation with analytic expressions for radial flow into drain tubes and for mass balance. The predictions of the approximate method were found to be in agreement with the numerical solution for drainage of an initially elliptic water table.

This paper develops an approach for assessing alternative strategies for improving an irrigation water delivery system in the context of multiple planning criteria. The methodology focuses on the exploration of distinct long‐term, system‐scale strategies to determine those which warrant further investigation in an implementation stage study. Alternatives that involve structural, managerial, and policy changes are discussed. A model is proposed for analyzing delivery system alternatives, using hydrologic simulation and incorporating water rights constraints. The model accounts for parameter uncertainty on both the supply and demand sides of the system resulting from temporal and spatial variability and inadequate data. Measures for the objectives of adequacy, efficiency, dependability, and equity of water delivery are used to evaluate system performance under each alternative considered. Expected values and coefficients of variation of these measures are considered over the planning horizon. Subjective notions attached to the evaluation of system performance are addressed by using fuzzy‐set membership functions to interpret the expected values of performance measures. Techniques of multicriterion decision making are presented as a means of achieving an aggregate ranking of alternative strategies according to criteria such as water delivery performance, relative cost, social acceptability, political and institutional acceptability, and environmental impact. A companion paper applies the developed methodology to the Alamosa River and La Jara Creek systems in the San Luis Valley of south‐central Colorado.

A companion paper (Gates et al. 1991) developed an approach for assessing alternative strategies for improving an irrigation water delivery system in the context of multiple planning criteria. The present paper applies this approach to the Alamosa River and La Jara Creek irrigation systems in the San Luis Valley of south‐central Colorado. Eleven distinct long‐term, system‐wide alternative strategies were explored, and their impacts upon irrigation delivery performance were predicted using a stochastic simulation model. The rehabilitation alternatives that were considered involved structural, managerial, and/or policy changes. Uncertainty due to spatial and temporal variability, inadequate data, and subjectivity was accounted for in the model. Two multicriterion decision‐making techniques were implemented to rank the alternative planning strategies according to criteria of relative cost, social acceptability, institutional acceptability, and environmental impact, as well as water delivery performance. Alternatives identified for further detailed consideration at an implementation stage were those involving reduced canal conveyance losses, increased irrigation application efficiencies, and a refinement in the prior appropriation scheme of water allocation.

Precipitation droughts were defined using monthly precipitation time series and different truncation levels—namely, 70%, 80%, 90%, and 95%—in which a 70% truncation level means that 70% of the recorded monthly precipitations are greater than or equal to the value. Conditional probabilities that droughts of certain truncation levels will advance to their next higher levels were estimated at each gage location. There are 21 precipitation stations selected in the Scioto River Basin, in Ohio, for the study of regional drought characteristics. The kriging method based on the minimum unbiased estimation was used to obtain a regional distribution of conditional drought probabilities at each truncated level. Contour lines of conditional probabilities for each truncation level were constructed to show possible drought‐prone distributions for the ungaged region. This reflects the areal severity of water shortage in case of drought.

One‐hundred‐year and 10‐year storm runoff volumes and peak discharges were computed with a Green‐Ampt infiltration model and the Soil Conservation Service's (SCS) TR‐20 hydrograph model. The results for six different standard design storm temporal distributions were compared with predictions made with the U.S. Geological Survey's (USGS) regional regression equations relating peak discharge to drainage area. Hypothetical watersheds similar to those in eastern Montana were used for the comparisons, which were made over a wide range of hydraulic‐conductivity values for drainage areas of one to 400 sq mi. Runoff volumes computed with the Green‐Ampt equation vary significantly with the rainfall distribution. The design storms, which included the independent duration (ID) frequency analysis for all durations within the event, resulted in estimates of both volume and peak discharge that were significantly greater than predicted by the USGS equations. The SCS type‐I distribution resulted in the best comparison over the range of frequencies and drainage areas examined.

A least-squares land-leveling program written for use on microcomputers allows flexibility in design that was not previously available. The user can specify six different types of land-leveling conditions common under surface irrigation. Slope and land leveling cut-depth limitations can also be specified. Computations can be made for irregularly shaped fields and rectangular fields with equal ease. The output gives full information on earthwork volumes and costs, and the maximum cut and fill depths. Cut and fill depths at the boundaries of the field are calculated for marking stakes to guide construction. If slopes, control elevations, or depths of cut are specified, the results will give the least cost but not the minimum cost that would result from a strictly least-squares solution.