Journal of Irrigation and Drainage Engineering

A finite element model is developed to simulate salt accumulation and transport as well as under drainage in irrigated croplands. The model simulates the flow of water and salts in a saturatedunsaturated soil system, including water uptake by plant roots. The finite element method is used to solve the mathematical equations that describe this problem, and the different components of the program are examined. An analysis is carried out on application efficiency, the quality of the applied water, and the irrigation schedule to ascertain their impacts on salinity levels in the soil profile. This model can evaluate the consequences of irrigation management decisions on the salt loads leached to the drainage system. An example of the model application is given.

A finite element model to simulate salt flow and underdrainage for shallow water table conditions (presented in Part 1) is used to simulate two illustrative cases. The first case involves a one‐dimensional problem in which cotton is growing under conditions typical of the San Joaquin Valley of California. The second case involves a two‐dimensional problem with a tile drain collecting excess water and salts from the root zone. The latter case evaluates the quantity of salts leached from the field through several drain diameters. The model is capable of handling a wide variety of problems that often need scientific assessment, including salinity control and drainage.

The effect of the October 1983 floods in southeastern Arizona, on a previously established generalized envelope for floods expected once in 100 years $\left(Q_{100}\right)\text{,}$ is studied. The design envelope is found to produce more conservative estimates of $Q_{100}$ than individual data sets find. The design envelope for $Q_{100}$ is revised to correct for some longer periods of record now available, and to be consistent with floods on a wider range of drainage area than previously considered. Additional design envelopes for floods expected once in 2 years $\left(Q_2\right)$ and once in 10 years $(Q_{10)}$ are prepared, and the three envelopes are used to provide conservative estimates of flood frequencies on ungaged watersheds in southeastern Arizona with drainage areas between $0.01{\mathrm{km}}^2$ and $10\text{,}000{\mathrm{km}}^2\text{.}$ A procedure is presented for developing regional flood frequency estimates that could be used in geographically and climatically homogeneous areas.

Classification systems for irrigation water conveyance facility components which directly af Fect the operation of open channel and pipeline systems are presented. Components are initially classified into four groups depending on the water management function they are intended to satisfy: diversion; depth‐control, transfer; and storage. These groups, as presented in various tables, are further classified into thirty‐two types of diversion structures, thirty‐two types of depthcontrol structures, four types of transfer structures, and three types of storage facilities. General equations describing the hydraulics and operation of diversion, depth‐control, and water transfer structures are also provided. Multiple outlet and compound and multifunction structures are also addressed.

A recirculating furrow infiltrometer is used to measure infiltration for simulated surge irrigation for various cycle times and cycle ratios. The data collected from three locations is used to evaluate two empirical surge infiltration models, one of which was developed by the authors. The effects of furrow geometry, surface depression storage, recession and advance time, and measurement error are considered. The evaluations indicate that a model based on the Kostiakov equation, the surge cycle ratio, and the surge cycle time work well in predicting infiltration during surge. This model, which considers surge cycle ratio and cycle time, is both simple and useful in surge irrigation design. Most importantly, it allows surge infiltration to be determined directly, without surge infiltration data, from continuous infiltration data.

Irrigation is an excellent use for sewage effluent because it is mostly water with nutrients. For small flows, the effluent can be used on special, well‐supervised “sewage farms,” where forage, fiber, or seed crops are grown that can be irrigated with standard primary or secondary effluent. Large‐scale use of the effluent requires special treatment so that it meets the public health, agronomic, and aesthetic requirements for unrestricted use (no adverse effects on crops, soils, humans, and animals). Crops in the unrestricted‐use category include those that are consumed raw or brought raw into the kitchen. Most state or government standards deal only with public health aspects, and prescribe the treatment processes or the quality parameters that the effluent must meet before it can be used to irrigate a certain category of crops. However, agronomic aspects related to crops and soils must also be taken into account. Quality parameters to be considered include bacteria, viruses, and other pathogens; total salt content and sodium adsorption ratio of the water (soil as well as crop effects); nitrogen; phosphorus; chloride and chlorine; bicarbonate; heavy metals, boron, and other trace elements; pH; and synthetic organics (including pesticides).

A complete furrow irrigation model was developed based on the cumulative solution of the volume balance equation, rather than the incremental solution. Space intervals were given (constant or variable) to allow infiltration characteristics to vary along the length of the furrow. The cumulative model predicted more accurately than the incremental model when compared to field data and the kinematic wave model results. Further, the incremental solution gave slower advance rates and higher runoff than the cumulative solution due to larger numerical error and oversimplification of postadvance phases. This deviation was larger for high‐intake soils. Field slope and roughness, however, did not influence the difference in accuracy between the two models as much as soil intake.

A linear optimization model for planning the management of Irrigation District No. 38, in the State of Sonora, Mexico, is presented. The model considers the yield, price, and production cost of twelve primary crops; the land restriction on cropped areas; the storage capacity of the existing reservoir and aquifer; the reservoir net inflows; the evaporation, releases, and spillages from the reservoir; the surface water and groundwater requirements of the crops; the quality of the mix of surface water and groundwater; and the requirements of other resources, such as fertilizer, pesticide, seed, equipment, and labor. The model gives the cropping pattern and the monthly schedule of reservoir releases and aquifer withdrawals that maximize the annual profit in the district. Solutions to the model facilitate an evaluation of the effects of net annual inflows on profits and cropped areas, and provide an indication of the levels of inflow that can be used for planning the operation of the irrigation district.

Consumptive use or evapotranspiration is an important element for estimating irrigation water requirements and in water resources management, especially under arid conditions where fresh water is a limited resource. In this study, 23 empirical methods have been selected from the literature and used for estimating consumptive use (or evapotranspiration) under the extremely arid conditions of central Saudi Arabia using the corresponding climatic data collected from the Hofuf station. The results from these methods have been evaluated with reference to actual measurements performed in the same area in two separate 12‐mo periods and then ranked according to five different rating criteria. The top six ranked methods obtained for the average as well as for Salih and Sendil's ratings are ranked in the following order of merit: Jensen‐Haise, class A pan, Ivanov, adjusted class A pan, Behnke‐Maxey, and Stephens‐Stewart.

A significant advantage attributed to surge flow irrigation is that for the same volume of water applied the stream will advance farther along the furrow than with continuous flow. Where this advance phenomenon exists, the reduction in runoff and deep percolation will improve uniformity and application efficiency. The mechanism for improvement in advance time has generally been ascribed to surface sealing and surface layer consolidation. However, these phenomena do not satisfactorily explain improved advance times in sandy soils. The Green‐Ampt model combined with a simple redistribution model is used to illustrate the effects of reduced wetting front suction due to intermittent wetting on the instantaneous infiltration rate on two soils. The effect of various surge cycle times on the volume infiltrated versus time is also predicted by the model. This approach to modeling infiltration under surge irrigation shows considerable potential for determining the effect of surge irrigation on a range of soil textures, and as a design tool for optimizing surge cycle times.

Tests were conducted to study the effects of nonlevel placement or construction on the calibration of long‐throated flumes. These flumes have previously been mathematically modeled to provide accurate $(\pm 2\%)$ calibrations for a variety of cross‐sectional shapes. While cross‐slope can be handled successfully by the modeling procedure, no assured procedure was apparent for longitudinal slope corrections. Laboratory tests were directed to determining errors that a user might experience if a flume crest were placed at various positive and negative longitudinal slopes. The magnitude of systematic errors are shown for portable flumes with either a sidewall gage or a translocated stilling well, and for permanently placed flumes. Correction procedures are suggested.

Equations expressed in explicit power algebraic forms for one‐dimensional upward water movement, cumulative infiltration, and infiltration rate have been derived. These equations are based on three derived consecutive time zones. The validity of the derived equations has been examined by using published data. The comparisons of computed and observed data show very good agreement.

The approximate theory of G. B. Witham, proposed in 1955, is often used to initialize numerical computation of border irrigation or a dam-break flood. However, this study shows that the main assumption of a uniform velocity profile is inconsistent with the law of mass conservation. Therefore that assumption is no longer employed in the development of the analytical solution of the continuity equation and a simplified momentum equation. The solution turns out to be a rather simple but powerful formula which can replace the more approximate solution of Witham without any increase of computational effort, but with a substantial gain of reliability.

An analytical solution for a trickle lateral is presented which includes velocity head in the basic formulation. The relative error when neglecting velocity head is greater for laminar than for turbulent flow. However, the magnitude of head loss is greater for turbulent regimes. Although the errors caused by neglecting the velocity in most cases are small, the analytical solution remains relatively simple and easy to use.

The nomograph developed by Frost is widely used to estimate sprinkler evaporation losses. The original data from over 700 tests used to develop the nomograph are apparently lost. An equation is developed to approximate the sprinkler evaporation losses found with the Frost nomograph. The equation is the result of regression analysis of measured nomograph dimensions. The equation is usable only on a limited range of values due to regression transformation non-linearity. The equation can usually estimate the Frost predicted evaporation within ±10%. The error is considerably higher for input values near the nomograph extremes.