Journal of Irrigation and Drainage Engineering

Based on a literature review, the suitability of the Manning equation to represent shallow flow in surface irrigation is questioned. This conclusion is supported by analysis of both experimental data of the present study on wheat and perennial pasture as well as other published and unpublished field data on wheat. The discharge‐depth equation (Eq. 2) fits the data of most tests with $R^2$ values greater than 0.9. The values of the exponent α in the equation varies generally between 1.0 and 3.0 depending on the type of vegetation, stem density, height, sowing pattern, stage of growth, and flow condition, and confirms that assuming a constant value (i.e., 5/3) for the exponent in the Manning equation is not appropriate. It is proposed that the discharge‐depth equation is a more suitable model than the Manning equation for shallow flow, and information is provided on the range of values of parameters for hydraulic resistance in surface irrigation flows.

A linear programming model was developed to optimize the use of different quality waters by alternative irrigations rather than by blending. In a situation of poor‐quality ground water and limited good‐quality canal water, the model decides how much land to put under each crop and how much ground water to abstract and apply to each crop in each time period. The irrigation system, based on an area in Pakistan, was modeled to maximize the net returns. These are the product of the area under each crop and the crop gross margin. The latter is a function of crop yield, which is itself a function of the irrigation water applied, i.e., the sum of canal water and ground water. Maximization of the net return therefore involves maximizing a function including the product of two variables, crop area and amount of ground water applied. To overcome the difficulties of nonlinearity and therefore greatly reducing computation time, a number of irrigation strategies were identified for each crop. Each strategy has a corresponding yield level, which becomes a coefficient in the objective function rather than a variable. The model developed presents a solution procedure wherever low rainfall and limited and different quality waters are the basic parameters governing the irrigation system. An example is given to illustrate the use of model.

Demand for water is increasing continually, whereas available supplies are more or less constant. Under these circumstances there is an urgent need to introduce efficient techniques in water resources management for optimal utilization of available water. Water management under drought conditions assumes great importance in a tropical region such as India, where one‐third of the cropped area is affected by frequent droughts. This paper deals with the development and application of an expert system for drought management. A linear programming model was used to generate optimal cropping patterns from past drought experiences as also from synthetic drought occurrences. These policies together with the knowledge of the experts were incorporated in an expert system. Using this, one can identify the degree of drought in the current situation and its similarity to the identified drought events and be able to get the corresponding management strategy.

Modeling the hydrology of a forest watershed is helpful to understand the interaction of the hydrologic components and the effects of management activities on the performance of the system. This study was conducted to develop a hydrologic model for a drained loblolly pine (Pinus taeda) plantation watershed. Rainfall interception, subsurface drainage, and evapotranspiration models characterizing a forest watershed are described and incorporated into DRAINMOD, a hydrologic model originally developed for poorly drained agricultural fields. Simulation results are compared with measured field data collected in 1988–1989 from an intensively monitored experimental forest watershed in the coastal plain of North Carolina. Estimated drainage rates and water‐table elevations agreed well with those measured on each of three watersheds. Estimated cumulative drainage volumes for a 672‐day period were within 2–9% of measured values. The estimated water‐table elevations had an absolute average deviation of 20–22 cm for the 672‐day period. The model adequately describes the hydrology of the forest system and will be an important analytical tool in evaluating alternative forest water management systems and in developing best management practices that take into account both off‐site and on‐site impacts.

Realistic estimates of hydraulic resistance are essential to the proper design and analysis of irrigation canal networks. Extensive field studies in Egypt provide data on the magnitude and variability of hydraulic resistance in earthen irrigation canals infested with aquatic weeds. Values of Manning resistance coefficient, n, were calculated from 280 measurements at selected cross sections in 23 stable canals with emergent ditch‐bank vegetation. The temporal sample mean of monthly values of n estimated within a year ranged from 0.017–0.062 for cross sections where five or more measurements were made. The temporal sample coefficient of variation ranged from 0.03–0.42. Dependence of n on flow regime (hydraulic depth and product of average velocity and hydraulic radius) was explored. Studies also were conducted on nine canals containing submerged vegetation. These studies included 312 sonar measurements of vegetation density as well as 156 measurements to estimate n at selected cross sections. Regression analysis revealed that variability in flow regime and in vegetation density contribute significantly to variability in n. Temporal mean of monthly values of vegetation density within a year ranged from 0.06–0.25, with temporal coefficient of variation between 0.17 and 0.92. Vegetation density at measured cross sections showed a clear seasonal pattern. The spatial mean and coefficient of variation in vegetation density along a canal were found to range monthly from 0.06–0.36 and from 0.18–0.76, respectively. Temporal mean values of n in canals with submerged vegetation ranged from 0.028–0.074, with temporal coefficient of variation between 0.13 and 0.57.

Two equations commonly used for estimating and modeling infiltration from pervious land surfaces are those developed by Horton and the U.S. Soil Conservation Service (SCS). The main shortcoming of the Horton equation is the strong dependence of the infiltration rate on the starting time of the rain, which becomes particularly critical when a rainfall event starts at a low intensity. The SCS equation, which was originally developed for runoff and later converted to compute infiltration losses, suffers from the complete lack of a time relationship. The problems associated with these shortcomings are discussed in this paper. The Horton equation is modified to make the infiltration rate a function of cumulative antecedent infiltration. A linear filter, in conjunction with two of the Horton parameters, is suggested to delay the SCS infiltration increments. In addition, a concept proposed by Bauer to incorporate deep percolation into infiltration equations is adopted to provide for the recovery of infiltration capacity between consecutive storm bursts.

Expansive soils are known for their moisture sensitiveness—they will swell when given access to water and shrink when they dry out. For a shallow foundation, this causes heave during the rainy season and settlement during the drought when the clays dry out. This causes cycles of up and down movement of the foundation, causing cracking and fatigue to the structure. Excess irrigation or poor drainage are often blamed as the causes of damage to structures on expansive soils. A more damaging situation could be the lack of irrigation during a drought, causing desiccation of the clays. If trees are located in close proximity to the building, the roots can grow beneath the shallow foundation and then extract moisture during the drought. This will cause the clay to shrink and the structure to settle. The goal in dealing with expansive soils is to try to maintain a constant soil moisture. The elimination of planter areas around the perimeter of the building or surrounding the building with concrete flatwork can help reduce moisture changes.

A method is presented for designing the outer‐end sprinkler of a center‐pivot irrigation system to prevent surface runoff. The smallest wetted diameter, for which soil infiltration capacity equalled or exceeded nozzle application rate, was determined by equating a nozzle application rate relationship and a Green and Ampt infiltration rate relationship. The computed wetted diameter was used in developing relationships to select nozzle type, orifice diameter, and operating pressure. Sensitivity analysis determined that wetted diameter is primarily sensitive to soil characteristics related to infiltration and to evapo‐transpiration rate. Examples illustrated variations in irrigation system design because of tillage system. Greater operating pressure of an irrigation system is necessary to prevent runoff if the tillage system does not prevent soil surface crusting or compaction by raindrops. Considering a no‐till system resulted in the selection of a low‐pressure impact sprinkler instead of a conventional high‐ or medium‐pressure impact sprinkler. Therefore, the developed methodology can be used to design the optimum center pivot irrigation system for given soil, climate, cropping pattern, and tillage practices.

The standard step method is one of the most widely accepted procedures for computing open‐channel water surface profiles. Conventional computer models that use the standard step method often have cumbersome data requirements for routine drainage analyses. An algorithm has been developed that executes the standard step method in prismatic open channels. This algorithm, presented in Newton‐Raphson form, is suitable for subcritical, supercritical, critical, adverse, and horizontal flow regimes. Transition channel sections having linearly variable bottom widths are easily accommodated. Numerical solution of the standard step equations, without using look‐up tables and interpolation procedures, results in fast execution times. A steady flow computer program that uses the algorithm has been developed to perform some “smart” checking of hydraulic profile logic and to facilitate simple data entry. The program is available to interested persons free of charge, upon receipt of a self‐addressed stamped envelope, disk mailer, and diskette.

The method of floodplain delineation on alluvial fans developed for the National Flood Insurance Program (NFIP) may be modified to provide estimates of peak flood flows of specified return periods at transportation alignments, such as aqueducts or railroads, crossing alluvial fans. The modified methodology divides the total length of the alignment into segments, and the peak flow expected in each segment during a flood event is estimated as a function of the return period of the event, the segment length, and the location of the alignment on the alluvial fan. This estimate of the peak flow can be used to properly size the facilities such as dikes, berms, and culverts to protect the alignment from flood damage. The proposed methodology has potential applications in any environment where transportation alignments must cross alluvial fans on which the hydraulic processes are similar to those for which the NFIP methodology was developed. An example of the use of this methodology is provided.