Journal of Irrigation and Drainage Engineering

Due to the impeding nature of the subsoil, drainage discharge from heavy land predominantly takes the forms of lateral interflow through the more permeable toplayer and of overland flow. Evidence is presented that this type of “shallow drainage” discharge can be described by the linear reservoir type rainfall‐discharge model. Procedures and guidelines are given, detailing how this model can be used to derive drainage coefficients for heavy land. Results are evaluated by comparison with other methods of deriving drainage coefficients and with commonly used design standards. It is concluded that the presented method is reliable and universally applicable.

A multipurpose water resource development project is proposed for construction and development in West Africa. The plans include the irrigation of an area of 274,805 ha by the year 2030 in three countries: Senegal, Mauritania, and Mali. Agricultural benefits are estimated and analyzed by means of a computer program. The factors influencing crop yields are analyzed and summarized including crop selection, water, fertility, and management. Some of the considerations that will produce higher crop yields in future years are presented. It is proposed that the irrigation project management should be responsible for calculating crop water requirements and insuring acceptable irrigation efficiencies. A method is presented for estimating crop water requirements from maximum and minimum air temperatures. The desirability of using irrigation supplemental to rainfall for maximizing benefits from limited water is presented. The maximum contribution of a unit of irrigation water to yield is possible during the rainy season. Projected revenues and costs and years required for each crop to become economically profitable arre presented in tables.

Petiole water content and canopy temperature of cotton were used as indicators of stress in cotton. A crop water stress index (CWSI) using remotely sensed canopy temperature was a better indicator of plant stress than was petiole water content. This point was most evident when using these parameters as a potential tool for scheduling irrigations and for estimating lint cotton yield.

Temporal and spatial changes in the flow profile decrease with time under furrow irrigation, and these slow changes suggest modifications in the simulation methods. Geometrically increasing time steps and node deletion at the upstream boundary are introduced and evaluated for accuracy and CPU time reduction. Increasing the time steps reduced CPU time without sacrificing accuracy significantly in predicting advance. The geometric progression factor is estimated automatically from the first two advance increments, thus incorporating site‐specific flow and infiltration characteristics. The moving left boundary method is slightly more accurate, but is less cost‐efficient than the varying time‐step modification for the example conditions. However, special infiltration conditions may favor the latter method.

Irrigation water demand, crop yield and water allocation models were developed and applied to a Philippine run‐of‐the‐river irrigation project. Dry season water supply hydrographs were classified into four levels of availability from high (I) to low (IV). These levels were correlated with the October stream flow volume to provide a prediction of dry season supply. Project net annual benefits could be increased for all four dry season water supply hydrograph levels by growing an upland crop (soybean) and rice, instead of only rice, providing that costs were less than $90/ha for additional drainage facilities. Net benefits for all rice were greater for reduced area planted, but fully irrigated when the available dry season water supply was lower than average as compared to planting the total area with inadequate irrigation. The irrigation system water demand model was calibrated to match actual diversions for the 1977‐78 dry season. The project irrigation efficiency, combined effects of conveyance and application, was determined to be 41%. Thus, a savings in available water supply could be realized by increased system efficiency both from physical modifications as well as from improved management.

A procedure is presented for designing a system to reuse runoff from free outflow irrigation borders. Runoff volumes are obtained using four dimensionless variables and runoff curves developed with a zero‐inertia mathematical model. An example of the design procedure includes different modes of reuse system operation.

In the unsaturated zone, fine-grained soils tend to exist in situ at higher degrees of saturation than coarse-grained soils. As a result, fine-grained soils generally will exhibit lower resistivities than coarse-grained soils. On this basis, empirical relations between recharge rates and electrical resistivities measured with surface techniques can be developed. An example using published data is given.