INTEGRATION OF GIS AND HYDROLOGICAL MODELS IN A FEASIBILITY STUDY OF IRRIGATION UNDER SALINITY

This study estimated net irrigation benefits from the proposed Cable Mountain Reservoir (CMR) on the North Fork of the Red River in Southwestern Oklahoma. Part of the benefits from the CMR might come from replacing the largely depleted groundwater in the Tillman Terrace area (TTA) of western Tillman County. The elevation of the TTA would be at or below level of the CMR. Construction of the CMR is dependent on a project to prevent loading of up to 400 tons of salt per day on the Elm Fork upstream of the proposed reservoir. ArcGIS was used to locate and calculate the area of potential irrigable soils (Class I, IIe, and IIw). The potential irrigable soils were restricted to those with a slope of less than three percent. The clusters of irrigable soils that could be covered by ¼ mile pivot systems within cadastral section boundaries while avoiding roads, buildings, and gullies were then calculated. Some 543 full or partial pivots containing over 68,000 acres were identified. Elevation contour maps constructed from USGS data sets were used to draw alternative pipeline routes that could deliver water to each of the pivots with minimal pumping. The EPANET hydrologic pipeline program was used to calculate energy costs and pipeline pressures necessary to deliver 600 to 800 gpm to each pivot with at least 35 PSI. Salinity of the surface water limited crop choice to currently grown cotton crop. The EPIC simulation program was used to develop soil type specific cotton response functions to irrigation, to salinity in the irrigation water, and to salt accumulation in the soil. These functions were used to estimate the yield of cotton to the amount of irrigation and the accumulation of soil salinity over a 50-year period. Examination of EPANET results indicated points in the system where pumps were necessary and where pipe diameters could be increased or decreased to reduce the sum of energy costs and annualized pipe costs. A series of nonlinear programming models were used to determine the annual amount of water to be applied to maximize discounted profit from each pivot given the individual soil types covered by pivot and the delivery cost of water from the pipeline. If all producers adopted 600 gpm pivots and operated simultaneously, the cost of the 360,000 gpm pipeline would be prohibitive. However, formation of an irrigation scheduling district that divided the irrigable areas in four subareas where producers use 800 gpm pivots requires a 120,000 gpm system at one-third of the fixed cost of the unrestricted 600 gpm system. The net present value (NPV) per acre for a 50-year period for designs 1, 2, and 3 were profitable over 75, 70, and 65 cents of cotton prices, respectively. The study suggests that economies can be obtained through a combination of pipe sizing and by increased cooperation or utilization of the pipeline.