Methods and Costs of Optimized Irrigation System Design Capacity Using Historical Evapotranspiration

Claire Marie  Melad Castillo

doi:10.7273/000005017

Irrigation systems need adequate capacity to be able to replace the water used by the crop to avoid crop water stress, yet not so much that the irrigation system costs are excessive, and the irrigator must make constant on-off decisions to avoid over-watering. A method for calculating the irrigation system design capacity was developed that considers the daily and interannual evapotranspiration (ET) of a crop and its variability by using historical data and soil information. Also, cost estimates for excessive irrigation system capacity were estimated, and a simple irrigation guidance tool was developed. To demonstrate the process, historical evapotranspiration data were gathered from three climatically different locations in Washington State: near Prosser, Puyallup, and Omak to represent arid, humid, and mountain valley areas, respectively. The mean and standard deviation of the ET for each day of the year was used in the calculation of the minimum design capacity with a selectable probability of adequacy due to natural weather variability. Using the developed method, the net irrigation design capacity was adjusted to its minimum value such that during hot portions of the season the soil water could be depleted without resulting in water stress. Including the effects of the ET variability and a probability of adequacy increases the computed, required irrigation design capacity. This resulted in greater confidence in the chosen design capacities compared with the traditional methods of selecting design capacity, which simply uses the peak ET or maximum of the mean month ET. Calculating the irrigation design capacity without considering daily and interannual variability can result in more years of crop water stress due to inadequate system design. A cost vs design capacity curve was established for different irrigation systems. Increasing the design capacity also increases the costs of the system. Drip irrigation systems had the highest cost followed by center pivots, side roll, and then surface irrigation systems. Furthermore, to use the computed optimized design capacity, a simple irrigation scheduling guidance tool was developed to eliminate the struggle of farmers in making frequent decisions about when to turn on and off the irrigation system that will make sure the crop will not suffer stress since crop water timings are subject to the availability of labor. This tool creates an irrigation schedule for an average year that can be printed once and used forever as a general guide.

Methods and Costs of Optimized Irrigation System Design Capacity Using Historical Evapotranspiration

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