This is often expressed as gallons per minute per acre (GPM/acre). Many federal and state regulatory agencies and design firms apply this relationship to a generic value of the flow rate per acre. Nonuniform sprinkler coverage reflects the gross amount, which must be determined by dividing the net volume by the irrigation or application efficiency. With the prior knowledge of the crop’s evapotranspiration rate, these values can be used to determine the daily, weekly, or seasonal volume of water needing to be applied to a given crop.įor example, if a crop has an evapotranspiration (uptake) rate of 0.25 inches per day, the net volume of water required per acre equals 27,150 gallons/acre-inch × 0.25 = 6,787.5 gallons per day.Īlways remember this represents the net volume of water, losses due to percolation, wind drift, and solar radiation. For convenience, this is often rounded to 325,800 gallons of water per acre-foot.Ĭonversely, if working in units of acre-inches, the unit commonly used for determining application depth equals 27,150 gallons of water per acre-inch (325,800 gallons/12 inches per foot). With the knowledge there are 7.48 gallons of water per cubic foot, one acre-foot of water would therefore equal 43,555.69 ft 2 × 7.48 gals./ft 3 = 325,796.56 gallons. Thus, since one acre of land in the United States is defined as an area with cross-sectional dimensions of 208.7 feet × 208.7 feet, the total area would equal 43,555.69 square feet. One acre-foot is defined as the volume of water that would occupy one acre of land to a uniform depth of 1 foot. The first includes the instantaneous and average application rates in inches per hour, and the second is the total application over the irrigated area in inches.īy applying the consumptive use with a further understanding of the correlation between the volume of water in 1 acre-inch or foot versus the elapsed time of application, a designer can then determine the amount of water required to irrigate a specific crop over any size of field.
There are two distinct types of applications that must be considered for an irrigation system. This month in the third part of our six-part series, we will expand with an overview of a few of the more important elements of an irrigation system: hydraulic considerations including allowable soil loading (i.e., water application rate and depth per soil type) water losses and application efficiency distribution uniformity irrigation system efficiencies and seasonal water volume and storage requirements. In the last two Engineering Your Business columns, we have discussed basic irrigation terms and definitions, decision-making criteria, and soil-water-plant relationships, including water-quality considerations and consumptive water use of plants.
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