The goal of any irrigation system is to apply exactly the same amount of water to every square inch under irrigation. The degree of uniformity that is achieved by an irrigation system is usually expressed as the Coefficient of Uniformity (CU). The CU is determined by laying out a grid of cans (for example, every 2 to 5 feet in a square pattern) on the area being irrigated by a single system. The irrigation system is operated for a known period of time after which the accumulated water in each can (catchment data) is measured (volume in ml or cc). If the top surface area of the can is known, the application rate (in/hr, mm/hr) can be determined. The catchment data is used to calculate the average quantity of water per can. The CU is then calculated by the following equation:
C.U. = 100 (1.0 - x--xi _______x-n)
x¯-xi = sum of the deviations of each observation (xi) from the mean (x¯) of the observations and n = the number of observations.
The CU has a maximum possible value of 100 which would indicate a perfectly uniform application of water. A CU less than 80 indicates a poorly designed irrigation system.
Catchment data can be used to determine application (precipitation) rates. The application rate is the average rate at which water is being applied to the area covered by the sprinkler layout. The application rate should be known for all irrigation systems. For drip or spaghetti systems the application rate can be measured by catching water from the system over a known period of time.
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