Irrigation Precipitation Rate, Calculating

Knowing the total amount of water which can be, or is, applied to an area is important so as to maintain plant health and sward durability.

It is important to know the rate at which water infiltrates and percolates through a soil to avoid surface water arising from the application of irrigation water. In addition, for irrigation design, the maximum amount of water required by a plant should be determined to ensure that adequate quantities of water can be supplied at peak plant requirements.

The precipitation rate of an installed irrigation system, or sprinkler, can be determined, in practice, by placing several collection trays or containers (maybe 10 to 20) within the area that a sprinkler will cover. Leave the sprinkler running for a set period of time, which might be 15 to 30 minutes, and then measure the depth of water in each container tray or container, producing an average depth. Convert this to be expressed in mm per hour, so if the sprinkler was run for 15 minutes and the average depth was 2mm, then the precipitation rate would be 8mm per hour, meaning that 8-litres would be applied per m².

Another method is to use figures provided by a manufacturer. For example, a sprinkler at a certain pressure and nozzle type might have a discharge rate of 2 litres per second. This would mean that over one-hour (2 x 60 (seconds) x 60 (minutes)) 7,200 litres (which is also 7.2m³) would be discharged. This figure is then divided by the total area over which it is applied. For simplicity, if the sprinkler had a throw radius of 20-m, and it operated as a full circle spray, then the area covered would be πr², or 1,256m². The precipitation rate would therefore be 7,200 ÷ 1,256 = 5.7mm per hour (per m²).