Irrigation is needed to provide water for a crop when the frequency and amount of rainfall is not sufficient to meet the crops water needs.
Evapotranspiration refers to the water that is evaporated from the soil and crop surface (evaporation) and the water that is released through the plant, (transpiration).
A crop's water requirement or water usage is directly related to ET and the crop's stage of development. A crop coefficient taking into account the specific crop and growth stage is applied to the ET value to determine the water requirement. The irrigation system can be scheduled using the adjusted ET values.
The following factsheets available from the Ministry of Agriculture, Food and Fisheries provide additional information.
For non-agricultural irrigators users the Turf and Landscape Irrigation Scheduling Guide is available from the Irrigation Industry Association of BC.