Evapotranspiration and Net Irrigation Needs
Evapotranspiration or ET is a measurement of the amount of water required for plant growth. ET measures the quantity of water transpired from plant tissues and evaporated from the surface of surrounding soil, expressed as a depth (usually in inches). ET is based on a number of factors that can include: local temperature, precipitation, cloud cover, solar radiation, and the type of plants being grown. Most of the ET calculations done in urban settings are for turf grass, and the local temperature, precipitation and solar radiation information is often collected by a local ET measurement station.
The American Society of Civil Engineers (ASCE) has standardized the calculation of ET by establishing the modified Penman-Monteith equation as the preferred method for determining ET. A detailed explanation of the ASCE-Penman-Monteith formula and methodology is available here. 
Additional information about calculating ET is below.
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