Reference Crop ET

Sensible and latent heat transfer equations combine with the energy balance equation, following simplifying assumptions by Penman, to result in the well-known Penman equation, which produces evapotranspiration (ET) for a defined reference crop. The common Penman-Monteith equation for estimating ET involves a two-step process: (1) determining a reference ${\mathit{ET}}_{\mathit{ref}}$ which has been for either clipped grass or alfalfa, and (2) by multiplying ${\mathit{ET}}_{\mathit{ref}}$ with a crop coefficient, ${\mathit{K}}_{\mathit{c}}$, to obtain an estimate of actual ET for a particular crop or surface condition. The chapter provides procedures for estimating solar radiation, vapor pressure, and wind speed data when they are missing or of questionable quality. Estimates of reference ET are produced by measured evaporation from atmometers and from evaporation pans. These measurement systems can represent conditions within a local environment, which may be useful for some applications.