ASCE Reference Evapotranspiration (ETo and ETr)

Description

Reference evapotranspiration represents ET from a well-watered idealized reference surface and is a function of solar radiation, air temperature, humidity, and windspeed. Reference ET is often considered an upper limit on actual ET, whereas actual ET (ETa) is usually estimated by scaling reference ET downward based on estimates of the fraction of reference ET (EToF) based on remotely sensed or simulated soil and vegetation moisture conditions, and vegetation type and phenology.

ASCE (American Society of Civil Engineers) Reference ET (ETo and ETr) estimates in Climate Engine are derived from the Penman-Monteith model (ASCE-EWRI, 2005; Allen et al., 1998) under ambient meteorological and radiative conditions derived from meteorological reanalyses, gridMET (Abatzoglou, 2013). ASCE Grass Reference ET (ETo) assumes a reference surface of short grass (0.12 m high), while ASCE Alfalfa Reference ET (ETr) assumes a reference surface of tall grass (or alfalfa). 

References

Abatzoglou, J. T., 2013, Development of gridded surface meteorological data for ecological applications and modeling. Int. J. Climatol., 33, 121–131

Allen, R.G., L.S. Pereira, D. Raes, and M. Smith, 1998. Crop Evapotranspiration: Guidelines for Computing Crop Requirements. Irrigation and Drainage Paper No. 56, United Nations Food and Agricultural Organization (FAO), Rome, Italy. 

ASCE-EWRI, 2005. The ASCE Standardized Reference Evapotranspiration Equation. ASCE-EWRI Standardization of Reference Evapotranspiration Task Committee Report. American Society of Civil Engineers, Reston, Virginia.