Operational Estimates of Reference Evapotranspiration

Abstract Many forms of the Penman combination equation have been proffered for estimating daily evapotranspiration (ET) by the agricultural reference crops grass and alfalfa ( Medicago sativa L.). This study was conducted to evaluate popular forms of the Penman equation, and to develop and evaluate general relationships for estimating daily average values of canopy and aerodynamic resistance parameters required by the Penman‐Monteith equation. For simplicity and ease of use, resistance relationships were expressed as linear and logarithmic functions of mean plant height. The Penman‐Monteith and other forms of the Penman equation were compared at 11 international lysimeter sites, with the Penman‐Monteith method and a Penman equation with variable wind function developed at Kimberly, ID providing the best estimates of reference ET across the sites. Ratios of computed alfalfa to grass reference ET during peak months at various locations averaged 1.32, and ranged from 1.12 to 1.43. Values of computed ratios were related to local wind and humidity conditions. The development of relationships for canopy and aerodynamic resistances as functions of reference crop height allowed use of the Penman‐Monteith equation in an operational mode, and improved transferability of this resistance form of the Penman equation to a wide variety of climates.