ASCE's standardized reference evapotranspiration equation.

The ASCE Evapotranspiration in Irrigation and Hydrology Committee (ASCE-ET) is recommending, for the intended purpose of establishing uniform evapotranspiration (ET) estimates and transferable crop coefficients, two Standardized Reference Evapotranspiration Surfaces : (1) a short crop (similar to grass) and (2) a tall crop (similar to alfalfa), and one Standardized Reference Evapotranspiration Equation . The standardized equation is derived from the ASCE-Penman Monteith equation, by simplifying several terms within that equation. The standardized equation, with appropriate constants provided in an accompanying table, is used to calculate evapotranspiration for the standardized short reference (ET os ) and/or evapotranspiration for the standardized tall reference (ET rs ). One constant is in the numerator and one is in the denominator. The constant in the right-hand side of the numerator (C n ) is a function of the time step and aerodynamic resistance (i.e., reference type). The constant in the denominator (C d ) is a function of the time step, bulk surface resistance, and aerodynamic resistance (the latter two terms vary with reference to type, time step, and daytime/nighttime).