Simple ET0 Forms of Penman’s Equation without Wind and/or Humidity Data. II: Comparisons with Reduced Set-FAO and Other Methodologies

In the companion paper, a set of simplified formulas was derived to approximate the Penman equation for estimating reference evapotranspiration (ET0) from limited data. The first suggested formula is explicit for routinely measured data: air temperature, solar radiation, relative humidity, and wind velocity. Simplifications of this formula were made for places where wind speed and/or relative humidity data are not available. The performance of the new derived formulas was tested under various climatic conditions using high-quality daily weather data from 17 agricultural weather stations across California obtained from the California Irrigation Management Information System (CIMIS) database. The daily CIMIS-Penman estimations, obtained by summing hourly computations over 24 h periods and by using the standardized FAO-56 Penman Monteith scheme applied on a daily basis, were used as the basis for comparisons at the 17 stations in California. The performance of the formula with a full set of data is excellent. The formula without wind data was compared with other common empirical methods (reduced-set FAO procedure not requiring wind data, Turk, and radiation-based Hargreaves), whereas the formula without wind and humidity data was compared with the radiation-based Hargreaves and the reduced-set FAO procedure that does not require wind and humidity. Comparisons of the methods indicated that the proposed formulas is a better option than the other empirical methods for estimating ET0 when wind and/or relative humidity data are missing.