A recommendation on standardized surface resistance for hourly calculation of reference ETo by the FAO56 Penman-Monteith method

Continued development of networks of electronic weather stations worldwide has increased the availability of weather data for calculating ETo on an hourly basis. There has been question and debate as well as studies on the appropriate expression and parameterization for the surface resistance (rs) parameter of the Penman-Monteith (PM) equation and the associated coefficient for the reduced form FAO-PM equation when applied hourly. This paper reviews the performance of the FAO-PM method using rs = 70 s m−1 for hourly periods and using a lower rs = 50 s m−1 value during daytime and rs = 200 s m−1 during nighttime. Variability in hour to hour trends in rs among locations and dates makes it difficult, if not impossible, to establish a consistent algorithm for rs. However, the relatively good and consistent accuracy in ETo when using a constant rs = 50 s m−1 during daytime gives good reason to recommend this value as a standardized parameter and coefficient for calculating ETo. Based on a national study in the U.S. and studies by European and American researchers, the authors recommend that the FAO-PM ETo method from FAO56, when applied on an hourly or shorter basis, use rs = 50 s m−1 for daytime and rs = 200 s m−1 for nighttime periods. This use will provide, on average, good agreement with computations made on a 24-h time step basis. No changes are suggested for the FAO-PM method for daily (24-h) time steps, where use of rs = 70 s m−1 should continue.