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

Abstract Continued development of networks of electronic weather stations worldwide has increased the availability of weather data for calculating ET o on an hourly basis. There has been question and debate as well as studies on the appropriate expression and parameterization for the surface resistance ( r s ) 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 r s  = 70 s m −1 for hourly periods and using a lower r s  = 50 s m −1 value during daytime and r s  = 200 s m −1 during nighttime. Variability in hour to hour trends in r s among locations and dates makes it difficult, if not impossible, to establish a consistent algorithm for r s . However, the relatively good and consistent accuracy in ET o when using a constant r s  = 50 s m −1 during daytime gives good reason to recommend this value as a standardized parameter and coefficient for calculating ET o . Based on a national study in the U.S. and studies by European and American researchers, the authors recommend that the FAO-PM ET o method from FAO56, when applied on an hourly or shorter basis, use r s  = 50 s m −1 for daytime and r s  = 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 r s  = 70 s m −1 should continue.