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 ET, 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 ETO 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.. Based on a national study in the U.S. and studies by European and American researchers, the authors recommend that the FAO-PM ET, method from FAO56, when applied on an hourly or shorter basis, use rs = 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. (c) 2005 Elsevier B.V. All rights reserved.