Rn G y ri Uzes ea

where the units for ET0 are mm h-1; Rn and G have units of MJ m-2 h-1; U2 is hourly wind speed at 2 m in ms-1; and T (°C), es (kPa), and ea (kPa) are measured at 2.0 m and are relative to the hour period. The only distinction between the daily Eq. (5.14) and this one is that the 900 coefficient has become 37. Calculation procedures are given by Allen etal. [60].

Calculation of ET0 on hourly or shorter time steps is generally better than using 24-h calculation time steps in areas where substantial changes in wind speed, dewpoint, or cloudiness occur during the day.

The hourly FAO-PM also assumed that bulk surface resistance rs remains constant throughout a day at 70 sm-1. As noted by many authors [4, 34, 35, 37], rs has been observed to change with solar radiation, VPD, leaf temperature, and soil moisture deficit. However, relationships determined at various sites and with various data sets do not provide more consistent estimates than a constant rs. This is discussed by Allen et al. [10, 60]. Thus, assuming a hypothetical grass reference with rs = 70 s m-1 for all hourly periods is realistic, especially because adjustments for atmospheric stability are also absent from ra in the FAO-PM equations.

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