A model for the class A pan coefficient

Abstract

The class A pan coefficient ( K p) has been found to be determined by the daily wind run ( WR), the average relative humidity ( RH), and the fetch distance ( FD) of the pan. While it is quite easy to measure WR and RH, FD has to be estimated. After a heavy rainfall or irrigation, FD is very large, but decreases rapidly as the field dries out. Therefore, it varies continuously. In practice, FD has been taken as a fixed distance between the pan and the border of the vegetated area. The values of K p recommended by the Food and Agriculture Organization (FAO) grossly overestimate the measured values for a tropical climate. Applying the Penman-Monteith equation to both a grass field and the pan resulted in an alternative simple model for K p which takes into consideration only the average air temperature and the grass/air resistance ratio. This ratio has been found here to vary with the wind speed, their relationship being described by a non-linear function. Bulk grass canopy resistance varied from 10 s m −1 to over 230 s m −1, on a daily time scale, and this does not support the FAO's recommendation of a constant canopy resistance of 69 s m −1 for estimating grass reference evapotranspiration. On a daily or longer time scale the bulk grass canopy resistance does not follow the atmospheric vapour pressure deficit as well as it does on shorter time scales.