Using Modified Bellani Plate Evapotranspiration Gauges to Estimate Short Canopy Reference Evapotranspiration

Modified Bellani plate atmometer has been offered as an alternative and simpler technique to combination-based equations to estimate evapotranspiration (ET) rate from green grass surface. However, there is a lack of information on its’ accuracy in humid climates. The evaporation rate $\left(E_A\right)$ from one type of atmometer marketed under the brand name ${\mathrm{ET}}_{\text{gage}}$ ® (or ETG) with a Number 30 green canvas cover that simulates the ET rate from a green grass surface was tested against the reference ET of a short green grass canopy $\left({\mathrm{ET}}_o\right)$ computed using the Food and Agriculture Organization of the United Nations Paper No. 56 Penman–Monteith (FAO56-PM) equation in two sites in north-central Florida. The ETG underestimated the ${\mathrm{ET}}_o$ as much as 27%. The root mean square error (RMSE) of daily estimates of $E_A$ ranged from 1.03 to $1.15\mathrm{mm}$ . Data analyses indicated that the most of the poor performances and underestimations of the ETG occurred on rainy days. Using only the nonrainy day $E_A$ versus ${\mathrm{ET}}_o$ relationship, the daily RMSE was as low as $0.47\mathrm{mm}$ and $r^2$ was as high as 0.89, and the underestimations were within 3% of the ${\mathrm{ET}}_o$ . Averaging daily ETG readings over 3 and 7 day periods considerably improved (lower RMSE and percent error, $\%E$ , and higher $r^2$ ) ${\mathrm{ET}}_o$ estimates. The ETG performed quite well on nonrainy days. The adjustment factors were developed and tabulated as a function of rainfall amount to adjust the $E_A$ values on rainy days. Results showed that an average adjustment factor of $0.84(E_A∕0.84=\mathrm{E}{\mathrm{T}}_o)$ can be used as a practical number if rainfall observations are not available. The underestimations of the ETG on rainy days were attributed, in part, to the wetting of the green canvas cover due to the rainwater accumulations on it and to the lower diffusivity (higher resistance) value of the canvas cover $\left(112–294\mathrm{s}{\mathrm{m}}^{-1}\right)$ compared to the diffusivity of a green grass surface used in the ${\mathrm{ET}}_o$ definition $\left(70\mathrm{s}{\mathrm{m}}^{-1}\right)$ . Although it is found that the ETG is feasible and practical device, the $E_A$ values measured on rainy days require careful interpretation in humid and rainy climates such as Florida. The rainy day $E_A$ values should be used cautiously with the proper regression equation and adjustment factors to estimate ${\mathrm{ET}}_o$ for irrigation scheduling if the input variables are not available to use the FAO56-PM equation for ${\mathrm{ET}}_o$ estimates.