Estimating Reference Crop Evapotranspiration with ETgages

Three years of daily reference evapotranspiration measured by atmometers $\left({\mathrm{ET}}_g\right)$ were compared to the values computed from the ASCE standardized Penman–Monteith equation $\left({\mathrm{ET}}_r\right)$ using co-located meteorological measurements at 19 locations across North Carolina. The atmometers underestimated daily ${\mathrm{ET}}_r$ by an average of 21% across the study area. Linear regression models between ${\mathrm{ET}}_g$ and ${\mathrm{ET}}_r$ had intercepts significantly different from zero and slopes different from one, but would generally yield a gauge error within $1\mathrm{mm}{\text{day}}^{-1}$ . The ${\mathrm{ET}}_g\text{-}{\mathrm{ET}}_r$ relationship was found to be highly sensitive to precipitation and wind speed, but rather insensitive to humidity, radiation, and temperature. Although wind speed is generally low in the study area, the insensitivity of ETgages to wind caused severe underestimation in those periods when wind speed was high. Mean absolute error increased from 17% when wind speed was below $1\mathrm{m}{\mathrm{s}}^{-1}$ to 64% when wind speed was greater than $5\mathrm{m}{\mathrm{s}}^{-1}$ . Precipitation can temporarily disrupt ETgage evaporation and cause underestimation of ${\mathrm{ET}}_r$ . The linear relationship between ${\mathrm{ET}}_g$ and ${\mathrm{ET}}_r$ in rainy days was significantly different than that of the clear days. Analysis of the local relationships suggested that they are sensitive to their major surrounding physiographic environment and to the strictly local surface conditions, but not to the intermediate mesoscale surface environment. As a result, different linear regression equations were developed to adjust ${\mathrm{ET}}_g$ to ${\mathrm{ET}}_r$ in three land regions and in dry or wet conditions.