[ET.sub.a] Differences between the METRIC Model and Atmometer. The difference between the daily [ET.sub.a] estimated with the METRIC model ([ET.sub.a]-METRIC) and [ET.sub.a] estimated by atmometer ([ET.sub.a]-atm) is presented in Figure 10.
The more negative values were presented early in the growing season (DOY 154) (V3) due to high [K.sub.c] value (0.51) used with atmometer method compared to the low [K.sub.c] value (0.38) used in the METRIC model method.
This discrepancy was due to not only the wind speed values but also [ET.sub.r] recorded in atmometer on DOY 194 (VT).
Also, at the Oak Lake site the weather station and automated atmometer were located in smooth hill.
Based on the results from our study, [ET.sub.r] values from atmometer need to be adjusted during the windy days.
A limitation of satellite derived ET is that [ET.sub.a] is estimated only for clear sky days, while [ET.sub.a] from atmometer is estimated for any weather condition.
In conclusion the results of this study can be used by researchers and producers to estimate actual evapotranspiration and improve irrigation water management at regional and field scales using satellite-based remote sensing METRIC model method and at field scale using atmometer method.
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