Measuring crop evapotranspiration

Published on by in Non Profit

One of the project development objective indicator is "Irrigation efficiency (Water withdrawn used for crop production) - This indicator measures crop evapotranspiration as percentage of the total amount of water supplied" (I copied it as it was in the TOR).

Question: how can it be measured in practice (the project area is about 30,000 ha)? If someone did similar job, please share your experience.