For ET, you should accumulate over the time step. Only if you are considering a flux rate (e.g. mm/hr) would you average (e.g. accumulate rainfall, but average rainfall intensity).

Thanks Barry

So, if we have evapotranspiration from a lysimeter every 15 minutes and I want to convert it into hourly measurement... I have to accumulate each 15 minutes measurement. Am I right?

Thanks again

Yes, assuming that the evapotranspiration is in mm (for each fifteen minutes). Accumulate this to get mm for each corresponding hour.

Treat aggregation of ET as you do in the case of rainfall.

Hi Barry

What is the minimum time step to aggregate ET data? Also, is that method valid for Eddy system?

There is no minimum time step for aggregation of ET data. The only issue is that the smaller the time step, the smaller the amount of ET. For eddy covariance data, this depends on whether it is expressed as an amount or a rate. You aggregate an amount, you average a rate.

Thank you very much Barry.