Can the energy balance residual be used to estimate boreal forest evapotranspiration successfully?

In principle, evapotranspiration (ET) can be estimated by solving for the residual of the energy balance measurements of net radiation, sensible heat flux density and ground heat flux density, without measuring the vapor flux directly. This method is often avoided because of the lack of energy balance closure of all components, which creates uncertainty for operational estimates of ET. We show that the residual can agree well with the direct eddy covariance measurement after data have been quality-controlled and gap-filled. This is tested using energy balance data from three forests in Saskatchewan Canada during both wet and dry years. The seasonal cumulative ET agrees within less than 5% and weekly agreement is very good. There is more scatter among the daily data and we do not recommend this method for shorter than daily averages unless all energy storage terms are measured very well. We conclude that forest ET can be estimated cost effectively using the energy balance residual technique as daily, weekly and seasonal averages. This would not replace sets of full flux measurements at permanent flux stations but can be used in many other applications where a full instrument suite is not available.