Rainfall interception – a year-round crucial component of evapotranspiration and potential consequences for eddy-covariance measurements. Comparing long-term measurements of canopy water budget and eddy-covariance fluxes at a Norway spruce site

Interception of a Norway spruce stand was analysed based on canopy water balance measurements E_mass and for Eddy Covariance (EC) related measurements. The study site is located in the Tharandt Forest (Germany) and the analysis covers a long period from 2008 to 2018. E_mass was calculated as residual between gross rainfall above and net rainfall below the canopy. EC related observations are based on the water equivalent either directly measured (uncorrected) by Eddy Covariance (ET_EC) or as residual in the Energy Balance equation (ET_EB). Additionally, evaporation of intercepted water was modelled with the Penman-Monteith equation, which was adapted for a gradually wetted canopy (E_Rutter) by the use of the Rutter model. The latter approach was used to integrate the time series of all methods over the duration of modelled interception events leading to different estimates of wet canopy evaporation.

The canopy water balance from 2008 to 2018 shows a mean annual gross precipitation of 936±173mm and a mean annual interception evaporation of 376±56mm (E_mass). The majority of rainfall events (81%) is characterized by a depth less than 5mm, which leads to a high fraction of annual precipitation being captured by the canopy surface (0.41). The application of the Rutter model yielded good results with a mean modelled annual interception E_Rutter of 361±47mm being very close to E_mass. Thus, the model served as a good standard to define interception events. The water equivalent of wet canopy evaporation as the residual of the energy balance ET_EB and from gas analyser measurements ET_EC are both systematically underestimating E_mass, to a higher extent for the winter than summer half-year. On a mean annual basis, ET_EB and ET_EC underestimate E_mass by 145mm and 288mm, respectively. Comparing the totals over the majority of interception events, ET_EB corresponds to only 72% and ET_EC to only 33% of canopy water balance based measurements.

One reason for this underestimation might be a scaling problem between the interception measurement site and the flux tower footprint, which could not be resolved by the application of a simple scaling factor. A more likely explanation is the underestimation of turbulent fluxes by the EC method. The data is most affected during raining conditions with the highest gap in winter. An annual analysis of the linear relation between the sum of turbulent fluxes and available energy shows the lowest slope (0.57±0.15) for measurements during rain, while the highest slopes occur under completely dry conditions (0.76±0.03). Wet canopy conditions without rainfall seem not to be as crucial for energy imbalance as rain, but affect the closure gap for the winter half-year.

Records of EC measurements are generally too low and the magnitude of supplied sensible heat and sustained latent heat flux rates during interception events remains unclear. We conclude that gap filling and correction of both turbulent fluxes should be done separately for rain (interception) and dry (transpiration) conditions in order to determine proper amounts of evapotranspiration with the eddy-covariance method.