Evaporation of intercepted rainfall–Comparing canopy water budget and energy balance related long term measurements at a Norway spruce site

Abstract

Rain interception of a Norway spruce stand was analyzed based on canopy water balance measurements Emass and for two eddy covariance (EC) related methods covering a long period (2008–2018). Emass was calculated as residual between gross rainfall above and net rainfall below the canopy. EC related observations are based on the water equivalent either (i) directly measured by eddy covariance (ETEC) or (ii) as residual in the Energy Balance equation (ETEB). To relate the different methods to wet canopy conditions, the Rutter model was used to dynamically calculate water storage in the canopy. Finally, the different time series were integrated over the duration of modeled interception events.The canopy water balance shows a mean annual gross precipitation of 936 ± 173 mm/year of which 376 ± 56 mm/year evaporated due to interception. The majority of rainfall events (81%) is characterized by a depth <5 mm, 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 agreement between observed and modeled throughfall and served as a reasonable standard to define interception events. Wet canopy evaporation as the residual of the energy balance ETEB and from gas analyzer measurements ETEC are both systematically underestimating Emass. The mean annual underestimation of Emass is 145 mm/year for ETEB and 288 mm/year for ETEC. Most of the discrepancy can be mainly explained by an underestimation of turbulent fluxes, at which data is most affected during raining conditions. An analysis of the linear relation between the annual sum of turbulent fluxes and available energy shows the lowest slope (0.57 ± 0.15) for measurements during rainfall, while the highest slope (0.76 ± 0.03) occurs under completely dry conditions. Both fluxes should be gap-filled and corrected separately for dry (transpiration) and wet (interception) conditions in order to determine proper amounts of evapotranspiration with the eddy covariance method.