Do eddy-covariance measurements systematically underestimate evapotranspiration of coniferous forests? Results from a paired catchment – flux tower observatory near Dresden (Germany)

We combine long-term hydro-meteorological data from the small research catchment Wernersbach (WB, 4.6 km², dominated by Norway spruce) in operation since 1967 and from two eddy-covariance (EC) flux towers, all located in the Tharandt Forest, Germany. This combination forms an observatory, addressing actual evapotranspiration ET from a water budget perspective (catchment) and from an energy perspective (EC flux towers). However, obvious differences exist in time resolution. The spruce dominated tower DE-Tha is located a few kilometres east of the catchment. After a windbreak of another spruce stand (situated inside the catchment) and planting of deciduous oaks, the tower DE-Hzd was set up in 2009. We recently reported systematically about the observatory and the long-term water budgets in Pluntke & Bernhofer et al. (https://doi.org/10.1016/j.jhydrol.2022.128873).

The catchment and both towers did not show any systematic differences in meteorological data (especially wind-loss corrected precipitation totals are almost identical), allowing us to address observed differences in ET as (i) due to different soil and hydrogeological characteristics as well as (ii) due to methodological aspects. The catchment term ET plus storage, derived from precipitation P minus runoff R, showed the expected high variability with a significant increase over the more than 50 years of operation. The older, spruce-dominated flux-tower DE-Tha showed much lower inter-annual variability in ET with an average annual total of 486 mm (1997 to 2019), but no significant trend. For the same period, average catchment ET was 734 mm/year. The younger flux-tower DE-Hzd showed ET values closer to catchment ET at the very dry end of the ten-year record (2010 to 2019).

For the 23 years of parallel measurements, annual ET from EC was about 250 mm lower than catchment ET, despite the careful correction of tower ET for energy balance closure. Catchment ET = P – R might have a small bias towards larger ET, as the subsurface catchment size of WB could be up to 0.4 km² smaller. In addition, precipitation and runoff may contribute to higher catchment ET. However, the difference is too large to be explained by measurement bias alone. Flux tower ET is compared to (i) independent measurements of ET components, and (ii) model output of BR90. There is evidence from interception and transpiration measurements at the flux tower that more than 100 mm of intercepted water could be missing in the annual ET from EC. Model results show a large additional contribution of interception due to negative sensible fluxes in fall and winter. The difference in ET between tower and catchment of 250 mm is probably due to a variety of reasons: overestimation of catchment ET (up to 50 mm), soil characteristics (50-100 mm), and underestimation of tower ET (100-150 mm).

We conclude that the EC closure correction during interception events needs to be revisited. Generally, results of ET monitoring of similar evergreen forests in a humid climate should be checked for missing contribution of interception, as EC records might be generally too low. This illustrates the necessity of redundant and complementary measurements when dealing with large system complexity.