Interception loss as an underestimated component of evapotranspiration modelling in a forested catchment

Norway spruce monoculture stands often represent a typical vegetation cover in Central Europe and these stands impact the hydrological pattern on a catchment scale significantly. The processes of canopy interception, throughfall, snowmelt and land surface evaporation represent an important part of the water cycle in forested catchments. This study uses a headwater catchment Lysina Critical Zone Observatory located in the western part of the Czech Republic. The catchment with a long-term monitoring (since 1989) has an area of 25 ha and vegetation cover is a Norway spruce (Picea abies) monoculture. We applied two versions of a deterministic, process oriented, lumped parameter hydrological model Brook90 that runs on daily time step. The original version and the upgraded version LWFBrook90R was fully transformed to R environment. Our primary focus was the ability of the models to capture the interception. Both model versions performed well on historic streamflow and in agreement with each other according to the catchment water budget. The annual interception on catchment was almost 30% (260 mm) of the open area precipitation in the period of 1994–2019, this was calculated based on precipitation collected on catchment in an open area (bulk precipitation) and under the forest canopy (throughfall) in monthly interval. Both model versions showed notably lower interception than observed. However, the LWFBrook90R performed much better with an interception of 15% compared to only 1% of interception in the original Brook90 version. The difference was in evapotranspiration components, especially transpiration, that compensated the lower interception loss compared to observed and thus the total evapotranspiration of LWFBrook90R and Brook90 did not differ notably. These preliminary results show a necessity to focus on better model parametrization in order to correctly capture complex hydrological processes such as interception loss in forested catchments.