Climate and land use induced changes in evapotranspiration - experimental evidence from a forested catchment in Germany

Climate changes are expected to trigger changes in all water budget components at any scale. For Central Europe, higher evapotranspiration (ET) rates are already observed, other factors like land use or land cover characteristics change in parallel, but experimental evidence of the interdcations is limited, as it requires challenging long-term measurements. We take advantage of the well-documented hydro-meteorological dataset from the forested research catchment Wernersbach in Saxony, Germany, covering 52 years between 1968 and 2019 (Pluntke & Bernhofer et al., 2023).

We analyzed hydro-climatological time-series for linear trends and for breakpoints. Significant positive trends were found for global radiation, mean air temperature and grass-reference evaporation, as well as for the difference between catchment precipitation and runoff (P-R; hydrological estimate of ET). Precipitation increased and runoff decreased over the 52 years, but not significantly.

Air temperature and global radiation show significant breakpoints around 1988 and 1996, respectively, with below average conditions before and above average conditions after the breakpoints. Temperature change is associated with global warming, and possibly with the independent regional effect of air pollution. Since the 1960s, large sulphur dioxide emissions from fossil fuel burning led to a high aerosol density in the troposphere reducing solar radiation over most of Europe and North America. While this effect was reduced by filtering the emissions elsewhere in the early 1980s, it continued in neighboring parts of today’s Germany, Poland, and Czech Republic until the early 1990s. Breakpoint of grass reference evaporation coincides with air temperature (1988), and the breakpoint of P-R is a few years later.

We attributed changes in ET to changes in land use and climate by applying an adapted Budyko framework and enabled insights into their interactions. The sulphur dioxide emissions triggered widespread forest dieback in regions over 600 m in Saxony. Consequences were decreasing ET in the 1970s/1980s. The Wernersbach catchment (390 m) shows a similar tendency (not significant). Since the 1990s, both climate (increasing atmospheric demand) and land use (healthier forest stands and improved management practices) led to an increase of ET. In 2010s, climate induced damages of forest stands (due to droughts, storms, snow load, and bark beetle infestations) led to a drastic decrease of ET in Wernersbach despite favorable climatic conditions for ET. Since the intensity and frequency of such extreme events are likely part of climate change, they may cause greater regional changes in the water balance than direct effects of climate change, and may cause lasting damage to Ecosystem Services of forests, like flood mitigation, or carbon sequestration.

Our results show the need for climate adaptation measures in forests, such as the establishment of a more site-specific mixed forest, and a sustainable  forest management.

 

References

Pluntke T. & Bernhofer C., Grünwald T., Renner M., Prasse H.: Long-term climatological and ecohydrological analysis of a paired catchment – flux tower observatory near Dresden (Germany). Is there evidence of climate change in local evapotranspiration? J Hydrol 617 (2023), https://doi.org/10.1016/j.jhydrol.2022.128873