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Meteorological Drivers of Mass and Energy Exchange between Inland Waters and the Atmosphere (MEDIWA)

Subject Area Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
Atmospheric Science
Term from 2020 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 445326344
 
Final Report Year 2024

Final Report Abstract

A proper quantification of energy exchange, evaporation rates and gas fluxes between inland waters and the atmosphere is crucial for sustainable water resource and ecosystem management. Direct measurements are possible but costly and technically challenging. Thus, modelling approximations are typically used. However, their reliability depends on model choice, proper parameterization, and data representativeness. This study addresses the improvement of model estimates in particular by taking into account the spatial and temporal variability of the relevant driver variables. Over three years, we analysed mass and energy fluxes at the Bautzen Reservoir (Lusatia, Germany). A floating outdoor laboratory equipped with an eddy covariance (EC) system was used to measure continuously gas, i.e., H2O, CO2 and CH4, and energy fluxes, i.e., H and LE. Hydrochemical probes monitored CO2, CH4 and O2 concentrations in different depths near the water surface. To assess spatial variations and the alteration of meteorological conditions along the fetch; atmospheric turbulence structures (power spectra and co-spectra), H and meteorological basic variables was measured at three additional EC stations installed on a transect in direction of the main wind direction. These long-term measurements were complemented by monthly water sampling and measurements using bubble traps and floating chambers. Joint experiments with cooperating research groups added valuable insights using a mobile EC system on an autonomous robot boat, wind LIDAR measurements, and an evaporation vessel. Our investigations revealed novel mechanisms of gas exchange under low wind conditions. We could show that a surface water film with elevated CO2 concentrations forms in calm nights, which significantly triggers the CO2 exchange. This process discovered helps to understand the discrepancies that have been often observed between hydrochemical and micrometeorological gas fluxes measurements. For further analyses, we developed a novel measurement system for probing gas concentrations in confined water layers. We figured out that the transition from terrestrial to aquatic atmospheric conditions occurs very closed to the shore and the adaptation is completed at a distance of less than 200 m. The combination of the different measurement methods revealed that the wind field above a water surface can be very heterogeneous with very complex spatial structures, which contradicts common ideas. Furthermore, main results of the predecessor project -TREGATA - were confirmed. It was proven that evaporation from open water surfaces is significantly lower than classic model concepts and prevailing doctrines claim. In the same way, the large inter-annual variability of greenhouse gas emissions (i.e., CO2 and CH4) was verified, which underpins the need for long-term monitoring.

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