NWG2 - Micro Meteorology


Aims

A full determination of the entire fluid system for the case study of the Thuringian basin requires to quantify the above-ground input quantities like precipitation, evaporation and the terms of the modulated energy budget. To provide a comprehensive picture of heat exchange, evapotranspiration of the land surface, runoff and groundwater recharge in deeper ground layers, it is needed to consider all driving forces in a temporal resolution of weather events on regional level. Due to its potentially large groundwater recharge rates, the forest ridges, surrounding the Thuringian basin, are of particular interest. Therefore, high quality evapotranspiration measurements are conducted above three representative forest sites, in the Hainich national park, the Dün-forest close to Leinefelde and the Wetzstein site, located in the south-eastern part of the Thuringian Forest as well as on an agricultural crop land near Gebesee.

Working area

These evapotranspiration measurements, which can be used as a basis for comprehensive extrapolations, can only be achieved with considerable technical effort. For this purpose, within the INFLUINS project, the eddy-covariance method is used to determine the evapotranspiration on four ecologically representative sites. With this method, the non-destructive detection of evaporation above a 1-2 km² large area can be achieved in the required temporal resolution.

Methods

Supportive sap flow measurements (Granier, 1985) were established on all forest sites in order to determine the transpiration. These additional measurements enable a separation between transpiration of trees and the understory transpiration, the evaporation of soil as well as the interception. The used sap flow method becomes one of the most used techniques in order to determine the transpiration rates of woody plants. The empirical approach enables a correlation between heat dissipation along a heated probe inside the sapwood of a tree and the water transport in the surrounding tissue. Up to now the development of wounded tissue as a consequence of tissue disturbance was rarely investigated but within the scope of the INFLUINS project it was able to detect and quantify experimentally the influence for the measurements. By the development of a new calibration-function, the number of sample-trees could be reduced and more reliable long-time measurements improves the accounting of evaporation rates in forest ecosystems.

NWG2-AC-vs

Figure: Correlation between sap flux density measurements, performed in freshly drilled holes (SFD no wound) and different stages of wound development (SFD wound). Up to a 40% lower sap flux densities (SFD) were found. But also obvious is that time since sensor installation (5, 11, or 22 weeks after installation) is not clearly correlated with the detected reduction.                    

Results

An optimized measuring setup, considering tree-species composition in combination with water flux measurements enables a regionalized determination of the evaporation rates in the area of the Thuringian basin.

Team

Members of the Team NWG2 you will find here.