Micrometeorology meets Tracer Hydrology in determining Forest Evapotranspiration

Since 1974, the Meteorological Institute of the University of Freiburg has been carrying out micrometeorological measurements in a pine forest in southwest Germany. Above all, the aim of these measurements is to determine the radiation, energy and water balance of an area at the interface of a forest ecosystem and of the atmosphere. The connected climatological evaluations resulted in the long-term determination of evapotranspiration, using various methods of calculation (Sverdrup, aerodynamic methods, eddy correlation method and water balance method). The agreement between these methods of determination is unsatisfactory.

In 1998, we started to join micrometeorological and tracerhydrological methods. For this, the following water and water vapour transports in the vertical profile of atmosphere - vegetation - soil - transition zone are determined from both sides:

- precipitation above canopy - interception gain (fog precipitation) - throughfall - stemflow - canopy transpiration - transpiration of the understory - interception - evaporation from the forest floor - storage in the ground (soil moisture) - lateral water movements - seepage.

Besides the micrometeorological methods mentioned above, sap flow measurements are used to determine the transpiration of the pine stand and small lysimeters for the physiological evaporation of the understory.

From the hydrological side, vertical fluxes of 18O are measured. The temporal pattern and the fraction of isotopes clarifies direction and quantity of water and water vapour transports in the continuum soil - vegetation - atmosphere. The isotope ratios of the water vapour in the air above the canopy and in the stand air are determined too.

Furthermore, the hydraulic conductivity of the unsaturated zone is determined by means of tracers in combination with pumping and infiltration experiments. The beginning and duration of the components of forest precipitation (collection of fog droplets, throughfall and stemflow are recorded online and quantitatively and temporally defined. This is also applied to soil moisture.

There are formulas for the aridity index, which classify the area under investigation to be semi-arid (mean yearly temperature average > 11 degree C, average yearly precipitation 500 - 600 mm). The local formation of ground water is problematic and is severely limited or prevented by the existing vegetation cover. Here, the investigation should define and obtain clarity, using methods to determine the energy balance and using hydrological methods.

Beside this, the ability to transfer and apply the investigations to arid areas which are not as monitored, is aimed for.