Proceedings zur 6. Fachtagung BIOMET - Deutsche ...

nearby climate station Grillenburg (mean annual temperature 7.2 °C, mean annual precipitation
853 mm; based on the period 1961 to 1990) is operated since 1955.
The EC flux measurements are performed at the Anchorstation Tharandt above a more than
100 yr old spruce forests. The site was selected to represent the dominating spruce forest in
the area and is judged to reflect typical growth rates due to its average status in water and
nutrient availability. The measurements include water and carbon fluxes by EC at a resolution
of 30 min at the measurement height of 42 m above ground. The results of 9 years of continuous
measurements indicate good quality of both water and carbon fluxes with low energy balance
closure gaps (around 10 to 25 percent of available energy for the individual years).
Fig. 1: Land-use in the Wernersbach catchment (based on recent data)
3 Results
The variability of ET between years and methods is large. The long-term watershed ET is
characterised by infrequent droughts with an average return period of 10 years. Obviously
storage effects reduce this variability somewhat; so we expect a larger interannual variability
in actual ET. However, between 1997 and 2005 eddy covariance measurements show a variability
of 129mm which is small compared to more than 400mm between the individual years
of watershed ET (Tab.1, see also GRÜNWALD & BERNHOFER, 2007). Bowen ratio as an indicator
of water availability was typically around 0.7, with 1.2 in the very dry year 2003.
Tab. 1: Water budget components in mm (uncorrected precipitation P UC , micrometeorological
evapotranspiration ET EC and catchment evapotranspiration P CO -R = ET HY ) and
Bowen ratio BR EC (dimensionless) for the Tharandt forest (dominated by Norway
spruce)
1997 1998 1999 2000 2001 2002 2003 2004 2005
PUC 714 909 826 803 938 1098 501 874 898
ET EC 485 499 518 537 487 445 389 438 478
ET HY 627 753 666 635 879 721 446 950
BR EC 0.66 0.56 0.59 0.59 0.53 0.64 1.16 0.76 0.72
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