Measuring Branch Deflection and Calculating
the Intercepted Snow Mass on Spruce Branches
Michael Bruendl 1
Snow interception plays an important role in the hydrological cycle of mountain forests because it
determines water supply for discharge in spring. A common way to measure the intercepted snow
mass on trees is to put a cut tree on a scale. This method yields accurate results with a high temporal
resolution but it destroys the natural system of tree, snowpack, and soil. Hence, it does not work
when water transport processes between the tree, the snowpack, and the soil are investigated. We
developed a nondestructive method to continuously observe and measure snow load on spruce
branches during a winter.
Throughout a winter season we continuously observed a spruce with a video camera. To quantify
the motion of branches during the snow interception process we suspended small illuminated balls
on branches at different distances from the trunk. The position of the balls at a certain time could be
analyzed by image analysis of the video image. The calculation of the position of the balls could be
partly done automatically. Images recorded at nighttime could be automatically analyzed, while for
daytime images the contrast between the balls and the background was too low. In this case we had
to extract the position of the balls by hand. The result of this analysis is a time series of branch
motion during an interception event.
Given the measured deflections, the intercepted mass on branches was calculated. The deformation
of branches under certain branch temperatures was calibrated with known weights. By using a
finite element model, the calibrations were used to calculate the linear relation between the branch
temperature and the Young's modulus. The calculations indicated that the mechanical properties of
a living branch can vary over the length of the branch. The relation between Young's modulus and
the branch temperature was used as input into a finite element model, allowing calculation of the
intercepted mass. For a fully snow-capped spruce branch, we found an intercepted snow mass of
4.7 kg. The calculated snow mass was compared with snow storage measurements at a spruce that
was put on a balance. The results show that it is possible to estimate the intercepted snow mass on
a spruce by knowing the snow mass intercepted on a single branch.
1
Swiss Federal Institut fr Snow and Avalanche Research, Flelastrasse 11, 7260 Davos Dorf, Switzerland
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