Measuring branch deflection of spruce branches caused by intercepted snow load

Abstract

Snow interception plays an important role in the hydrological cycle of mountain forests since 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 non-destructive method to continously observe and measure snow load on spruce branches during winter. Throughout a winter season we continously 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 given time were measured by analysis of the video image. 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. The calibrations were used to calculate the linear relation between the branch temperature and the Young's modulus of a branch. This relation was used as input into a finite element model which calculated the intercepted mass. For a fully snowcapped 220 cm long spruce branch we found an intercepted snow mass of 4.7 kg. The calculated snow mass was compared with snow storage measurements of different branches and showed that this method allows estimation of snow mass intercepted by a single branch.