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OBSERVATION OF SNOW INTERCEPTION IN SPRUCE CROWNS 
Michael Bründl, Swiss Federal Institute for Snow and Avalanche Research, Flüelastrasse 11, 
CH-7260 Davos-Dorf, Switzerland, phone: ++41 81 4170 172, fax: ++41 81 4170 110, e-mail: bruendl@slf.ch 
and 
Martin Schneebeli, Swiss Federal Institute for Snow and Avalanche Research, Flüelastrasse 11, 
CH-7260 Davos-Dorf, Switzerland, phone: ++41 81 4170 171, fax: ++41 81 4170 110, e-mail: mschneebGslf.ch 
KEY WORDS: time-lapse video recording, image analysis, forest 
ABSTRACT 
Snow interception plays an important part in the hydrological cycle of subalpine forest stands. Processes in 
an ecosystem in winter consisting of tree, snowpack and soil probably change under different meteorological 
conditions. To study the processes in spruce crowns and to determine the amount of snow in the crown a 
touchless method is necessary. 
During two winters a subalpine spruce was observed by a color video camera. Images were recorded with a 
time-lapse video recorder during the whole winter. Illuminated colored balls were fixed on the branches which 
could be recognized day and night in the video images so that the images could be analysed quantitatively. 
The video images were automatically digitized from the video tape with a modular frame grabber including a 
color acquisition module. After image enhancement (filtering, thresholding) the images were analysed with a 
commercial image analysing programme (BioScan OPTIMAS). The illuminated balls could be extracted and 
coordinates of their centers were digitized which showed the movement of the branches during a snow fall event. 
The angle of the branch changed very strongly due to the weight of the snow. We discuss the specific problem 
occuring by subsequent automated analysis. A method with adaptive thresholding will be proposed. 
1 INTRODUCTION 
The hydrological cycle in subalpine forest stands during winter and spring is mainly governed by the amount of 
snow which is kept by tree crowns during snow fall (Lundberg and Halldin, 1994). Snow which is not sublimated 
falls either as snow or as meltwater to the forest floor and is immediately or later available for discharge in 
winter and spring. To judge the conditions under a changing climate a better understanding of this processes is 
required. It is desirable to leave the ecosystem snowpack under the tree undisturbed, therefore a non-invasive 
method for observations and measurements in tree crowns is necessary. Time lapse video is today the method of 
choice. In past investigations with a similar purpose time lapse photography was used (Tennyson et al., 1974). 
In the last years video technology has become an important tool in environmental studies. Especially ob- 
servations of dynamic processes were conducted by this method (Beyer, 1990; Maas, 1992). Storing a lot of 
information on a quite cheap storage medium is another advantage of this method. 
In this investigation observation of snow interception by a video camera yielded the possibility of a continuous 
observation during snowfall events. The goal of this paper is a presentation of our instrumental setup and a 
discussion of possibilities and problems concerning image processing in environmental studies. 
2 METHODS 
2.1 INSTRUMENTATION SETUP 
Field instrumentation: To observe the dominant factors of snow interception a 15 m high spruce was selected 
in a forest near Davos. At a distance of 10 m a color CCD video camera (Sony SSC-C350P Hyper HAD) with 
a resolution of 500 by 582 pixel was mounted on a pole. The camera was equipped with a 12.5-75 mm zoom 
objective. The recording plane in the most interesting region has a size of 5.4 m in the horizontal and 4.1 m in 
the vertical axis. The camera was connected to a time-lapse video recorder (Panasonic AG-6720) which recorded 
IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop “From Pixels to Sequences”, Zurich, March 22-24 1995