Development and testing of a snow interceptometer to quantify canopy interception in the rain/snow transition zone of the North Cascades, Washington, USA

Tree canopy snow interception is a significant hydrological process, capable of removing up to 60% of snow from the ground snowpack. Our understanding of canopy interception has been limited by our ability to measure whole canopy interception in an undisturbed forest setting. This study presents a relatively simple and inexpensive technique for directly measuring snow canopy interception using an interceptometer, adapted from Friesen et al. [2008]. Changes in canopy mass due to interception result in a measureable trunk compression, according to Hooke's law of elasticity. The interceptometer is composed of four linear motion position sensors distributed evenly around the tree trunk, which directly measure trunk displacement caused by interception. Through calibration techniques, the amount of canopy snow required to produce the measured displacements can be calculated. We incorporate a trunk laser-mapping installation method for precise sensor placement to reduce signal noise attributed to sensor misalignment. The simple design of the interceptometer allows for installation in relatively remote locations with limited vehicle and power access. Through interceptometer development and installation, we demonstrate instrument performance on a western hemlock (Tsuga heterophylla) for a snow interception event in late November, 2011. Our study finds a snow interception efficiency of 83 ±15% of total accumulated ground snowfall with a maximum interception capacity of 50 ±8 mm snow water equivalent (SWE). The observed interception event is compared to simulated interception represented by the Variable Infiltration Capacity (VIC) hydrologic model. The model generally underreported interception magnitude by 33% using an LAI of 5 and 16% using an LAI of 10. The interceptometer was also able to capture periods of intrastorm accumulation of up to 3 mm SWE hr-1 and drip melt rates of 0.75 mm SWE hr-1, which the model failed to represent. While further validation is necessary to reduce instrument error, our results indicate that forest interception magnitude may be underestimated in maritime areas such as the Pacific Northwest. Through further instrument development and deployment, this method may prove to be a valuable tool capable of providing direct canopy snow interception estimates within intact canopy structures at high temporal resolution and sensitivity.