20 Years of Rainfall Interception in a Young Coastal Sitka Spruce Forest

Interception of rainfall by forests and its evaporation is an important component of the water balance. Throughfall troughs, roving below-canopy storage gauges, and stemflow collars were used to measure how interception changed as a young forest grew. The study took place at the Carnation Creek watershed on the west coast of Vancouver Island, BC, Canada (48°54'N, 125°0'W, 50 m) mean annual precipitation of 3050 mm of which over 95% falls as rain, mean annual temperature 9.8 C. In the late 1990s, the 20-year old forest was 5 to 8 m tall, 1500 stems/ha and approximately 70% canopy cover, and had an annual throughfall of 78% and stemflow 9% of the rainfall. By 2013, the forest was 15-20 m-tall and throughfall had decreased to 68% and stemflow remain at 10% of the rainfall. In the late 90's stemflow was initiated by storms greater than 5 mm, while it now requires storms of over 10 mm. Consequently, annual interception loss increased from 13 to 22±4% of the rainfall. Interception loss is now similar in size to transpiration. The fraction of the storm intercepted decreases with an increase in storm size. The small storms and warmer conditions in the summer mean that interception is much greater percentage of the rainfall in this period than in winter. Interception loss is approaching that was measured in a nearby old growth stand.