Rainfall Interception by Mature Coastal and Interior Forests in British Columbia

Abstract

Interception of rainfall by forest canopies and its evaporation back to the atmosphere is an important component of the hydrological balance. We quantify the influence of six mature forests at two coastal and one southern interior locations in British Columbia on interception loss. Drainage from the canopy was measured with throughfall troughs and stemflow collars that emptied into tipping buckets monitored by data loggers. Interception loss was evaluated on an event basis and summed to monthly and seasonal totals. The throughfall coefficients increased with a decrease in canopy volume with values of 0.3 for the south coast forest, 0.5 for the north coast forests, and 0.6 for the southern interior forests. The respective saturation capacities were 2.0, 1.1, and 0.6 mm. The average evaporation rates during events varied from 0.05 to 0.3 mm h-1 at all sites. Consequently, for large events, interception loss was dominated by evaporation and replenishment of water in storage during the event. Increasingn event size increased interception loss, which tended to plateau at large events. The coastal forests had throughfall 75±2 percent, stemflow 1±0.2 percent, and interception loss 24±2 percent of the May to November rainfall. The respective values for the interior forests were 72±3 percent, 0.05±0.05 percent, and 28±3 percent of the late May to October rainfall. The similarity between coastal and interior sites in the partitioning of the seasonal rainfall resulted from the differing distributions of event size and similar evaporation rates of intercepted water during events.