Runoff and phosphorus export patterns in large forested watersheds on the western Canadian Boreal Plain before and for 4 years after wildfire

Abstract

We examined water and phosphorus export patterns in four large (130 to 247 km2) Boreal Plain watersheds (two burned and two reference) before and for 4 years after wildfire and the influence of precipitation intensity and timing on these patterns. Time series analysis of the one burned and one reference watershed monitored before and after fire demonstrated that relative to changes in the reference watershed over the same time period, runoff and dissolved and particulate phosphorus exports were higher in the burned watershed during the four post-fire years than before the fire (P = 0.001). Comparison of post-fire means in all four watersheds monitored for the post-fire years demonstrated that mean water and particulate phosphorus exports were 1.6 (P = 0.01) and 3.7 (P = 0.03) times higher in burned than reference watersheds, respectively. A similar pattern existed for dissolved phosphorus exports, but differences were not significant (P = 0.13). Thus, the pre- vs. post-fire comparison was consistent with, and more powerful than, the post-fire treatment vs. reference comparison. As of year 4 post-fire, burned watersheds continued to export more water and particulate phosphorus per unit area than reference watersheds, particularly during peak flow periods. Key words: watershed disturbance, boreal forest, stream, water quality, fire, phosphorus, runoff.