Impact of wildfire on discharge and phosphorus export from the Sakwatamau watershed in the Swan Hills, Alberta, during the first two years

Water and phosphorus (P) exports during the peakflow season increased after a fire in early summer 1998 burned 89% of an upland watershed on the Boreal Plain of western Canada. The change in water export between pre(1983) and post-fire (1998 to 2000) periods was higher in the burned (4th order) than reference stream (3rd order) (P = 0.01). Burned to reference stream ratios of particulate P (PP) flow-weighted mean concentrations (FWMC) and export were 1.5 and 2.8, respectively, in year 1 and 2.8 and 6.7, respectively, in year 2 post-fire. Particulate P comprised a similar proportion of total P export in the burned stream before fire and in the reference stream (65%), but a higher proportion after fire during the peakflow season only (77%) (P < 0.02). Phosphorus concentration and discharge (Q) were positively related in both streams, across all Q intensities measured in the case of dissolved P, but only at Q > 1.5 m(3) s(-1) for PP. Changes in P export after fire were evident during peakflow and were largely restricted to the PP fraction. These changes appear to be driven by higher discharge, which enhanced loading of P-rich particulates from the watershed and (or) from the stream channel.