Extreme Water Quality Degradation Following Catastrophic Forest Fires

Abstract:

Global change is impacting the forests of the western United States through rising temperatures, earlier snowmelt, more rain and less snow, greater vapor pressure deficits in spring and autumn, forest dieback and increasing forest fire frequency and severity. A catastrophic forest fire (Las Conchas fire) occurred in central NM, USA, in 2011 burning about 634 km² with an estimated 46% of the fire being of severe or moderate intensity. We used remote sensing to link precipitation events occurring in the burn scar to extreme water quality excursions observed in the Rio Grande downstream of the burned catchments. Charcoal, ash, and sediment was routed through intermittent streams and rivers to the Rio Grande after thunderstorms producing black water events. At four sites along the river, in situ sensors captured the responses of water temperature, specific conductance, turbidity, and dissolved oxygen in late summer to these flood events following the fire. Runoff from burn scars caused turbidity peaks (to >2500 NTU), dissolved oxygen sags (to 0.0 mg L^-1), pH sags (up to 0.75 units) and conductivity changes (both increases and decreases). These water quality excursions extended at least 50 km downstream, with significant implications for the ecosystem health of this crucial river that supplies water to cities and agriculture. In addition, sensors continuously measuring nitrate and phosphate in streams and rivers impacted by the forest fire have been deployed both before and after the fire. Enrichment in nutrients stimulated metabolism in streams once enhanced sediment routing following the fire waned. Sudden, dramatic changes to forested catchments from severe forest fires and forest dieback are very likely to be among the strongest impacts of global change on stream and river ecosystems throughout the western United States.