8 Forest Fire Impacts on Hydrochemistry and Hydrology

Monday, 30 April 2001
1:00 PM - 5:00 PM

SOIL & WATER EFFECTS ANALYSIS OF MANAGEMENT ALTERNATIVES FOR THE

Paper number: 8-12
Program: THEME SESSIONS
Time: 3:45 PM – 4:00 PM

Authors

Abstract

Hydrologic and soil response in the Santa Fe Municipal Watershed (“the Watershed”) were analyzed for a variety of conditions as part of an Environmental Impact Statement (EIS) prepared to assess management alternatives (“proposed actions”). The primary objective of the proposed actions is to reduce the risk a catastrophic fire poses to the water supplies for the city of Santa Fe, including post-fire sedimentation of reservoirs in the Watershed (which provide the city with roughly 40% of the City’s annual water supply). The management alternatives range from an aggressive Mechanical Thinning with Prescribed Low Intensity Burning Alternative to the No Action Alternative. The No Action Alternative includes two separate conditions: the near-term condition in which the tree canopy densities continue to increase, and the condition following to a catastrophic stand-replacement fire in the Watershed. The high probability of a catastrophic fire in the relatively near-term supports considering a stand-replacement fire as part of the No Action Alternative.

Our analyses focussed on three key issues: Erosion and Sediment Yield (including potential for movement of sediments into water supply reservoirs), Peak Flood Flows (including projecting peak discharges downstream from reservoirs on the Santa Fe River), and Watershed Water Yield. To quantify each summary effects measure, we used data and observations from similar watersheds that have burned, and for the sediment yield and peak flow analyses we also utilized mathematical models to compute the relevant effects measures. In all cases, we employed models that have been previously demonstrated to be applicable for analysis of natural hydrologic systems. The results of the analyses indicate that a catastrophic fire (part of No Action Alternative) would seriously threaten the City of Santa Fe’s surface water supply and leave large portions of the historic plaza district susceptible to severe flooding. On the other hand, the proposed Watershed treatments are likely to impart only minor impacts on the three key issues considered.