Quantifying Watershed-Scale Effects of Wildland Fires on Hydrologic Processes

The Mediterranean-type climate of coastal California is characterized by highly seasonal precipitation and temperature regimes. Plant adaptations to prolonged hot, dry periods, such as the hardened “sclerophyllous” leaves of many chaparral shrubs, make the plant communities of this area especially fire-prone. Periodic extreme fire events can remove all vegetation, diminish water infiltration, increase storm runoff, and generate substantial sediment erosion. Most of the studies of fire effects on watershed hydrology have either focused on individual storm events following wildland fires or the response of smaller scale test plots where trees and other vegetation were removed. In this effort we utilized a database of USGS stream gauging records, NOAA precipitation stations, PRISM precipitation maps, and fire perimeter data in the Santa Lucia Mountains of central coastal California to compare precipitation vs. runoff relationships pre- and post-fire from two moderate scale watersheds (100 to 500 square kilometers) that were nearly completely burned with one nearby that did not burn. PRISM data allowed for the extrapolation of station precipitation data to the scale of the entire watersheds, and watershed totals were compared with runoff on an annual water-year basis. The differences were calculated for the burned and unburned watersheds, both before and after the fire, to assess whether the lack of vegetation in the burned watersheds reduced water consumption, at least during the first few years after the wildland fire. Despite extensive stream gauge and precipitation data, there was considerable statistical variability in the results, making it difficult to distinguish pre- and post-fire hydrologic response. As climates shift to possibly hotter and drier conditions, wildland fire frequency and watershed response are of immediate interest. We therefore examined additional approaches needed to quantify the expected impacts at the watershed scale.