Pacific Southwest

Title: Pre- and postfire distribution of soil water repellency in a steep chaparral watershed

Author: Hubbert, K. R.; Wohlgemuth, P. M.; Preisler, H. K.

Date: 2008

Source: In: Narog, Marcia G., tech. coord. 2008. Proceedings of the 2002 Fire Conference: Managing fire and fuels in the remaining wildlands and open spaces of the Southwestern United States. Gen. Tech. Rep. PSW-GTR-189. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. p. 99-106

Station ID: GTR-PSW-189

Description: The development and nature of water repellent soils and their spatial distribution on the landscape are not well understood relative to evaluating hillslope response to fire. Soil water repellency is particularly common in chaparral communities, due in part to the coarse-textured soils, and the high resin content of the organic litter. Objectives of this study were 1) to investigate pre- and post-fire distribution of soil water repellency on the landscape, and 2) to determine if species composition affected changes in soil water repellency. A prescribed burn was conducted on a 1.28 ha mixed chaparral study site located in the San Dimas Experimental Forest. We sampled pre- and post-fire soil water repellency using the water drop penetration time method at 105 sites. At each site, measurements were taken at the surface, 2 cm, and 4 cm depths within a 15 x 15 cm square plot. Thirty-eight percent of the pre-fire soil surface exhibited moderate to high repellency. At 7 days post-fire, moderate to high repellency in the surface soil increased to 66 percent. After 76 days, post-burn surface soil water repellency returned to near pre-fire values. Variability in water drop penetration time among replicates within a given 15 x 15 cm plot was as large as the variability seen among sites over the whole watershed. At the 2 cm depth, 7 day post-fire moderate and high repellency increased 14 percent. Beneath chamise, soils exhibiting high repellency increased by 17 percent after the burn, and by 38 percent under ceanothus, whereas high repellency beneath sugar bush decreased by 23 percent. Greater understanding of the pre- and post-fire spatial distribution of water repellency is necessary for land management decisions in steep chaparral watersheds.

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Citation

Hubbert, K. R.; Wohlgemuth, P. M.; Preisler, H. K.  2008.  Pre- and postfire distribution of soil water repellency in a steep chaparral watershed.   In: Narog, Marcia G., tech. coord. 2008. Proceedings of the 2002 Fire Conference: Managing fire and fuels in the remaining wildlands and open spaces of the Southwestern United States. Gen. Tech. Rep. PSW-GTR-189. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. p. 99-106.