Forests in the western U.S. are generally over-dense, a result of nearly a century of fire suppression. Thinning forests can reduce the incidence of high-severity wildfire and potentially increase stream flow. We estimate forest water use changes due to mechanical thinning and fire using a relation between Landsat satellite data and ground-based evaporation measurements. This tool offers the potential for land managers to quantify hydrologic benefits of forest treatment over the period of the Landsat record (1985 to present) in areas that otherwise lack detailed hydrologic measurements.
Increased forest evapotranspiration (ET) coupled with snowpack decreases in a warming climate is likely to decrease runoff and increase forest drought stress. Field experiments and modeling suggest that forest thinning can reduce ET and thus increase potential runoff relative to untreated areas. We investigated the potential magnitude and duration of ET decreases resulting from forest-thinning treatments and fire using a robust empirical relation between Landsat-derived mean-annual normalized difference vegetation index (NDVI) and annual ET measured at flux towers. Among forest treatments, the minimum observed NDVI change required to produce a significant departure from control plots with NDVI of about 0.70 was -0.07 units, corresponding to a basal-area reduction of 3.1 m2 ha-1, and equivalent to an estimated ET reduction of -102 mm yr-1. Intensive thinning in highly productive forests that approached pre-fire-exclusion densities reduced basal area by 40-50%, generating estimated ET reductions of 152-216 mm yr-1 over five years following treatment. Between 1990 and 2008, fires in the American River basin generated more than twice the ET reduction per unit area than those in the Kings River basin, corresponding to greater water and energy limitations in the latter and greater fire severity in the former. A rough extrapolation of these results to the entire American River watershed, much of which would have burned naturally during this 19-year period, could result in ET reductions that approach 10% of full natural flows for drought years and 5% averaged over all years. This work demonstrates the potential utility to estimate forest ET change at the patch scale, which in turn may allow managers to estimate thinning benefits in areas lacking detailed hydrologic measurements.
Goulden, M.; Bales, R. (2017): Estimating evapotranspiration change due to forest treatment and fire at the basin scale in the Sierra Nevada, California. Fall Meeting, American Geophysical Union, December 2017. Abstract H21B-1443..