Estimating evapotranspiration change due to forest treatment and fire at the basin scale in the Sierra Nevada, California

Abstract

We investigated the potential magnitude and duration of forest evapotranspiration (ET) decreases resulting from forest‐thinning treatments and wildfire in west‐slope watersheds of the Sierra Nevada range in California, USA, using a robust empirical relation between Landsat‐derived mean‐annual normalized difference vegetation index (NDVI) and 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 −0.09 units, corresponding to a basal‐area reduction of 29.1 m2/ha (45% reduction) and equivalent to an estimated ET reduction of 153 mm/year (21% change; approximate mean annual precipitation = 1,000 mm). Intensive thinning in highly productive forests that approached prefire‐exclusion densities reduced basal area by 40–50%, generating estimated ET reductions of 153–218 mm/year (21–27% change) over 5 years following treatment. Low‐intensity underburn treatments resulted in no significant change in ET. Examining the cumulative impact of wildfires on ET between 1990 and 2008, we found that the lower and wetter American River basin (5,310 km2) generated more than twice the ET reduction per unit area than those in the higher and drier Kings River basin (4,790 km2), 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 suggests that ET reductions due to forest thinning by wildfire could approach 10% of full natural flows for dry years and 5% over all years.

Fuels treatment and wildfire effects on runoff from Sierra Nevada mixed‐conifer forests

“…(1,200-2,100 m) in the Kings River basin directly south (Goulden et al, 2012). Using the same top-down approach, Roche et al (2018) showed similar results to those in our study, in that thinning treatments reduced evapotranspiration in the American, but not in the Forest vegetation growth following treatment or fire disturbance also varies by forest type and disturbance event. Roche et al (2018) found that following wildfire, evapotranspiration in the central Sierra…”

Spatial patterns of meadow sensitivities to interannual climate variability in the Sierra Nevada

“…Given the recent drought and associated tree mortality in the Sierra Nevada (Tree Mortality Task Force, ), contemporary studies have addressed the impacts of tree density reductions due to forest thinning or fire on ET and surface water availability (Boisramé, Thompson, Collins, & Stephens, ; Roche, Goulden, & Bales, ). We included fire return interval departure—an indicator of tree density relative to historic conditions and associated fuels loading and ET—to assess how meadow sensitivity varies among departure classes.…”

“…Sensitivity of meadows in watersheds with the majority of vegetation classified as departed due to decreased fire frequency (suggesting increased tree densities) relative to historical conditions was not detectably different from those in watersheds in which fire frequency is consistent with historic regimes. It is possible that any signal in these data may simply be overwhelmed by the strong environmental gradients that exist across the study area, as fire effects may differentially affect watershed‐scale ET rates across gradients of water availability (Roche et al, ). Moreover, fire return interval departures do not explicitly account for fire severity and thus may not necessarily be indicative of ET water use.…”

From Drought to Flood: A Water Balance Analysis of the Tuolumne River Basin during Extreme Conditions (2015 – 2017)

“…Previous work has estimated ET amounts in the Sierra Nevada using water balance approaches (Henn, Painter, et al, 2018; Kattelmann & Elder, 1991) and regression relationships between eddy covariance flux tower measurements and satellite products (Fellows & Goulden, 2017; Goulden et al, 2012; Roche, Goulden, & Bales, 2018). These approaches focused on different applications but concluded that annual ET in the upper elevations of the Sierra Nevada ranged between 150 and 400 mm, with most of the losses occurring below tree line.…”