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

Abstract

We applied an eco‐hydrologic model (Regional Hydro‐Ecologic Simulation System [RHESSys]), constrained with spatially distributed field measurements, to assess the impacts of forest‐fuel treatments and wildfire on hydrologic fluxes in two Sierra Nevada firesheds. Strategically placed fuels treatments were implemented during 2011–2012 in the upper American River in the central Sierra Nevada (43 km2) and in the upper Fresno River in the southern Sierra Nevada (24 km2). This study used the measured vegetation changes from mechanical treatments and modelled vegetation change from wildfire to determine impacts on the water balance. The well‐constrained headwater model was transferred to larger catchments based on geologic and hydrologic similarities. Fuels treatments covered 18% of the American and 29% of the Lewis catchment. Averaged over the entire catchment, treatments in the wetter central Sierra Nevada resulted in a relatively light vegetation decrease (8%), leading to a 12% runoff increase, averaged over wet and dry years. Wildfire with and without forest treatments reduced vegetation by 38% and 50% and increased runoff by 55% and 67%, respectively. Treatments in the drier southern Sierra Nevada also reduced the spatially averaged vegetation by 8%, but the runoff response was limited to an increase of less than 3% compared with no treatment. Wildfire following treatments reduced vegetation by 40%, increasing runoff by 13%. Changes to catchment‐scale water‐balance simulations were more sensitive to canopy cover than to leaf area index, indicating that the pattern as well as amount of vegetation treatment is important to hydrologic response.

Wildfire controls on evapotranspiration in California’s Sierra Nevada

“…Practically, forest restoration can be limited by cost, accessibility, and ownership, which may limit the area and pace of restoration (Lydersen et al 2019;North et al 2015). We found that much of the evapotranspiration reduction (equivalent to 8±1% of the current annual evapotranspiration) occurs even in low-severity fires, implying that less-severe treatments may provide a meaningful increase in water yields (Saksa et al 2020;Saksa et al 2017).…”

“…Likewise, analyses of actual management projects are needed. Additional factors may impact the ET after forest treatments, including changes in biomass, density, tree size and species composition can influence the post-treatment transpiration (Bart et al 2016;Roche et al 2018;Saksa et al 2020;Saksa et al 2017); treatment methods (mechanical thinning, clear cutting, with/without prescribed fires) and removing versus leaving woody debris on the ground can change evaporation demand by altering the land surface albedo and wetness (Knapp et al 2017;Stephens and Moghaddas 2005;Walker et al 2006).…”

“…These forest management practices have also led to the accumulation of ladder and surface fuels, which coincide with climate warming, has contributed to progressively larger and more-severe wildfires (Collins et al 2017a;Westerling et al 2006). Mitigating the wildfire hazard in these highly altered forests by reducing tree density and fuel loads will also change the water balance; and these potential water cobenefits should be considered in designing and evaluating restoration treatments (Roche et al 2020;Saksa et al 2020). Uncharacteristically dense forests have not only reduced overall water yield but also increased inter-tree competition for water, making contemporary forests more vulnerable to drought and insect attack Goulden and Bales 2019;Liu et al 2019;Stephens et al 2018;Young et al 2017).…”

Evapotranspiration Mapping for Forest Management in California's Sierra Nevada

“…As forests are dynamic systems, water use by forests can respond in multiple ways to reductions in biomass (Tague et al, 2018). For example, Saksa et al (2019) reported a significant reduction in evapotranspiration following fuel treatment in a densely forested central Sierra Nevada area but no significant reduction in a comparable but more water-limited southern Sierra area. In the southern site, reductions in forest biomass apparently stimulated growth of remaining vegetation.…”

Hydrologic Response of a Montane Meadow from Conifer Removal and Upslope Forest Thinning

“…The low level of forest thinning and contributing area treated might explain the lack of response in meadow groundwater levels. A recent modelling study of fuels treatment in the Sierra Nevada found a 3% increase in water yield in the mid-regions of the Sierra Nevada associated with an 8% reduction in vegetation [27]. However, modelling studies do not encounter the variability of field measurements.…”