Long‐term hydrologic recovery after wildfire and post‐fire forest management in the interior Pacific Northwest

Abstract

Elevated wildfire activity in many regions in recent decades has increased concerns about the short-and long-term effects on water quantity, quality, and aquatic ecosystem health. Often, loss of canopy interception and transpiration, along with changes in soil structural properties, leads to elevated total annual water yields, peak flows, and low flows. Post-fire land management treatments are often used to promote forest regeneration and mitigate effects to terrestrial and aquatic ecosystems.However, few studies have investigated the longer-term effects of either wildfire or post-fire land management on catchment hydrology. Our objectives were to quantify and compare the short-and longer-term effects of both wildfire and post-fire forest management treatments on annual discharge, peak flows, low flows, and evapotranspiration (AET). We analyzed ten years of pre-fire data, along with post-fire data from 1 to 7 and 35 to 41 years after wildfire burned three experimental catchments in the Entiat Experimental Forest (EEF) in the Pacific Northwest, USA. After the fire, two of the catchments were salvage logged, aerially seeded, and fertilized, while the third catchment remained as a burned reference. We observed increases in annual discharge (150-202%), peak flows (234-283%), and low flows (42-81%), along with decreases in AET (34-45%), across all three study catchments in the first seven year period after the EEF wildfire. Comparatively, annual discharge, peak flows, lows flows, and AET had returned to pre-fire levels 35-41 years after the EEF fire in the two salvage logged and seeded catchments. Surprisingly, in the catchment that was burned but not actively managed, the annual discharge and runoff ratios remained elevated, while AET remained lower, during the period 35-41 years after the EEF fire. We posit that differences in long-term hydrologic recovery across catchments were driven by delayed vegetation recovery in the unmanaged catchment. Our study demonstrates that post-fire land management decisions have the potential to produce meaningful differences in the long-term recovery of catchment-scale ecohydrologic processes and streamflow.

Scientists' warning on extreme wildfire risks to water supply

“…For instance, regions where fire suppression turned historically open tree‐savannas into dense closed‐canopy forests, ecosystem water use increased significantly due to increased evapotranspiration (Boisramé et al, 2019; Roche et al, 2018), leading to reduced streamflow. Safely using naturally‐occurring wildfires to restore overstocked source watersheds can increase water security by reducing the likelihood of extreme fire events while simultaneously increasing streamflow and subsurface water storage (Boisramé et al, 2019; Roche et al, 2018); this option must, however, be carefully reviewed, as water demand from post‐fire regrowth can offset gains in water availability (Brookhouse et al, 2013; Niemeyer et al, 2020).…”

Comparison of Satellite and Drone-Based Images at Two Spatial Scales to Evaluate Vegetation Regeneration after Post-Fire Treatments in a Mediterranean Forest

“…More specifically, LEBs are built by felling burned trees, while CFDs consist of branch and small-felling burned trees, and both are laid on the ground along the slope contour. Many studies have assessed the effectiveness of hillslope stabilization treatments on soil hydrological response (namely runoff and erosion) (e.g., [12][13][14][15] as well as reductions in CO 2 emissions and carbon sequestration [16]. In some cases, the impact of these actions in reducing runoff and trapping sediments is limited to the less intense rainfall events (e.g., [11]).…”

Hillslope sediment production after wildfire and post‐fire forest management in northern California

“…As such, concerns have grown regarding the immediate and longer‐term effects on forest resilience and the water supply originating in forests (Hallema, Robinne, & Bladon, 2018; Stevens‐Rumann et al, 2018). Increasingly severe wildfires have produced substantial and long‐lasting (>10 years) effects on annual streamflow and peak flows (Hallema, Sun, et al, 2018; Niemeyer, Bladon, & Woodsmith, 2020; Saxe, Hogue, & Hay, 2018), debris flows (Langhans et al, 2017; Nyman et al, 2015), physical and chemical water quality (Rhoades et al, 2019; Rust, Hogue, Saxe, & McCray, 2018), aquatic ecosystem health (Bixby et al, 2015; Emelko et al, 2016), and downstream drinking water supply (Emelko, Silins, Bladon, & Stone, 2011; Hohner, Terry, Townsend, Summers, & Rosario‐Ortiz, 2017).…”