Wildfires can dramatically alter vegetation cover and soil properties across large scales, resulting in substantial shifts in runoff generation, streamflow, and water quality. In September 2020, extensive and high-severity wildfires burned more than 490,000 ha of forest land on the westside of the Cascade Mountain Range in the Pacific Northwest. Much of the area impacted by these fires is critical for the provision of water for downstream aquatic ecosystems, agriculture, hydropower, recreation, and municipal drinking water. We undertook a study to evaluate the effects of four of the large high severity wildfires from 2020 (Riverside, Beachie Creek, Lionshead, and Holiday Farm) on streamflow in nine burned catchments in western Oregon. We also included four unburned, reference catchments in our analysis to enable us to assess post-fire streamflow changes in the burned catchments. To quantify the effects of wildfire on the catchment water balance we used publicly available streamflow data and estimated precipitation, potential evapotranspiration (PET), and actual evapotranspiration (ET), using satellite-based meteorological data. We quantified catchment area burned and burn severity with the average differenced normalized burn ratio (dNBR). We compared hydrologic conditions for the pre-fire (2001–2020) and post-fire (2021–2022) periods by analyzing catchment runoff ratios, ET ratios (evaporative index: quotient of ET divided by precipitation, referred to as EI hereafter), and Budyko curves. We also used random forest models to explore factors influencing the variability in EI. During the post-fire period, we observed decreases in EI and increases in runoff ratio in the burned catchments. Post-fire declines in EI were positively related to burn severity ( = 0.70 in 2021; 0.76 in 2022) and area burned ( = 0.91 in 2021; 0.95 in 2022), and were primarily driven by decreases in ET. Declines in ET were highly variable, ranging from 10.7–40.2 % in the first year after the fires and 6.1–32.0 % in the second year after the fires, and were generally related to catchment burn severity and area burned. The greatest increases in runoff (16.1 % in 2021 and 19.8 % in 2022) occurred in the same catchment. These results were reinforced by the random forest analysis, which illustrated the importance of burn severity as a predictor of EI. Interestingly, the variability in changes in EI during the post-fire period was also associated with other geomorphic factors such as catchment slope, elevation, geology, aspect, and pre-fire vegetation type. Since the duration and seasonality of post-fire impacts on hydrology remain uncertain, our findings bring new insights and guide future studies into the post-fire responses on hydrology that are crucial for water and forest management.

中文翻译：

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美国俄勒冈州西部野火的水文响应


野火可以极大地改变大范围的植被覆盖和土壤特性，导致径流产生、水流和水质发生重大变化。 2020年9月，太平洋西北地区喀斯喀特山脉西侧发生大范围、高强度的野火，烧毁了超过49万公顷的林地。受火灾影响的大部分地区对于下游水生生态系统、农业、水电、娱乐和市政饮用水的供水至关重要。我们开展了一项研究，评估 2020 年发生的四场严重的大型野火（Riverside、Beachie Creek、Lionshead 和 Holiday Farm）对俄勒冈州西部 9 个被烧毁的集水区水流的影响。我们还在分析中纳入了四个未燃烧的参考集水区，以便我们能够评估燃烧集水区火灾后水流的变化。为了量化野火对流域水平衡的影响，我们使用了公开的水流数据以及基于卫星的气象数据估计的降水量、潜在蒸散量 (PET) 和实际蒸散量 (ET)。我们用平均差异归一化燃烧比 (dNBR) 量化了烧毁的流域面积和烧伤严重程度。我们通过分析流域径流比率、ET比率（蒸发指数：ET除以降水量，以下简称EI），比较了火灾前（2001-2020年）和火灾后（2021-2022年）时期的水文条件。和布迪科曲线。我们还使用随机森林模型来探索影响 EI 变异性的因素。在火灾后时期，我们观察到烧毁集水区的 EI 下降，径流比增加。火灾后 EI 下降与烧伤严重程度呈正相关（2021 年 = 0.70；0.2022 年 = 76）和烧毁面积（2021 年 = 0.91；2022 年 = 0.95），主要是由蒸散量减少驱动的。 ET 的下降变化很大，火灾后第一年的下降幅度为 10.7-40.2%，火灾后第二年的下降幅度为 6.1-32.0%，并且通常与流域烧伤严重程度和烧毁面积有关。径流增幅最大（2021 年为 16.1%，2022 年为 19.8%）发生在同一流域。随机森林分析强化了这些结果，说明了烧伤严重程度作为 EI 预测因子的重要性。有趣的是，火灾后时期EI变化的变异性也与其他地貌因素有关，例如流域坡度、海拔、地质、坡向和火灾前植被类型。由于火灾后对水文影响的持续时间和季节性仍然不确定，我们的研究结果带来了新的见解，并指导未来对水文火灾后反应的研究，这对水和森林管理至关重要。