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 enable us 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, refer to 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 (R ² = 0.70 in 2021; 0.76 in 2022) and area burned (R ² = 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.

Keywords: Budyko curve; Potential evapotranspiration; Random Forest; Streamflow; Wildfire.

野火可以显著改变大范围的植被覆盖和土壤特性，从而导致地表径流生成、溪流流量和水质的显著变化。2020年9月，太平洋西北部喀斯喀特山脉西侧的大规模且高严重程度的野火烧毁了超过49万公顷的森林土地。这些火灾影响的区域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。我们进行了一项研究，评估2020年四场大型高严重程度野火（Riverside、Beachie Creek、Lionshead 和 Holiday Farm）对俄勒冈州西部九个烧毁流域溪流流量的影响。我们还在分析中包括了四个未燃烧的参考流域，以便我们可以评估烧毁流域的火灾后溪流流量变化。为了量化野火对流域水量平衡的影响，我们使用公开可用的溪流流量数据，并利用基于卫星的气象数据估算降水量、潜在蒸散量（PET）和实际蒸散量（ET）。我们使用平均差分归一化燃烧比（dNBR）来量化流域燃烧面积和燃烧严重程度。我们通过分析流域径流比、ET比（蒸发指数：ET除以降水量的商，以下简称EI）和Budyko曲线，比较了火灾前（2001-2020）和火灾后（2021-2022）的水文条件。我们还使用随机森林模型探索影响EI变异性的因素。在火灾后期间，我们在烧毁流域中观察到EI下降和径流比增加。火灾后EI的下降与燃烧严重程度（2021年R²=0.70；2022年0.76）和燃烧面积（2021年R²=0.91；2022年0.95）呈正相关，主要由ET减少驱动。ET的下降高度可变，火灾后第一年范围为10.7-40.2%，第二年为6.1-32.0%，通常与流域燃烧严重程度和燃烧面积相关。最大的径流增加（2021年为16.1%，2022年为19.8%）发生在同一流域。随机森林分析的结果进一步证实了这些结果，表明燃烧严重程度作为EI预测因子的重要性。有趣的是，火灾后期间EI变化的变异性也与其他地貌因素相关，如流域坡度、海拔、地质、朝向和火灾前植被类型。由于火灾后对水文学的影响持续时间和季节性仍不确定，我们的研究发现带来了新的见解，并指导未来关于对水文学的火灾后响应的研究，这对于水资源和森林管理至关重要。