It is generally accepted that year-to-year variability in moisture conditions and drought are linked with increased wildfire occurrence. However, quantifying the sensitivity of wildfire to surface moisture state at seasonal lead-times has been challenging due to the absence of a long soil moisture record with the appropriate coverage and spatial resolution for continental-scale analysis. Here we apply model simulations of surface soil moisture that numerically assimilate observations from NASA's Gravity Recovery and Climate Experiment (GRACE) mission with the US Forest Service's historical Fire-Occurrence Database over the contiguous United States. We quantify the relationships between pre-fire-season soil moisture and subsequent-year wildfire occurrence by land-cover type and produce annual probable wildfire occurrence and burned area maps at 0.25-degree resolution. Cross-validated results generally indicate a higher occurrence of smaller fires when months preceding fire season are wet, while larger fires are more frequent when soils are dry. This result is consistent with the concept of increased fuel accumulation under wet conditions in the pre-season. These results demonstrate the fundamental strength of the relationship between soil moisture and fire activity at long lead-times and are indicative of that relationship's utility for the future development of national-scale predictive capability.

中文翻译：

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美国野火发生对生态系统季前土壤水分条件的敏感性

人们普遍认为，水分条件和干旱的逐年变化与野火发生率增加有关。然而，由于缺乏用于大陆尺度分析的具有适当覆盖范围和空间分辨率的长期土壤水分记录，因此在季节性提前期量化野火对地表水分状态的敏感性一直具有挑战性。在这里，我们应用地表土壤水分的模型模拟，在数值上将 NASA 的重力恢复和气候实验 (GRACE) 任务的观测结果与美国林务局在美国毗连的历史火灾数据库中的观测结果同化。我们通过土地覆盖类型量化火灾季节前土壤水分与随后年份野火发生之间的关系，并以 0.25 度的分辨率生成年度可能发生的野火和烧毁面积图。交叉验证的结果通常表明，当火灾季节前几个月是潮湿的时候，较小的火灾发生率较高，而当土壤干燥时，较大的火灾发生率更高。这一结果与季前潮湿条件下增加燃料积累的概念是一致的。这些结果证明了土壤水分和火灾活动之间关系在很长一段时间内的基本实力，并表明这种关系对未来发展国家规模预测能力的效用。交叉验证的结果通常表明，当火灾季节前几个月是潮湿的时候，较小的火灾发生率较高，而当土壤干燥时，较大的火灾发生率更高。这一结果与季前潮湿条件下增加燃料积累的概念是一致的。这些结果证明了土壤水分和火灾活动之间关系在很长一段时间内的基本实力，并表明这种关系对未来发展国家规模预测能力的效用。交叉验证的结果通常表明，当火灾季节前几个月是潮湿的时候，较小的火灾发生率较高，而当土壤干燥时，较大的火灾发生率更高。这一结果与季前潮湿条件下增加燃料积累的概念是一致的。这些结果证明了土壤水分和火灾活动之间关系在很长一段时间内的基本实力，并表明这种关系对未来发展国家规模预测能力的效用。