Seven days of extreme rainfall during September 2013 produced more than 1100 debris flows in the Colorado Front Range, about 78% of which occurred on south-facing slopes (SFS). Previously published soil moisture (volumetric water content) observations suggest that SFS were wetter than north-facing slopes (NFS) during the event, which contrasts with soil moisture patterns observed during normal conditions. Various causes have been hypothesized for the preferential saturation of SFS, but those hypotheses remain largely untested. Here, we analyze the soil moisture patterns using additional soil moisture observations, determine the hydrologic processes controlling the preferential saturation of SFS, and evaluate the importance of soil moisture in predicting the debris flow initiation sites. Soil moisture patterns are simulated using the Equilibrium Moisture from Topography, Vegetation, and Soil (EMT + VS) model. Five hypotheses are tested that may have influenced the soil moisture reversal including higher rainfall rates, lower interception rates, lower saturated water content, thinner soils, and reduced deep drainage on SFS. The EMT + VS model is coupled with an infinite slope stability model to produce factor of safety maps. The hypotheses are tested by comparing the modeled soil moisture to soil moisture observations and the debris flow initiation sites. The results suggest that differences in interception and deep drainage between SFS and NFS were primarily responsible for producing wetter SFS, but the soil moisture pattern likely played a smaller role than vegetation and slope in determining where debris flows initiated. The final model predicts instability at approximately 72% of the observed debris flow initiation sites.

中文翻译：

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模拟与 2013 年 9 月科罗拉多锋区风暴期间土壤饱和度和泥石流发生相关的水文过程

2013 年 9 月的 7 天极端降雨在科罗拉多前沿山脉产生了 1100 多条泥石流，其中约 78% 发生在朝南的斜坡 (SFS)。先前公布的土壤水分（体积含水量）观测结果表明，在事件期间，SFS 比朝北斜坡 (NFS) 更湿润，这与在正常条件下观察到的土壤水分模式形成对比。已经假设了 SFS 优先饱和的各种原因，但这些假设在很大程度上仍未得到检验。在这里，我们使用额外的土壤水分观测来分析土壤水分模式，确定控制 SFS 优先饱和的水文过程，并评估土壤水分在预测泥石流起始位点中的重要性。土壤水分模式使用来自地形、植被和土壤的平衡水分 (EMT + VS) 模型进行模拟。测试了可能影响土壤水分逆转的五个假设，包括更高的降雨率、更低的截留率、更低的饱和含水量、更薄的土壤和减少 SFS 的深层排水。EMT + VS 模型与无限边坡稳定性模型相结合以生成安全系数图。通过将模拟的土壤水分与土壤水分观测值和泥石流起始点进行比较来检验这些假设。结果表明，SFS 和 NFS 之间截留和深层排水的差异是产生更湿的 SFS 的主要原因，但土壤水分模式在确定泥石流起始位置方面的作用可能比植被和坡度要小。