In the western US, most rangelands receive snowfall. Yet, a commonly used tool to assess rangeland’s vulnerability to erosion, the USDA’s Rangeland Hydrology and Erosion Model (RHEM) is run using long-term simulated climate inputs that assumes that all precipitation occurs as rainfall. This can be problematic for areas that receive heavy snowfall or substantial rain-on-snow events. In this research, we have developed an efficient snow module for RHEM, called RHEM-Snow, which partitions precipitation between rainfall and snowfall, simulates snowpack accumulation and ablation, and passes net water input (consisting of rainfall, snowmelt, or both) to RHEM. In some areas, the inclusion of the snow module can reduce annual overland flow runoff and erosion estimates by more than 20 % of the total annual overland flow runoff and erosion produced without the snow module (or by as much as 10–50 mm/year for overland flow runoff or >100 kg/ha-yr for erosion). The reclassification of precipitation events from rainfall in RHEM to snowfall in RHEM-Snow tends to reduce overland flow runoff and erosion, but this reduction can be partially counterbalanced by increases from snowmelt and rain-on-snow. However, hydrologic responses to rain-on-snow events can either be enhanced or muted depending on the characteristics of the storm and the snowpack, as sometimes the snowpack can absorb the precipitation inputs, and sometimes snowmelt enhances the precipitation inputs. Because of this mixed impact, the average difference in erosion caused by rain on snow events is relatively small compared to corresponding events where only the liquid phase is considered. Further study is needed of the complex erosion processes under snowpack and frozen soil/variable saturation conditions. Overall, RHEM-Snow provides more realistic timing and magnitude of overland flow runoff and erosion in cold environments, better satisfying the conditions for RHEM applications.

中文翻译：

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牧场水文和侵蚀模型的积雪模拟


在美国西部，大多数牧场都有降雪。然而，美国农业部的牧场水文和侵蚀模型 （RHEM） 是评估牧场易受侵蚀的常用工具，它使用长期模拟气候输入运行，该输入假设所有降水都以降雨的形式发生。对于大雪或大量降雪事件的地区，这可能是个问题。在这项研究中，我们为 RHEM 开发了一种名为 RHEM-Snow 的高效造雪模块，它将降水分为降雨和降雪，模拟积雪堆积和消融，并将净水输入（包括降雨、融雪或两者兼而有之）传递给 RHEM。在一些地区，包括降雪模块可以减少估计的年度陆上流径流和侵蚀估计值，比没有降雪模块产生的年度陆上流径流和侵蚀总量减少 20% 以上（或陆上流径流高达 10-50 毫米/年，或侵蚀减少 >100 千克/公顷）。将降水事件从 RHEM 中的降雨重新分类为 RHEM-Snow 中的降雪往往会减少陆上流动径流和侵蚀，但这种减少可以通过融雪和雨雪的增加来部分抵消。然而，根据风暴和积雪的特性，水文对雨雪事件的响应可以增强或减弱，因为有时积雪可以吸收降水输入，有时融雪会增加降水输入。由于这种混合影响，与仅考虑液相的相应事件相比，雨雪事件引起的侵蚀平均差异相对较小。需要进一步研究积雪和冻土/可变饱和度条件下复杂的侵蚀过程。 总体而言，RHEM-Snow 在寒冷环境中提供了更真实的陆上流径流和侵蚀的时间和幅度，更好地满足了 RHEM 应用的条件。