The North American Monsoon (NAM) is an important source of precipitation across the southwestern United States. The approximate northern boundary of this feature crosses the Navajo Nation (NN), in the Four Corners region, where NAM rains have long been important to the livelihoods of Native Americans. Relatively little is known about the characteristics and hydrological significance of the NAM in this region. Here, we report a new 4 yr record of stable H and O isotope ratios in monsoon-season rainfall and water resources across the NN. Monthly precipitation samples collected at 39 sites document a characteristic pattern of ²H- and ¹⁸O-enrichment associated with monsoonal precipitation. These changes are weakly correlated with local precipitation amount, however, and the correlation that does exist is dominated by sub-cloud evaporation effects. In contrast to precipitation amount, monsoon-season isotopic values exhibited little spatial variability across the region, and after correction for sub-cloud evaporation NN values were similar to those from a site in southern Arizona. Airmass back trajectory analysis suggests that the uniformly high NAM precipitation water isotope ratios across the region may reflect (a) a region-wide shift from mid-latitude to low-latitude moisture sources at the onset of the peak monsoon, and (b) substantial land-surface recycling of NAM moisture in upwind regions. Comparison of precipitation water isotope data with surface and groundwater values implies that, despite its hydroclimatic significance, monsoon rainfall contributes little to subsurface water resources. This highlights the monsoon's importance for warm-season land-surface ecology and hydrology critical to residents in the Four Corners region.

中文翻译：

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降水和流星水中的稳定同位素：在亚利桑那州、新墨西哥州和犹他州寻找和追踪北美季风

北美季风 (NAM) 是美国西南部降水的重要来源。该特征的大致北部边界跨越纳瓦霍族 (NN)，位于四角地区，在那里 NAM 降雨长期以来对美洲原住民的生计很重要。对该地区NAM的特征和水文意义知之甚少。在这里，我们报告了 NN 季风季节降雨和水资源中稳定的 H 和 O 同位素比率的 4 年新记录。在 39 个地点收集的每月降水样本记录了² H- 和¹⁸与季风降水相关的 O 富集。然而，这些变化与局地降水量的相关性较弱，而且确实存在的相关性以亚云蒸发效应为主。与降水量相比，整个地区的季风季节同位素值几乎没有空间变化，并且在对亚云蒸发量进行校正后，NN 值与亚利桑那州南部一个地点的值相似。气团反向轨迹分析表明，该地区一致高的 NAM 降水水同位素比可能反映了 (a) 在季风高峰期开始时区域范围内从中纬度向低纬度水分源的转变，以及 (b)逆风地区 NAM 水分的地表循环。降水水同位素数据与地表水和地下水值的比较表明，尽管季风降雨具有水文气候意义，但对地下水资源的贡献很小。这凸显了季风对暖季地表生态和水文的重要性，这对四角地区的居民至关重要。