McGill L M, Steel E A, Brooks J R, Edwards R T, Fullerton A H
Quantitative Ecology and Resource Management, University of Washington, Seattle, WA 98105, USA.
Pacific Northwest Research Station, USDA Forest Service, 400 NW 34th Street, Suite 201, Seattle, WA 98103, USA.
J Hydrol (Amst). 2020 Apr 1;583. doi: 10.1016/j.jhydrol.2020.124610.
The stable isotope ratios of stream water can be used to trace water sources within river basins; however, drivers of variation in water isotopic spatial patterns across basins must be understood before ecologically relevant and isotopically distinct water sources can be identified and this tool efficiently applied. We measured the isotope ratios of surface-water samples collected during summer low-flow across five basins in Washington and southeast Alaska (Snoqualmie, Green, Skagit, and Wenatchee Rivers, and Cowee Creek) and compared models (isoscapes) describing the spatial variation in surface-water isotope ratios across a range of hydraulic and climatic conditions. We found strong correlations between mean watershed (MWE) elevation and surface-water isotopic ratios on the windward west side of the Cascades and in Alaska, explaining 48-90% of variation in δO values. Conversely, in the Wenatchee basin, located leeward of the Cascade Range, MWE alone had no predicative power. The elevation relationship and predictive isoscapes varied between basins, even those adjacent to each other. Applying spatial stream network models (SSNMs) to the Snoqualmie and Wenatchee Rivers, we found incorporating Euclidean and flow-connected spatial autocovariance improved explanatory power. SSNMs improved the accuracy of river water isoscapes in all cases; however, their utility was greater for the Wenatchee basin, where covariates explained only a small proportion of total variation. Our study provides insights into why basinscale surface-water isoscapes may vary even in adjacent basins and the importance of incorporating spatial autocorrelation in isoscapes. For determining source water contributions to downstream waters, our results indicate that surface water isoscapes should be developed for each basin of interest.
河水的稳定同位素比率可用于追踪流域内的水源；然而，在识别出具有生态相关性且同位素特征不同的水源并有效应用该工具之前，必须先了解跨流域水同位素空间格局变化的驱动因素。我们测量了华盛顿州和阿拉斯加东南部五个流域（斯诺夸尔米河、格林河、斯卡吉特河、韦纳奇河和考伊溪）夏季低流量期间采集的地表水样本的同位素比率，并比较了描述一系列水力和气候条件下地表水同位素比率空间变化的模型（等比线图）。我们发现，卡斯卡德山脉迎风西侧和阿拉斯加地区的平均流域（MWE）海拔与地表水同位素比率之间存在很强的相关性，可解释δO值变化的48 - 90%。相反，在位于喀斯喀特山脉背风处的韦纳奇河流域，仅MWE没有预测能力。即使是相邻的流域，其海拔关系和预测等比线图也有所不同。将空间河流网络模型（SSNMs）应用于斯诺夸尔米河和韦纳奇河，我们发现纳入欧几里得和流量连接的空间自协方差提高了解释力。在所有情况下，SSNMs都提高了河流水等比线图的准确性；然而，它们在韦纳奇河流域的效用更大，因为协变量仅解释了总变化的一小部分。我们的研究深入探讨了为什么即使在相邻流域，流域尺度的地表水等比线图也可能不同，以及在等比线图中纳入空间自相关的重要性。对于确定水源对下游水体的贡献，我们的结果表明，应该为每个感兴趣的流域绘制地表水等比线图。
