Nickolas Lydia B, Segura Catalina, Brooks J Renée
Forest Engineering, Resources, and Management, Oregon State University, 201 Peavy Hall, Corvallis, OR, USA 97331.
Water Resources Graduate Program, Oregon State University, 116 Gilmore Hall, Corvallis, OR, USA 97331.
Hydrol Process. 2017 May 15;31(10):1913-1925. doi: 10.1002/hyp.11156.
Understanding the temporal and spatial variability of water sources within a basin is vital to our ability to interpret hydrologic controls on biogeochemical processes and to manage water resources. Water stable isotopes can be used as a tool to determine geographic and seasonal sources of water at the basin scale. Previous studies in the Coastal Range of Oregon reported that the variation in the isotopic signatures of surface water did not conform to the commonly observed "elevation effect", which exhibits a trend of increasing isotopic depletion with rising elevation. The primary purpose of this research is to investigate the mechanisms governing seasonal and spatial variations in the isotopic signature of surface waters within the Marys River Basin, located in the leeward side of the Oregon Coastal Range. Surface water and precipitation samples were collected every 2-3 weeks for isotopic analysis for one year. Our results confirmed the lack of elevational variation of surface water isotopes within this leeward basin. While we find elevational variation in precipitation in the eastern portion of the watershed, this elevation effect is counteracted by rainout with distance from the Pacific coast. In addition we found significant variation in surface water isotope values between catchments underlain predominantly by basalt or sandstone. The degree of separation was strongest during the summer when low flows reflect deeper groundwater sources. This indicates that baseflow within streams drained by each lithology is being supplied from two distinctly separate water sources. In addition, the flow of the Marys River is dominated by water originating from the sandstone water source, particularly during the low flow summer months. We interpreted that the difference in water source results from sandstone catchments having highly fractured geology or locally tipping to the east facilitating cross-basin water exchange from the windward to the leeward side of the Coast Range. Our results challenge topographic derived watershed boundaries in permeable sedimentary rocks; highlighting the overwhelming importance of underlying geology.
了解流域内水源的时空变异性对于我们解释水文对生物地球化学过程的控制以及管理水资源的能力至关重要。水稳定同位素可作为一种工具，用于确定流域尺度上的水的地理和季节来源。先前在俄勒冈州海岸山脉的研究报告称，地表水同位素特征的变化不符合通常观察到的“海拔效应”，即随着海拔升高，同位素贫化程度呈增加趋势。本研究的主要目的是调查位于俄勒冈海岸山脉背风侧的玛丽斯河流域内地表水同位素特征的季节和空间变化机制。在一年的时间里，每2 - 3周采集一次地表水和降水样本进行同位素分析。我们的结果证实了这个背风流域内地表水同位素缺乏海拔变化。虽然我们在流域东部的降水中发现了海拔变化，但这种海拔效应被距太平洋海岸距离产生的降雨冲刷抵消了。此外，我们发现主要由玄武岩或砂岩覆盖的集水区之间地表水同位素值存在显著差异。在夏季，当低流量反映更深层的地下水源时，分离程度最强。这表明由每种岩性排水的溪流中的基流由两个明显不同的水源供应。此外，玛丽斯河的水流主要由来自砂岩石源的水主导，特别是在夏季低流量月份。我们推断水源差异是由于砂岩集水区地质高度破碎或局部向东倾斜，促进了跨流域的水从海岸山脉的迎风侧向背风侧交换。我们的结果挑战了渗透性沉积岩中由地形得出的流域边界；突出了底层地质的压倒性重要性。
