DWCZ-MEF-Data from Warix et al. (2023), Local topography and hydraulic conductivity influence riparian groundwater age and groundwater-surface water connection

The western United States is experiencing increasing rain-to-snow ratios due to climate change, and the resulting effects on groundwater–surface water connections remain uncertain. We examined how watershed topography and streambed hydraulic conductivity influence groundwater age and stream discharge at eight sites along a headwater stream within the Manitou Experimental Forest, Colorado, USA.

To do so, we measured:

continuous stream and groundwater discharge, water level, and specific conductivity from April to November 2021;

biweekly stream and groundwater chemistry;

groundwater chlorofluorocarbons (CFCs) and tritium in spring and fall;

streambed hydraulic conductivity; and

local slope.

We used the chemistry data to calculate fluorite saturation states that informed an end-member mixing analysis of streamflow sources. We then combined CFC and tritium data to estimate the age composition of riparian groundwater.

Our results suggest that future stream drying is more probable where local slope is steep and streambed hydraulic conductivity is high. In these areas, groundwater source shifted seasonally—as indicated by age increases—and we observed a high fraction of groundwater in streamflow, primarily interflow from adjacent hillslopes. In contrast, where local slope is flatter and streambed hydraulic conductivity is low, streamflow is more likely to persist, with seasonally constant groundwater age buffered by storage in alluvial sediments.

Paired measurements of groundwater age and streamflow, together with characterization of watershed topography and subsurface properties, identify likely controls on future stream-drying patterns.

受气候变化影响，美国西部地区的降雨转降雪比例持续升高，由此对地下水-地表水交互过程造成的影响仍不明确。我们以美国科罗拉多州曼图阿实验森林内一条源头溪流沿线的8个监测点位为研究对象，探究了流域地形与河床水力传导度如何影响地下水年龄与河道径流量。

为此，我们开展了如下观测与测定工作：
1. 2021年4月至11月期间，持续监测河道与地下水径流量、水位及比电导率；
2. 每两周开展一次河道与地下水化学组分测定；
3. 在春季与秋季采集地下水样品，测定其中的氯氟烃（chlorofluorocarbons, CFCs）与氚含量；
4. 测定河床水力传导度；
5. 测定局地坡度。

我们利用化学组分数据计算了萤石饱和状态，以此为基础开展了径流来源的端元混合分析；随后结合氯氟烃与氚的测定数据，估算了河岸带地下水的年龄组成。

研究结果表明，在局地坡度较陡且河床水力传导度较高的区域，未来河道干涸的可能性更大。在这类区域，地下水来源呈现季节性变化——表现为地下水年龄随季节升高——且河道径流中地下水占比偏高，主要来自邻近山坡的壤中流。与之相反，在局地坡度较缓且河床水力传导度较低的区域，河道径流更易持续，冲积沉积物的储水作用使得地下水年龄保持季节恒定。

本研究通过地下水年龄与河道径流的配对观测，结合流域地形与地下介质特征的表征分析，明确了调控未来河道干涸模式的潜在关键因子。