Data_Sheet_1_Determining How Critical Zone Structure Constrains Hydrogeochemical Behavior of Watersheds: Learning From an Elevation Gradient in California's Sierra Nevada.PDF

Concentration-discharge (C-Q) relations can provide insight into the dynamic behavior of the Critical Zone (CZ), as C-Q relations integrate the spatial distribution and timing of watershed hydrogeochemical processes. This study blends geomorphologic analysis, C-Q relations and reactive-transport modeling using a rich dataset from an elevation gradient of eight watersheds in the Southern Sierra Nevada, California. We found that the CZ structure exerts a strong control on the C-Q relations, and on the hydrogeochemical behavior of headwater watersheds. Watersheds with thin regolith, a large stream network, and limited water storage have fast mean transit times along subsurface flow lines, and show limited seasonal variability in ionic concentrations in streamflow (i.e., chemostatic behavior). In contrast, watersheds with thicker regolith, a small stream network and more water storage have longer transit times along subsurface flow lines, and exhibit greater chemical variability (i.e., chemodynamic behavior). Independent estimates of mean transit times and water storage from other isotopic, hydrologic and geophysical studies were consistent with results from modeling C-Q relations. The stream chemistry and its variability were controlled by lateral flow within the regolith, and no mixing with deep groundwater was needed to explain the observed chemical variability. This study opens the possibility to estimate water-storage capacity and mean transit times, and thus drought resistance in watersheds, by using quantitative modeling of C-Q relations.

浓度-流量（Concentration-discharge, C-Q）关系可揭示临界带（Critical Zone, CZ）的动态行为，因其整合了流域水文地球化学过程的空间分布与时间演化特征。本研究结合地貌分析、C-Q关系与反应性输运建模，采用美国加利福尼亚州南部内华达山脉8个流域的高程梯度丰富观测数据集。研究发现，临界带结构对C-Q关系及源头流域的水文地球化学行为具有显著调控作用。风化层较薄、河网密集且储水能力有限的流域，其地下径流路径的平均滞留时间更短，径流离子浓度的季节变异性较弱，即化学稳态行为（chemostatic behavior）。与之相反，风化层较厚、河网稀疏且储水能力更强的流域，其地下径流路径的平均滞留时间更长，径流离子浓度表现出更强的季节变异性，即化学动态行为（chemodynamic behavior）。来自同位素、水文与地球物理研究的平均滞留时间与储水量独立估算结果，与C-Q关系建模的结果高度一致。溪流化学特征及其变异性受控于风化层内的侧向径流，无需与深层地下水混合即可解释观测到的化学变异性。本研究通过C-Q关系的定量建模，为估算流域储水能力、平均滞留时间乃至抗旱能力提供了可行途径。