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

Abstract

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.

Quantifying contributions of snowmelt water to streamflow using graphical and chemical hydrograph separation

“…These factors would lead to more variability in flowpath development and larger hysteresis indices observed at annual and daily scales. These results are consistent with Ackerer et al (2020) who observed greater water storage, longer transit times along subsurface flow lines, and greater seasonal variability in ionic concentrations in streamflow in watersheds with thicker regolith relative to watersheds with thinner regolith.…”

From Hydrometeorology to River Water Quality: Can a Deep Learning Model Predict Dissolved Oxygen at the Continental Scale?

“…Basins with flashy hydrology tend to have larger model errors (SI Figure S2). This indicates watersheds with steep slope, highly conductive soil, or thin regolith and low water storage (thus flashy streamflow) should be sampled more frequently and over the full range of discharge and DO variability. Hydrological studies have shown that when streamflow data are carefully chosen, 10% of streamflow measurements are sufficient for parameter calibration, meaning that approximately 90% of data are redundant .…”

Hydrogeochemical Dynamics and Response of Karst Catchment to Rainstorms in a Critical Zone Observatory (CZO), Southwest China

“…Solute concentrations normally vary inversely with discharge, which can be well reflected by power-law function (Godsey et al, 2009;Musolff et al, 2015;Zimmer et al, 2019;Ackerer et al, 2020). To demonstrate relationships between concentrations and discharge, power-law function was adopted as follows:…”