Abstract
Modifications to the rates of water flowing from the surface to groundwater (groundwater recharge) due to climate variability are the most difficult to assess because of the lack of direct long-term observations. Here, we analyze the chloride salt distribution below the surface soil on a plateau near Los Angeles to reconstruct the amount of recharge that occurred in the last five centuries. Over this time interval, periods of major high and low recharge with different duration follow each other and this cyclicity is consistent with long-term atmospheric forcing patterns, such as the Pacific Decadal Oscillation. This study determines the range and the natural variability of recharge to groundwater, which sustains local freshwater flow system, and helps forecast future availability of groundwater resource in southern California, where water scarcity is critical to both local and global populations.
Highlights
Modifications to the rates of water flowing from the surface to groundwater due to climate variability are the most difficult to assess because of the lack of direct long-term observations
The pore water found at different depths in this zone represents the layering of successive inputs of rainfall from the most recent, just below land surface, to the oldest situated at the water table
The Cl approach is applicable in recharge areas in semi-arid climate regions where there is no Cl released from the geologic media and no contributions from anthropogenic sources[9]
Summary
To investigate the vertical variability of Cl, we obtained two porewater profiles from high-resolution depth-discrete sampling of continuous cores at two locations, RD-103 and RD-106 (Fig. 1). This method is suitable for application in semi-arid regions where porous fractured sedimentary bedrock occurs in recharge areas and where the vadose zone is sufficiently thick and has moderately matrix permeability for retaining insights of past hydrologic conditions over the long term. Unlike other proxies, such as lacustrine sediments and tree-rings, the Cl profiles represent a more powerful tool because they show most directly the variability of the groundwater component of the hydrological cycle, which is generally the most difficult to assess because of the lack of direct observation. This observed multidecadal trend must be considered by stakeholders and regulators when planning measures to avoid groundwater over-exploitation and to achieve sustainable management of water resources, so important to both local and global populations
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