Abstract
Abstract. This study combines major ion and isotope chemistry, age tracers, fracture density characterizations, and physical hydrology measurements to understand how the structure of the critical zone (CZ) influences its function, including water routing, storage, mean water residence times, and hydrologic response. In a high elevation rhyolitic tuff catchment in the Jemez River Basin Critical Zone Observatory (JRB-CZO) within the Valles Caldera National Preserve (VCNP) of northern New Mexico, a periodic precipitation pattern creates different hydrologic flow regimes during spring snowmelt, summer monsoon rain, and fall storms. Hydrometric, geochemical, and isotopic analyses of surface water and groundwater from distinct stores, most notably shallow groundwater that is likely a perched aquifer in consolidated collapse breccia and deeper groundwater in a fractured tuff aquifer system, enabled us to untangle the interactions of these groundwater stores and their contribution to streamflow across 1 complete water year (WY). Despite seasonal differences in groundwater response due to water partitioning, major ion chemistry indicates that deep groundwater from the highly fractured site is more representative of groundwater contributing to streamflow across the entire water year. Additionally, the comparison of streamflow and groundwater hydrographs indicates a hydraulic connection between the fractured welded tuff aquifer system and streamflow, while the shallow aquifer within the collapse breccia deposit does not show this same connection. Furthermore, analysis of age tracers and oxygen (δ18O) and stable hydrogen (δ2H) isotopes of water indicates that groundwater is a mix of modern and older waters recharged from snowmelt, and downhole neutron probe surveys suggest that water moves through the vadose zone both by vertical infiltration and subsurface lateral flow, depending on the lithology. We find that in complex geologic terrain like that of the JRB-CZO, differences in the CZ architecture of two hillslopes within a headwater catchment control water stores and routing through the subsurface and suggest that shallow groundwater does not contribute significantly to streams, while deep fractured aquifer systems contribute most to streamflow.
Highlights
Understanding the interconnections of groundwater and surface water is fundamental to water resource management as groundwater and surface water should be considered a single resource (Winter, 1998); their interactions in different hydrogeologic settings are varied and complex (Winter, 1999)
As temperatures increased above freezing again, maximum flows were reached in La Jara flume because streamflow remained high above baseflow conditions between snowmelt peaks, and a local streamflow maximum was reached in the ZOB flume
While streamflow peaks were greatest in response to the spring snowmelt, there were obvious, smaller peaks in ZOB and La Jara surface waters following summer monsoons and fall storms
Summary
Understanding the interconnections of groundwater and surface water is fundamental to water resource management as groundwater and surface water should be considered a single resource (Winter, 1998); their interactions in different hydrogeologic settings are varied and complex (Winter, 1999). Discerning stream water sources and groundwater dynamics are even more important in the context of a changing climate, especially in the semiarid, mountainous environment of the western United States, where warming trends are expected to threaten water supply (Barnett et al, 2005). Identifying compartmentalized groundwater stores is necessary to sufficiently account for all components of the water balance (McDonnell, 2017). Characterizing localized water stores and the hydrologic connection of those aquifers to streams in mountainous environments that act as water towers (Viviroli et al, 2007) has important implications for water resource availability of large population centers downstream. A. White et al.: Distinct stores and the routing of water in the deep critical zone
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