Storage and routing of water in the deep critical zone of a snow dominated volcanic catchment

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 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 a perched aquifer in consolidated collapse breccia and deeper groundwater in a fractured tuff aquifer, enabled us to untangle the interactions of these groundwater stores and their contribution to streamflow across one complete water year. 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, comparison of streamflow and groundwater hydrographs indicates hydraulic connection between the fractured welded tuff aquifer and streamflow while the perched aquifer within the collapse breccia deposit does not show this same connection. Furthermore, analysis of age tracers and stable water isotopes 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 as vertical infiltration and subsurface lateral flow, depending on lithology. We find that in complex geologic terrain like that of the JRB-CZO, differences in CZ architecture of two hillslopes within a headwater catchment control water storage and routing through the subsurface and suggest that the perched aquifer does not contribute significantly to streams while deep fractured aquifers contribute most to streamflow.