The role of watershed storage dynamics mediating runoff generation is well known yet remains poorly understood. A growing body of research is increasing recognition of the role that critical zone structure plays in partitioning precipitation between multiple storage pools and watershed-scale fluxes. New watershed studies are being performed in deep, critical zones where the subsurface is heterogeneous in its hydrologic properties. We built a network of shallow groundwater monitoring wells within a headwater watershed in the Southern Piedmont of the United States at the Calhoun Critical Zone Observatory. This region is characterized by deep weathering profiles and stratified soil structure which includes relatively shallow, clay-rich horizons. We distributed shallow monitoring wells spatially through this watershed and also vertically above and below these clay soil horizons. Additionally, we monitored a 70 m deep groundwater well to characterize regional scale groundwater dynamics. We observed spatial heterogeneity of shallow groundwater response depending on the depth, hillslope position, and local terrain: some wells developed seasonally persistent water tables while others responded only to proximate inputs from precipitation. The perennial deeper groundwater largely followed seasonal trends of potential evapotranspiration. By relating these distinct measurements of local storage dynamics to runoff, we determined that runoff occurs not only when shallower soil horizons are saturated, but also that a minimum threshold of storage below clay horizons must be exceeded to allow the former process to occur. Concurrent dynamics of deeper groundwater indicated broad, seasonal linkages to runoff generation but also suggests inertia in the connection between hydroclimatic forcing and regional scale groundwater dynamics. This scale-dependent shift in dominant groundwater dynamics for runoff generation has strong ramifications as headwater hydrology is aggregated and scaled up to larger basins. Additionally, these findings emphasize the importance of understanding storage heterogeneity in three dimensions in all but the simplest watersheds and how distinct storage zones can control distinct scales and components of hydrologic response in these deep, highly weathered landscapes.
Mallard, John McDevitt, Brian L McGlynn, Daniel deB. Richter (2019): Storage volume and depth drives runoff generation in a deep and highly weathered headwater watershed. American Geophysical Union Fall Meeting, San Francisco, CA, December 9-13, 2019.
This Paper/Book acknowledges NSF CZO grant support.