Our understanding of runoff generation and hydrologic storage dynamics in regions characterized by deep, highly weathered soils is poorly constrained, despite the prevalence of these landscapes worldwide. One such region is the southern Piedmont of the United States: the region east of the Appalachians that extends from Maryland to Alabama. Although regional relief is modest, the landscape is highly dissected and local slope is variable, resulting in steep headwater catchments. Soils are predominantly ultisols, with hydrologic properties controlled by argillic Bt horizons. The humid subtropical climate leads to consistent precipitation intra-annually and seasonal variability in energy availability. Consequently, the mixed deciduous and coniferous tree cover drives seasonal variations in evapotranspiration, which in turn leads to wet and dry seasons. While moist soils and longitudinally extended stream networks are typical in the fall through spring, relatively dry soils and contracting stream networks prevail in the summer. Here, we explore the connections between the internal heterogeneity of watershed structure and runoff generation at the watershed scale. Additionally, we examine how these dynamics vary as a function of season, and the implications of that variability for storage dynamics in these watersheds. We measured precipitation and discharge in a first-order watershed in South Carolina, and installed a nested, shallow groundwater wells and soil water content probes within it to measure its subsurface dynamics across soil horizons and landscape position. Most landscape positions exhibited minimal water table response to precipitation throughout dry summer periods, and infrequent response did not result in streamflow generation. In contrast, during the fall through spring period, streamflow was driven by transient perched water tables and topographically mediated redistribution of shallow groundwater. Coupled to these internal watershed results we observed minimal changes in storage throughout the year and flashy event response. Our findings suggest that interpreting streamflow and storage dynamics in regions possessing complex terrain and vertical soil structure is enhanced by characterization of both terrain mediated water redistribution and subsurface soil hydrology.
Mallard, J.M., B.L. McGlynn, D.deB. Richter (2018): Terrain and subsurface influences on runoff generation and storage dynamics in a steep, deep, highly weathered landscape. American Geophysical Union 2018 Fall Meeting, Washington, DC, 10-14 Dec 2018.
This Paper/Book acknowledges NSF CZO grant support.