A comparison of the spatial distribution of vadose zone water in forested and agricultural floodplains a century after harvest

To improve quantitative understanding of the long-term impact of historic forest removal on floodplain vadose zone water regime, a study was implemented in fall 2010, in the Hinkson Creek Watershed, Missouri, USA. Automated, continuously logging capacitance-frequency probes were installed in a grid-like formation (n=6) and at depths of 15, 30, 50, 75, and 100 cm within a historic agricultural field (Ag) and a remnant bottomland hardwood forest (BHF). Data were logged at thirty minute intervals for the duration of the 2011, 2012, and 2013 hydrologic years. Results showed volumetric water content (VWC) to be significantly different between sites (p<0.01) during the study, with site averages of 33.1 and 32.8% at the Ag and BHF sites, respectively. Semi-variogram analyses indicate the presence of strong (<25%) horizontal and vertical spatial correlation of VWC at the Ag site, and a relatively short-range (25 cm) vertical spatial correlation at the BHF, but only indicate horizontal VWC spatial correlation in the top 30 cm of the BHF profile. Likely mechanisms contributing to patterns of observed differences are contrasting rates and depths of plant water use, and the presence of preferential flow paths in the below ground BHF. Results suggest historic forest removal and cultivation of the Ag site lead to an effective homogenization of the upper soil profile, and facilitated the development of strong VWC spatial dependency. Conversely, higher hydraulic conductivity of the more heterogeneous BHF subsurface likely results in a wetting of the deeper profile (75 cm) during climatically wet periods, and thus a more effective processing of hydrologic inputs. Collective results highlight the greater extent and degree to which forest vegetation impacts subsurface hydrology, relative to grassland/agricultural systems, and point to the value of reestablishing floodplain forests for fresh water routing, water quality, and flood mitigation in mixed-land-use watersheds.