Heterogeneity of Infiltration Rates in Alluvial Floodplains as Measured with a Berm Infiltration Technique

Hydrologic heterogeneities (e.g., macropores and gravel outcrops) in floodplains are hypothesized to play an integral role in impacting flow and leaching between the soil surface and shallow alluvial aquifers, which are intricately connected to streams. Infiltration is often assumed to be uniform, but this neglects the spatial variability common in anisotropic, heterogeneous alluvial floodplain soils. The objective of this research was to quantify infiltration and hydraulic conductivity across a range of scales (point to 100 m) using a berm infiltration technique. Plot-scale leaching experiments were performed across a range of soil types at each of three floodplain sites in northeastern Oklahoma and northwestern Arkansas. Plots maintained a constant head of 2 to 9 cm for up to 52 h. Effective saturated hydraulic conductivity (Keff), based on plot-scale infiltration rates and a one-dimensional Darcy flow equation, ranged between 0.6 and 68 cm h and varied considerably even within a single floodplain. The Keff was also calculated at the point scale using particle size distributions and Retention Curve (RETC). Point-scale estimates were significantly lower than plot-scale Keff and also failed to capture the variability of Keff. The estimated permeability of the limiting layer reported in soil surveys was consistent with point-scale estimates of Keff but was lower than plot-scale Keff at most sites. Tension infiltrometers showed that macropores accounted for approximately 84% to 99% of the total saturated hydraulic conductivity. The plot scale (1 to 100 m) generally appears to be within the representative elementary volume (REV), but drift in Keff occurs beyond the REV due to changing geomorphic formations. Plot-scale infiltration tests are recommended over point-scale estimates, although only small plots (1 m × 1 m) are necessary.