Revealing in-Situ Unsaturated Soil Hydraulic Conductivity at Fine Depth Scale.

Unsaturated soil hydraulic conductivity (K_u) plays a significant role in processes important for ecological, agricultural, and hydrological applications. In general, K_u is measured in the laboratory, or more commonly, predicted using soil water characteristics and saturated hydraulic conductivity. In the field, K_u can be estimated through infiltration experiments. None of these approaches are capable of continuously tracking in-situ K_u dynamics under natural conditions. In this study, we investigated an approach to continuously estimate fine depth-scale K_u dynamics under field conditions. Heat pulse, time domain reflectometry, and water potential sensors were installed at multiple depths within the 0-16 cm layer of a loamy sand field soil to monitor evaporation rates, water contents, and water potential gradients, respectively, during several natural wetting-drying cycles. We estimated soil water flux at different depths from evaporation rate and water content data using a simple water balance. Then, K_u at 2, 4, 8, and 13 cm depths were derived using the Darcy equation from water fluxes and measured water potential gradients. The approach provided reasonable K_u estimates, which agreed well with those measured in the laboratory using a Hyprop technique and those predicted using the van Genuchten–Mualem model. The K_u varied with depth and time following changes in soil water content. At shallower depths, K_u varied over a larger range than at deeper depths because surface soil water content changed rapidly. At the same water content, K_u at 2 and 4 cm depths were greater than those at 8, and 13 cm, which is likely due to differences in soil bulk density. Fine-scale in situ K_u estimates have potential to reveal the influence of natural soil conditions on hydraulic properties as they change with depth and over time.