Climate Variability and Vadose Zone Controls on Damping of Transient Recharge Fluxes

Abstract:

We investigate the effects of interannual to multidecadal climate variability on groundwater resources by exploring the physical processes in the vadose zone that partially control transient infiltration and recharge fluxes. The vadose zone connects climate variability modes to groundwater systems by influencing infiltration events. Infiltration events become time-varying water flux through the vadose zone and are controlled by highly nonlinear, complex interactions between mean infiltration flux, infiltration period, soil textures, and depth to water table. We focus on the behavior and damping depth of water flux in the vadose zone. The damping depth is defined as the depth that the flux variation damps to 5% of the land surface variation. When the damping depth is above the water table, recharge may be considered steady; when the damping depth is below the water table, recharge may be considered transient. Previous work shows that the damping depth is sensitive to the frequency of the infiltration pattern and the unsaturated hydraulic properties of the media. We examine controls on the damping depth by modeling transient water fluxes at the land surface using the Gardner-Kozeny soil model for diffuse unsaturated flow in HYDRUS 1-D. Results for homogeneous profiles show that shorter-period oscillations, smaller mean fluxes, and finer-grained soil textures generally produce damping depths closer to land surface. Modeling layered soil textures indicates similar, but more complicated responses in the damping depth. Model results indicate that finer-textured layers in a coarser soil profile generally result in damping depths closer to land surface, while coarser-textured layers in a coarser soil profile result in damping depths deeper in the vadose zone. Findings from this study will enhance understanding of the vadose zone’s influence on transient water flux and improve the simulation of recharge processes and climate variability effects in groundwater models.