Climate variability and vadose zone controls on damping of transient recharge fluxes

I 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. Climate signal flux propagation and variation with depth is influenced by highly nonlinear interactions of vadose zone controls, specifically the local layer texture, period (time interval), and mean amplitude of the flux. The depth at which the flux variation damps to 5% of its initial variation at land surface is defined as the damping depth. When the damping depth is above the water table, recharge may be considered steady; when the damping depth is not reached at the water table, recharge may be considered transient. I examined controls on the damping depth by modeling transient water fluxes through the vadose zone, using the Gardner-Kozeny soil model for diffuse unsaturated flow in the subsurface flow model, HYDRUS 1-D. Homogeneous profiles show that shorter-period oscillations, smaller mean fluxes, and finer-grained soil textures with poor sorting, cause shallower damping depths. Twolayered systems show similar, but more complicated responses. In two-layered systems, coarse-grained soils cause the greatest deviations from homogeneous damping depths, most evidently if they make up the lower layer of the profile. Flow simulations show that coupled finer-grained soil profiles cause damping depths that are more similar to their homogeneous counterparts and are closer to land surface, relative to coupled finegrained/ coarse-grained profiles. Linear superposition is possible in simulations of flux variations with short periods and finer-grained soils. Flux simulations with longer periods and coarser-grained soils fail to provide evidence for linear superposition. Findings from this study will enhance our understanding of how the vadose zones influences the teleconnections between climate variability modes and groundwater levels.