Damping of multi-frequency infiltration with depth in the vadose zone

Abstract:

Hydrologic processes operate at various frequencies that often range from daily to annual and interdecadal time scales. As a result, water fluxes at the land surface can vary cyclically at multiple simultaneous frequencies. These time-varying fluxes combine with vadose zone processes to produce time-varying flux and water content in the vadose zone. Previous work has defined the dominant characteristics that control the damping of a single time-varying cycle. This work resulted in a numerical tool that can be used to determine whether recharge can be approximated to be steady state in subregions of an aquifer. Single-frequency analyses suggest that the variability from shorter period events (e.g. the rapid changes in water content that result from episodic precipitation and runoff) are damped at shallow depths. However, given the nonlinear character of unsaturated flow, it is not always appropriate to assume that the effects of multiple, simultaneous time-varying cycles will add linearly. Therefore, we extend our previous analysis to identify areas of steady recharge in aquifers in response to multi-frequency cyclical infiltration. Results show that wet periods of the longer cycle (e.g. interannual and interdecadal variations related to global-scale oceanic-atmospheric phenomena) can “carry” the shorter cycle infiltration to greater depths. In contrast, the shorter cycle damps more rapidly during a dry part of a long cycle. This suggests that long-term changes in precipitation and ET may have more complicated impacts on the spatial and temporal patterns of recharge than has been considered to date.