Effects of variable rainfall intensity on the unsaturated zone response of a forested sandy hillslope
2007
Tymchak, M.P. | Torres, R.
1 The purpose of this study was to determine the effects of single-storm rainfall variability on the timing of the unsaturated zone soil water content response. To investigate this, we monitored rainfall rate and soil water content at three soil profiles about 8 m apart along a downslope transect on a sandy forested hillslope. We also conducted one-dimensional numerical simulations of soil water content response in order to compare observed and expected wetting velocities. Results show that during relatively low intensity storms, wetting rates were comparable in each soil profile and were adequately represented in the numerical results. However, storms with highly variable, high-intensity rainfall rates produced an increase in wetting velocity with soil depth in two of three soil profiles, and the numerical simulations did not adequately replicate this response. Therefore it appears that variability in rainfall intensity, and in particular the sequence of rainfall variability, may cause all or part of the soil profile undergoing wetting to switch from a slow displacement process to a much faster accelerating process. We speculate that this transition is related to the initiation of preferential flow. This switching process may help reconcile the disparate findings of old versus new water in stormflow reported by Mosley (1979, 1982), Pearce et al. (1986), and Sklash et al. (1986), and it may help explain the rapid development of high pore pressures that lead to slope instability (e.g., Montgomery et al., 2002; Iverson, 2000).
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