Comparative Analysis of Nonlinear Dynamics of Water Flow through Variably Saturated Soil and Rocks

The goal of this presentation is to compare the dynamic instabilities in unsaturated-saturated liquid-flow processes, using the results of ponded infiltration tests in soil (silt and loam) and fractured rock (basalt and tuff). Flow through structured soil and rock is affected by a combination of factors, such as redistribution of water and entrapped air, as well as biofilm and colloidal sealing of macropores, fractures and the matrix. The analysis of infiltration rates measured in both soil and rock shows identical patterns of infiltration-rate changes on three temporal scales: (1) a macro-scale trend of overall decreasing flow, (2) a meso-scale trend of variation in the flow rate—decreasing, increasing, and (again) decreasing flow rate, mainly caused by the redistribution and removal of entrapped air and biofilm formation, and (3) a micro-scale trend of high-frequency fluctuations, which could be caused by multiple threshold effects and water-air redistribution among flow channels and the matrix. For both soil and fractured rock, the flow-process dynamics are affected by (1) flow through conductive flow channels (macropores or fractures), (2) dead-end channels, (3) connecting flow-through channels, and (4) the low-permeability matrix. The unstable flow processes are hysteretic and depend on both the intrinsic media properties and boundary conditions. Based on our analysis of experimental data, the conceptual model of flow through variably saturated soil and rock should incorporate the time dependent parameters of water redistribution in subsurface fractured-porous system. These relationships are characteristics of the water-flow system feedback and nonlinear dynamic processes in soil and rock. Using the results of field experiments, the author will demonstrate that the temporal flow-rate decrease can accurately be described by a four-parameter Weibull's distribution function, and the downward water travel time can be simulated using a fuzzy-systems approach.

This work was supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.