Unsaturated zone processes and the hydrologic response of a steep, unchanneled catchment

As part of a larger, collaborative study, we conducted field experiments to investigate how rainfall signals propagate through an unsaturated soil profile, leading to a rapid pore pressure response and slope instability. We sprinkler-irrigated an entire, unchanneled headwater basin in the steep, humid Oregon Coast Range, and we drove the system to quasi steady state as indicated by tensiometers, piezometers, and discharge. During initial wetting some of the deeper tensiometers responded before the arrival of an advancing head gradient front. With continued irrigation most tensiometers attained near-zero pressure heads before most piezometers responded fully, and a stable unsaturated flow field preceded the development of a stable saturated flow field. Steady discharge occurred after the last piezometer reached steady state. With the onset of steady discharge the unsaturated zone, saturated zone, and discharge became delicately linked, and a spike increase in rain intensity led to a response in the saturated zone and discharge much faster than could have happened through advection alone. We propose that the rain spike produced a slight pressure wave that traveled relatively rapidly through the unsaturated zone, where it caused a large change in hydraulic conductivity and the rapid effusion of stored soil water. An important control on the hydrologic response of this catchment lies with the soil-water retention curve. In general, below pressure heads of about -0.05 m, soil-water contents change slightly with changes in pressure head, but above -0.05 m the soil-water content is highly variable. Minor rainstorms upon a wet soil can produce slight changes in pressure head and corresponding large changes in soil-water content, giving rise to the passage of pressure waves in response to increased rain intensity and a relatively rapid response in the unsaturated zone. This rapid unsaturated zone response led to a rapid rise in the saturated zone, and it may be the underlying mechanism enabling short bursts of rain to cause slope instability.