Laboratory and numerical investigations of hillslope soil saturation development and runoff generation over rainfall events

Runoff is a key process that controls the behaviour of a hillslope hydrological system. The study presented here aims to examine the mechanisms of runoff generation due to saturation excess by investigating the development of the subsurface saturated zone. Experiments were conducted on a hillslope system with a uniform slope (0.2) and a relatively homogeneous and highly permeable medium (Ks = 1.28 cm/min), subjected to high and constant surface recharges (0.51–0.69 cm/min). Two initial conditions prior to the rainfall events were set up in the experiments to represent relatively dry and wet antecedent soil conditions, respectively. Measurements showed that during vertical infiltration, local pressure head and soil moisture remained constant for a certain period, showing a ‘waiting’ behaviour. The saturated area formed initially at the slope toe, quickly rose to the surface and subsequently expanded to the upslope. When propagating in the upslope direction, the wetting front caused the pore-water flow to deflect in areas above the wetting front and at the slope base. With a wetter initial condition, the soil responded to the rainfall more quickly. The initial moisture conditions also altered the relation between the subsurface discharge and pressure head. Under the applied rainfall rates, the system reached a fully saturated condition and produced surface runoff. The rainfall intensity was found to affect the temporal variations and magnitude of surface runoff characteristics; however it did not seem to impose any significant effect on the maximum subsurface discharge rate. These results provide insight into the behaviour of the hillslope system in response to rainfall.