Hillslope hydrology: From patterns to processes

This dissertation re-examines the now standard perceptual model of hillslope hydrological response to rainfall, which includes the growth of a saturated wedge at the soil-bedrock interface or impeding layer. It also challenges the notion of bedrock impermeability and the assumption that the pattern of subsurface stormflow is determined by the soil moisture pattern. The results presented in this dissertation challenge the status quo model and show that at the Panola Mountain research hillslope, subsurface storm flow is a threshold-function of precipitation. This threshold is a result of a disconnection between transient saturated areas and the slope base during small to medium size storms (<55 mm). Water must fill bedrock depressions before it can flow further downslope. Once filled, connectivity between subsurface saturated areas and the slope base is established and subsurface flow initiates. Transient saturation does not have the form of a saturated wedge but starts first in the shallow soil areas. The pattern of transient saturation does not resemble the pre-event soil moisture pattern but is a function of soil depth and bedrock micro-topography. Flow through bedrock is a major component of the hillslope water balance. Soil moisture distribution across the slope shows two distinct states: wet and dry. Soil moisture in the shallow soil areas of the hillslope limits transpiration during the late growing season but transpiration is not soil moisture limited on deeper soil sections. Overall, these findings represent a new perceptual model of hillslope hydrological response during and between rainfall events.