CONTROLS ON SOIL VARIABILITY ON HILLSLOPES

Spatial variability of soils on hillslopes is a first order control on hillslope hydrology. Current soil maps are produced at too coarse a scale to adequately capture the nature of soil variability on most hillslopes. Characterization of spatial soil variability requires an understanding of the drivers of variability in soil landscapes which can be broadly grouped into temporal or spatial controls. In a temporally controlled soil landscape, soil properties are determined primarily by the age of the surface on which the soil is forming. In a spatially controlled landscape, soil properties reflect the influence of other soil forming factors such as position on a hillslope, parent material, slope orientation, and vegetative cover. Spatially controlled landscapes are common in areas with high erosion rates and the pattern of soil distribution is determined by the type of erosion processes. In a spatially controlled soil landscape, soil catenary relationships are common and differences due to slope orientation and parent material are apparent. These concepts can be applied to soil landscapes at different scales from entire drainage basins to individual landforms.

Identification of the major drivers of spatial soil variability is essential when sampling soils on hillslopes and even more so if statistical analyses of soil properties is contemplated.