Terrain-Shape Indices for Modeling Soil Moisture Dynamics

This study examined spatial and temporal relationships between measured soil moisture and terrain-based proxies for soil moisture dynamics. Two catenas were intensively sampled reflecting a mosaic of differences in degree of soil development in California's Sierra Foothill Region. A catena containing weakly developed soils (Haploxerepts + Haploxeralfs) formed from granitic parent materials was compared to a catena of well-developed soils (Haploxeralfs and Palexeralfs) formed from metavolcanic parent materials. Soil moisture was monitored at 10-, 30-, and 50-cm depths in 15 profiles in the granitic catena and 100 profiles in the metavolcanic catena. Seven post-rainfall periods during the 2008-2009 water year were selected to compare terrain shape indices and measured soil moisture. No single terrain index (slope, tangential curvature, profile curvature, mean curvature, topographic prominence, terrain characterization index, and compound topographic index [CTI]) consistently described variability in mean water content or dry-down rates, across depth or space. However, within the granitic catena, a combination of CTI and modeled beam radiance consistently accounted for 30 to 70% of the total variance in mean water content at 10 cm, and 10 to 40% at 30- and 50-cm depths. The predictive capacity of digital elevation model (DEM)-derived terrain shape indices for soil moisture dynamics varied widely in time and space, and was influenced by spatial patterns in the degree of soil development. Efforts to describe soil moisture variability are an important attribute of digital soil mapping (DSM). Moreover, soil variability influences soil moisture dynamics, thus synergistic activities are needed to integrate landscape scale variability with digital soil mapping.