Chronotopographic analysis directly from point-cloud data: A method for detecting small, seasonal hillslope change, Black Mesa Escarpment, NE Arizona

The advent of high-resolution, precise, back-pack portable terrestrial lidar scanners (TLS) provides a revolutionary new tool for obtaining quantitative, high-resolution (2mm to 30-mm point spacing) measurements of landscape surface features.Moreover, data collected using these instruments allow observation of geomorphic processes in systems that can experience change on a daily basis.We have introduced TLS techniques in ongoing investigations of semiarid landscapes associated with weakly cemented sandstones along part of the Black Mesa escarpment of NE Arizona.Clay-cemented, Jurassic sandstones exposed along this escarpment are sensitive to moisture, and thus climate, via hydration-expansion weathering of interstitial clay.Sediment shed from weathered slopes has caused locally rapid valley fl oor aggradation and upper basin slope vertical denudation rates of 2-3 mm/yr over 10-to 100-yr timescales, as indicated by dendrochronology coupled with soil geomorphic analysis.These rates suggest rapid hillslope denudation rates.Employing the University of New Mexico Lidar Laboratory Optech Ilris 3D TLS, we are constructing a high-resolution model of two major basins along the escarpment.Focusing on a single, small (30 × 60 m) area of a mostly non-vegetated, steep slope (>35°), we demonstrate in this paper a method of comparative analysis of pointcloud data sets that can detect subcentimeter change resulting from a single season of monsoon precipitation along the escarpment.Using repeat scans can provide an empirical evaluation of single season erosion rates in the study site, and because our observations are geospatial in nature, we can also document the parts of the slopes that make the greatest contribution to local valley fl oor aggradation.In demonstrating the utility of this method, we expect that continued investigation of this site will provide insight to the key processes associated with soil-mantled versus bedrockdominated slopes during modern escarpment retreat and hillslope modifi cation, which, in turn, may further elucidate the impacts of Holocene climate change on this rapidly evolving landscape.