CHRONOTOPOGRAPHIC ANALYSIS TO DETECT SMALL, SEASONAL HILLSLOPE CHANGE USING TLS LIDAR DATA, BLACK MESA ESCARPMENT, NE ARIZONA

Change detection using airborne or terrestrial lidar scanner (TLS) data sets is not unusual. However, the most common approach is to grid the data to derive a model based on irregular point cloud data. Models based on scans taken at different times are then compared to derive a difference volume between the two data sets. The resulting calculation is a bulk volume estimate of change where the exact position and magnitude of change is not recorded. Alternatively, the differences in surface elevation at each node of two grids having the same resolution and dimension can be calculated to give the spatial variation of dz. Although reasonable, these are essentially modeling approaches and the errors related to gridding the data are rarely documented or calculated. Moreover, the spatial context of the variation of error is essentially ignored (i.e., gridding errors are potentially lower over a flat surface). In a project where TLS techniques have been introduced in on-going investigations of semiarid landscapes, we are attempting to document the day-to-day hillslope processes that lead to pulses of erosion and sedimentation associated with weakly cemented sandstones along part of the Black Mesa escarpment of NE Arizona. Dendrochronology coupled with soil geomorphic analysis indicates that abundant sediment is being shed from weathered slopes at rates of 2-3 mm/year over 10-100 yr timescales, causing rapid localized valley floor aggradation. Employing the UNM LiDAR Laboratory Optech Ilris 3D terrestrial LiDAR scanner, we have devised a method for chronotopographic analysis where subsequent point-cloud data sets are compared directly to each other. Focusing on a single patch, we demonstrate a method of comparative analysis that can detect sub-centimeter change resulting from a single season of monsoon precipitation along the escarpment. The long-term goal of this work is to demonstrate the utility of this technology in documenting the effects of key slope-forming processes (e.g. diffusive vs. advective) associated with escarpment retreat and hillslope modification. Such research products may in turn further elucidate the impacts of Holocene climate change on this rapidly evolving landscape.