Measuring spatial and temporal variation in surface moisture on a coastal beach with a near-infrared terrestrial laser scanner

Wind-alone predictions of aeolian sand deposition on the most seaward coastal dune ridge often exceed measured deposition substantially. Surface moisture is a major factor limiting aeolian transport on sandy beaches, but existing measurement techniques cannot adequately characterize the spatial and temporal distribution of surface moisture content. Here, we present a new method for detecting surface moisture at high temporal and spatial resolution using a near-infrared terrestrial laser scanner (TLS), the RIEGL VZ-400. Because this TLS operates at a wavelength (1550 nm) near a water absorption band, TLS reflectance is an accurate parameter to measure surface moisture over its full range. Five days of intensive laser scanning were performed on a Dutch beach to illustrate the applicability of the TLS. Gravimetric surface moisture samples were used to calibrate the relation between reflectance and surface moisture. Results reveal a robust negative relation for the full range of possible surface moisture contents (0%-25%), with a correlation-coefficient squared of 0.85 and a root-mean-square error of 2.7%. This relation holds between 20 and 60 m from the TLS. Within this distance the TLS typically produces O(106-107) data points, which we averaged into surface moisture maps with a 1 × 1 m resolution. This grid size largely removes small reflectance disturbances induced by, for example, footprints or tire tracks, while retaining larger scale moisture trends.