Measuring and Modeling the Effect of Surface Moisture on the Spectral Reflectance of Coastal Beach Sand

Surface moisture is an important supply limiting factor for aeolian sand transport, which is the primary driver of coastal dune development. As such, it is critical to account for the control of surface moisture on available sand for dune building. Optical remote sensing has the potential to measure surface moisture at a high spatio-temporal resolution. It is based on the principle that wet sand appears darker than dry sand: it is less reflective. The goals of this study are (1) to measure and model reflectance under controlled laboratory conditions as function of wavelength ([Image: see text]) and surface moisture ([Image: see text]) over the optical domain of 350–2500 nm, and (2) to explore the implications of our laboratory findings for accurately mapping the distribution of surface moisture under natural conditions. A laboratory spectroscopy experiment was conducted to measure spectral reflectance (1 nm interval) under different surface moisture conditions using beach sand. A non-linear increase of reflectance upon drying was observed over the full range of wavelengths. Two models were developed and tested. The first model is grounded in optics and describes the proportional contribution of scattering and absorption of light by pore water in an unsaturated sand matrix. The second model is grounded in soil physics and links the hydraulic behaviour of pore water in an unsaturated sand matrix to its optical properties. The optical model performed well for volumetric moisture content [Image: see text] [Image: see text] 24% ([Image: see text] [Image: see text] 0.97), but underestimated reflectance for [Image: see text] between 24–30% ([Image: see text] [Image: see text] 0.92), most notable around the 1940 nm water absorption peak. The soil-physical model performed very well ([Image: see text] [Image: see text] 0.99) but is limited to 4% [Image: see text] [Image: see text] [Image: see text] 24%. Results from a field experiment show that a short-wave infrared terrestrial laser scanner ([Image: see text] = 1550 nm) can accurately relate surface moisture to reflectance (standard error 2.6%), demonstrating its potential to derive spatially extensive surface moisture maps of a natural coastal beach.