Measurement of Beach Surface Moisture Using Surface Brightness

Abstract

This article describes the testing and calibration of a digital video camera system used to measure surface moisture over a section of beach extending tens of metres alongshore using the brightness of the sand surface. The system consisted of two networked video cameras mounted on a mast on the foredune crest about 14 m above the beach surface and oriented obliquely alongshore in either direction. Photographs and video clips were stored on a notebook computer. Over a period of 4 weeks in May and June 2006, a series of field tests was carried out; these tests were designed to calibrate the surface brightness recorded at locations visible in the digital images against gravimetric surface moisture measured by scraping the surface at the same location. While there was a significant correlation between surface brightness and gravimetric moisture content, the strength of this relationship was improved by normalising against a white board present in the field of view of the camera. An even stronger relationship was obtained by normalising against an area of dry sand present within a portion of the photograph. Rectification of the images and application of the calibration equation permitted mapping of surface moisture variations on a timescale of hours to days. The approach appears to provide a useful means of estimating surface moisture on the beach on a scale of tens of metres and over timescales of seconds to months.