Sensitivity of Near-Infrared Permanent Laser Scanning Intensity for Retrieving Soil Moisture on a Coastal Beach: Calibration Procedure Using In Situ Data

Abstract

Anthropogenic activities and climate change in coastal areas require continuous monitoring for a better understanding of environmental evolution and for the implementation of protection strategies. Surface moisture is one of the important drivers of coastal variability because it highly affects shoreward sand transport via aeolian processes. Several methods have been explored for measuring surface moisture at different spatiotemporal resolutions, and in recent years, light detection and ranging (LiDAR) technology has been investigated as a remote sensing tool for high-spatiotemporal-resolution moisture detection. The aim of the present study is the assessment of the performance of a permanent terrestrial laser scanner (TLS) with an original setting located on a high position and hourly scanning of a wide beach area stretching from a swash zone to the base of a dune in order to evaluate the soil moisture at a high spatiotemporal resolution. The reflectance of a Riegl-VZ2000 located in Noordwijk on the Dutch coast was used to assess a new calibration curve that allows the estimation of soil moisture. Three days of surveys were conducted to collect ground-truth soil moisture measurements with a time-domain reflectometry (TDR) sensor at 4 cm depth. Each in situ measurement was matched with the closest reflectance measurement provided by the TLS; the data were interpolated using a non-linear least squares method. A calibration curve that allowed the estimation of the soil moisture in the range of 0–30% was assessed; it presented a root-mean-square error (RMSE) of 4.3% and a coefficient of determination (R-square) of 0.86. As an innovative aspect, the calibration curve was tested under different circumstances, including weather conditions and tidal levels. Moreover, the TDR data collected during an independent survey were used to validate the assessed curve. The results show that the permanent TLS is a highly suitable technique for accurately evaluating the surface moisture variations on a wide sandy beach area with a high spatiotemporal resolution.

Permanent Laser Scanner and Synthetic Aperture Radar Data: Correlation Characterisation at a Sandy Beach

“…Several studies have shown that remote sensing data on coastal environments are affected by variables such as wind condition and surface roughness. This has been shown independently for both PLS [ 10 , 15 , 16 ] and SAR data, where the backscattering behaviour depends on the roughness in relation to the wavelength [ 32 , 33 ] and is affected by wind speed and direction [ 34 ]. Therefore, notably, meteorological conditions in terms of wind speed and direction, rain, and surface roughness, will be taken into consideration in the evaluation of the correlation between PLS and SAR data.…”

“…Among its applications, forestry [ 1 , 2 , 3 ], river systems [ 4 , 5 ] and geomorphology [ 6 , 7 ] have been investigated. In recent years, our knowledge on coastal environments has been enriched by information provided by TLS systems, which show significant potential for examining coastal processes [ 8 , 9 , 10 ]. Among the coastal applications, TLS has been used in order to generate Digital Elevation Models (DEM) and to evaluate accurate volumetric changes on beaches, dunes and cliffs, Ref.…”

“…[ 11 ]: thanks to the high density of the point clouds with high accuracy/precision, TLSs are suitable for the detailed DEM mapping of features on hundreds of meters of beach–dune systems [ 12 ]. Several studies have also demonstrated the potential for estimating other beach features, such as the surface moisture, using both short and long-range TLS [ 10 , 13 , 14 , 15 , 16 ].…”

Environmental Strain on Beach Environments Retrieved and Monitored by Spaceborne Synthetic Aperture Radar

“…Optical remote sensing techniques are based on the principle that wet sand is less reflective than dry sand, due to pore water surrounding the sand grains [24]. Their potential for aeolian research was demonstrated by [25][26][27][28] and while widely used, these techniques are susceptible to cloud, rain, and light conditions. This is not convenient in areas with high average annual cloud cover.…”

“…One of the most common characterizations of surface roughness is its root-mean-squared height (RMSH). For this purpose, Terrestrial Laser Scanning (TLS) has been investigated for its huge potential for coastal processes [25,50], as it allows fast and repeatable measurements of surface topography [23] and surface moisture. Several studies demonstrated how soil roughness can be evaluated using TLS [51,52].…”