Figure 1.
Spectral power distribution of sunlight (in Wm−2) outside the atmosphere (yellow line) and at sea level (brown line), to illustrate the effect of absorption of sunlight by water.
Included in the top panel: spectral Fresnel reflectance (-) for a water surface (green line) and absorption coefficients (in cm−1) for pure water (blue line). Bottom panel: overview of the studies (including this study) that demonstrate the potential of relating reflectance to surface moisture. The orange bars indicate the spectral range in which the measurements were taken.
Figure 2.
Conceptual representation of the non-linear decrease of spectral reflectance as moisture content in the sand matrix increases.
At wilting point (1) there is almost no absorption of light in water as the optical path length () is close to zero. At field capacity (2) the optical path length in water (
) increases with increasing absorption as result. At saturation (3) the optical path length in water (
) is at its maximum and certain wavelengths may be completely absorbed.
Figure 3.
Water retention curve for a coarse grained sand matrix (M50 = 350–500 m), created using the Van Genuchten equation (Eq. 5).
Shown in the graph are the residual water content (), saturated water content (
), air entry value (1/
) at the inflection point, and (
) which is related to the slope at inflection point and indicates pore-size distribution. Data taken from [63].
Figure 4.
Measurement setup of the laboratory spectroscopy experiment.
An ASD Fieldspec Pro spectrometer (Analytical Spectral Devices, Boulder, CO), fitted with a 1° FOV foreoptic, was directed at nadir at 40 cm distance from the sample. A 900 watt Quartz Tungsten Halogen (QTH) lamp was placed 70 cm from the sample at a 30° zenith angle. The spectral reflectance was measured over a range of 350–2500 nm, at 1 nm intervals.
Figure 5.
Measured spectral reflectance over a range of 350–2100 nm at 4% volumetric moisture content interval, between 32% (saturation) and 0.01% (air-dry).
A non-linear decrease in reflectance upon wetting is observed over the full range of wavelengths. Notable dips in reflectance occur at water absorption peaks at 1470 and 1940 nm (blue bands). The dashed red lines show the spectral reflectance as calculated by fitting the optical model (Eq. 6) to measured spectral reflectance.
Figure 6.
Performance of the optical model (Eq. 6).
Top panel: goodness of fit of the optical model as function of volumetric moisture content. Bottom panel: trajectories of the regression parameters
and
of the optical model, describing the proportional contribution of scattering (fraction
) and absorption (optical path length in water
) as moisture levels increase. Shaded areas indicate 95% confidence intervals.
Figure 7.
Measured spectral reflectance upon wetting at the water absorption peaks of 760, 970, 1200, 1470, and 1940 nm.
The dashed red lines show volumetric moisture content as calculated by fitting the soil-physical model (Eq. 7). With sand matrix parameters
= 0.04 and
= 0.29. Non-linear regression parameters
and
are shown in the plot at corresponding wavelength.
Figure 8.
Spectral reflectance upon wetting (red dashed lines) obtained by fitting the soil-physical model (Eq.7) to measured spectral reflectance at 1 nm interval.
The blue bands indicate spectral absorption peaks for water. Between 4.5–24% volumetric moisture content the spectral reflectance reconstructed with Eq. 7 has a goodness of fit of
0.99.
Figure 9.
Averaged measured spectral reflectance upon wetting at visible wavelengths (400–700 nm), illustrating the limited contrast in reflectance between a dry and a wet beach composed of quartz sand.
Figure 10.
Example gravimetric moisture content (in %) map, estimated from the reflectance collected with a RIEGL VZ-400 3D terrestrial laser scanner at Egmond aan Zee, The Netherlands, from the upper dry beach (cross-shore distance m) to the water line (
to 10 m) [62].
The local co-ordinates are positive in the seaward direction and to the south. The scanner was located at m. The slightly drier sand immediately around the scanner position is an artifact of the conversion from intensity to reflectance; close to the scanner, as also noted by [39], the imposed
correction does not apply. The narrow bands with apparent lower moisture content (e.g. at alongshore
distances
to
m and
m) correspond to car tracks.