Relating soil surface moisture to reflectance

Abstract

The objective of this study was to explore the relationship between soil reflectance in the solar domain (400–2500 nm) and soil moisture. Ten soils covering a large range of composition have been sampled. To decrease the large dimensionality of the data set, we reduced the number of wavebands investigated thanks to a simple stepwise linear regression. Seven wavebands were selected, which represent the whole spectral domain for the 10 soils and all moisture conditions with a root mean square error (RMSE) better than 0.002, close to the experimental uncertainties. Each soil reflectance spectrum was normalized by the corresponding reflectance spectrum observed under the driest condition. This allows to minimize effects due to soil type, as well as those of other undesirable multiplicative factors such as roughness and measurement configuration. The relationship between the normalized soil reflectance and moisture was then investigated. For all the wavelengths and all the soils, results show that for low soil moisture levels, the reflectance decreased when the moisture increased. Conversely, after a critical point, soil reflectance increased with soil moisture. For some soils, the reflectance of the wettest conditions can overpass that of the driest conditions. The position of the critical point was related to soil hydrodynamic properties. For both low and high soil moisture levels, and the seven wavelengths selected, the relative reflectance was strongly correlated with moisture. Adjustment of the relationships over individual soil types provides better soil moisture retrieval performances. It also shows that the relationships are generally nonlinear. These results are discussed with regards to the underlying physical processes, as well as for application to soil moisture estimates from reflectance measurements.