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Monitoring Surface Changes of Bare Soils due to Slaking Using Spectral Measurements

Institut National de la Recherche Agronomique (INRA), Laboratoire de Science du Sol, Unité de Bioclimatologie, 66 rue de St Brieuc, 35042 Rennes cedex, France

Patrick Bertuzzi

INRA, Laboratoire de Science du Sol, Domaine St Paul, BP 91, 84143 Montfavet cedex, France

Michel-Claude Girard

Laboratoire de Science du Sol, INAPG, 78830 Thiverval-Grignon, France

*Corresponding author.

ABSTRACT

The structural degradation of the soil surface due to rainfall makes the surface more susceptible to erosion. Loamy soils are particularly vulnerable to slaking, seal development, and crusting. The sealing process occurs in different stages. This study examined the possibility of using visible and near-infrared reflectances for detecting these surface changes due to rainfall, and for defining optimal conditions for spectral measurements. Experimentation was conducted under field-controlled conditions using a SPOT satellite simulation radiometer. Radiometric measurements were made at three solar altitudes: 0600, 1200, and 1800 h GMT, at five different stages of surface degradation following simulated rainfall. Ground measurements consisting of Munsell color, water content, and surface roughness were made simultaneously to characterize surface changes. The influence of these various factors on reflectance was analyzed. Soil water was the main source of variation in reflectance, while surface roughness was a factor of secondary importance. Reflectance increased with increased roughness and decreased soil water content. A combination of red and near-infrared reflectances made it possible to identify different soil lines according to soil water content, solar angle, and slaking stage. Optimal conditions for monitoring surface changes were those where the solar altitude is the lowest for dry soils. Different slaking stages can also be detected by reflectances performed at 1200 h under dry conditions.

Received for publication August 28, 1992.