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An optical wetness sensor (OWS) was developed for continuous surface wetness measurements. The sensor is an all-weather instrument that does not interfere with the surface wetting and drying process and is unaffected by solar radiation. It is equipped with its own light source with which it can scan a surface and analyse its spectral reflectance. The backscattered radiation is detected around two wave bands: 1.70 mu m (170 nm waveband) and 1.94 mu m (80 nm waveband). The optical design is such that the ratio ofthe two signals is not dependent on the sampling distance, hence making it possible to study leaves under field conditions. Field and lab testing showed that the OWS was also capable of resolving small changes in surface soil water content such as from dew. In addition, the change of leaf water content could be detected (accuracy 5%). The OWS was also capable of measuring small changes in water content in the upper soil layer (<2 mm) of a loamy and fine sandy For plant leaves it must be noted that the OWS responds to both internal and external soil. water and thus cannot always discern between them. (c) 2007 Elsevier B.V. All rights reserved.