Limnology and remote sensing: present situation and future developments
Remote sensing has been used successfully in oceanography for many years, whereas applications in limnology have been comparatively modest. We attempt to discover why from a review of the literature. After a brief description of remote sensing and of satellites in operation, we stress the specificity of inland waters targets compared with "solid" targets : low reflectivity in the visible and near infrared, complex water composition (chlorophyll, mineral suspensoids and gelbstoff which are not covariant) and the variable depth of investigation. Hence, simple chlorophyll and suspended solid retrieval algorithms, developed for type-I waters, cannot be applied to inland waters. Nonetheless, remote sensing may be, and has been applied effectively to the study and management of drainage basins and to the mapping of lake limits, aquatic vegetation, water masses, currents, thermal structures and water colour. Site-specific, regression-type models have been computed to express suspended solid concentration and water quality parameters as functions of radiante. So far, most of the applications have been performed on large lakes (Laurentian Great Lakes, Tahoe, Chad, Biwa, Balaton, Léman), where not only LANDSAT but also CZCS and AVHRR data have been in operation. A typology of present difficulties in applying remote sensing to limnology is proposed : intrinsic (complexity of inland waters), technological (land resource satellite bands not designed for water sensing, revisit time too long) and institutional difficulties (cost of data and equipment, and scarcity of limnologists proficient in remote sensing techniques).