As a result of human activities, the pressure exerced on the environment bas never been so important and diverse. Watersheds, the worlds major resource of freshwater, are subjected to severe environmental threat. Post war agriculture has had a number of significant repercussions on aquatic environments, particularly an increase in trophic pollution. Only recently, agriculture was recognized as an activity contributing to degradation of the environment and is continually expanding. The management of water resources is a political, economical and technical challenge. This situation is now recognized by communities throughout the world and research is being conducted to develop new tools for the management and purification of freshwater.

In this context, the GISWAQ project, elaborated in 1989 by UNEP/GRID and ILEC, aims to evaluate the potential of Geographic Information Systems (GIS) and Remote Sensing (RS) in the analysis and management of lakes and their watersheds. Later on, these methods may be applied in developing countries. Two pilot studies were carried out, one by ILEC on lake Biwa in Japan, and the other by UNEP/GRID on lake Geneva in Switzerland.

The main objectives of this research can be summarized as follows:

• localize, at a large watershed scale (several thousands of km²), the regions susceptible to generate diffuse pollution induced by nutrient exportation. This localization procedure is supported by an adaptation of the Universal Soil Loss Equation (USLE; Wischmeier & Smith, 1978).

• establishing methods of watershed managment based on the use of GIS and RS exportable to developing countries. Therefore, the computing techniques must remain affordable, user friendly and versatile. [...]