Water management is complex because it concerns different spatial and temporal scales and multiple stakeholders with varying goals. Computer-model tools have been developed by a research team of systemic agronomists to assist water managers, from farmers to water agencies. The common basis of these tools is the use of bio-decisional models that take into account farmers' adaptive behaviour. This paper presents these tools which deal with the irrigation block, farm scale, and water basin levels. Illustrations of their potential use are provided. The research team positions its modelling approaches in the general integrated assessment modelling approach, based on the use of several quantitative models representing the interacting sub-components of the human-environment-technology system acting at different levels. Lessons from the use of the different tools demonstrate the importance of modular models, multi-criteria analysis assessment, participatory and interdisciplinary research and to coherently combine soft and hard tools.