WEAP is a software tool that provides a comprehensive, flexible and user-friendly framework for water systems planning and policy-making.
Created by the Stockholm Environment Institute, WEAP can be applied to municipal and agricultural systems, a single watershed or complex transboundary river basin systems.
It takes an integrated approach, placing water supply projects in the context of demand-side management, water quality, and ecosystems, and simulating both natural and engineered components of water systems.
SEI offers support to users and holds WEAP workshops around the world.
http://www.weap21.org/