Freshwater management challenges are increasingly common. Allocation of limited water resources between agricultural, municipal, and environmental uses requires the full integration of supply, demand, water quality, and ecological considerations. The Water Evaluation and Planning system, or WEAP, aims to incorporate these issues into a practical yet robust tool for integrated water resources planning. WEAP was developed by the Stockholm Environment Institute's U.S. Center.

WEAP operates on the basic principle of a water balance and can be applied to municipal and agricultural systems, a single watershed, or complex transboundary river basin systems. Moreover, WEAP can simulate a broad range of natural and engineered components of these systems, including rainfall runoff, baseflow, and groundwater recharge from precipitation; sectoral demand analyses; water conservation; water rights and allocation priorities; reservoir operations; hydropower generation; pollution tracking and water quality; vulnerability assessments; and ecosystem requirements. A financial analysis module also allows the user to investigate cost-benefit comparisons for projects.

The WEAP system offers:

• Integrated Approach—A unique approach for conducting integrated water resources planning assessments.
• Stakeholder Process—WEAP's transparent structure facilitates engagement of diverse stakeholders in an open process.
• Water Balance—A database maintains water demand and supply information to drive mass balance model on a link-node architecture.
• Simulation Based—The system calculates water demand, supply, runoff, infiltration, crop requirements, flows, and storage, and pollution generation, treatment, discharge, and instream water quality under varying hydrologic and policy scenarios.
• Policy Scenarios—WEAP evaluates a full range of water development and management options, and takes account of multiple and competing uses of water systems.
• User-Friendly Interface—A graphical drag-and-drop GIS-based interface with flexible model output as maps, charts, and tables.
• Model Integration—Includes dynamic links to other models and software, such as QUAL2K, MODFLOW, MODPATH, PEST, Excel, and GAMS.

An evaluation version of the software is available for download from the WEAP website. Before downloading, users must first join the WEAP User Forum (at no cost). To fully activate the downloaded copy of WEAP, a valid license is required; license fees vary depending on the type of user. More information on licensing and fees can be obtained from the Licensing page of the WEAP website.