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Compendium on methods and tools to evaluate impacts of, and vulnerability and adaptation to, climate change

Water Evaluation and Planning System (WEAP)

This is a PC based surface and groundwater resource simulation tool, based on water balance accounting principles, which can test alternative sets of conditions of both supply and demand. The user can project changes in water demand, supply, and pollution over a long-term planning horizon to develop adaptive management strategies.

WEAP is designed as a comparative analysis tool. A base case is developed, and then alternative scenarios are created and compared to this base case. Incremental costs of water sector investments, changes in operating policies, and implications of changing supplies and demands can be economically evaluated.

Appropriate Use What-if analysis of various policy scenarios and long-range planning studies. Adaptive agriculture practices such as changes in crop mix, crop water requirements, canal linings; changes in reservoir operations; water conservation strategies; water use efficiency programs; changes in instream flow requirements; implications of new infrastructure development. Strengths include detailed demand modeling.
Scope All locations, surface- and groundwater systems; national, international or site-specific.
Key Output Mass balances, water diversions, sectoral water use; benefit/cost scenario comparisons; pollution generation and pollution loads.
Key Input

Configuration of system (can use GIS layers for background) and component capacities and operating policies.

Water demand: Spatially explicit demographic, economic, crop water requirements; current and future water demands and pollution generation.

Economic data: Water use rates, capital costs, discount rate estimates.

Water supply: Historical inflows at a monthly timestep; groundwater sources.

Scenarios: Reservoir operating rule modifications, pollution changes and reduction goals, socioeconomic projections, water supply projections.

Ease of Use Relatively easy to use. Requires significant data for detailed analysis.
Training Required Moderate training/experience in resource modeling required for effective use.
Training Available On-line tutorial available at Contact SEI for details regarding available training.
Computer Requirements 200 MHz or faster Pentium class PC with Microsoft Windows 95 or later (a 400 MHz PC with Windows 98 or later is recommended). A minimum of 32 MB of RAM and 50 MB of free hard disk space is also required (64 MB of RAM recommended). In addition Microsoft Internet Explorer version 4.0 is required for viewing WEAP’s HTML Help. Monitor should be set to a minimum resolution of 800x600, but preferably even higher (e.g., 1024x768 or 1280x1024), to maximize the presentation of data and results.
Documentation WEAP21 User Guide; available online at pdf-icon

Has been used for projects in the Aral Sea; Beijing, China; Rio San Juan, Mexico; Rajasthan, India; South Africa; West Africa; California, Texas, and Southeast, USA; Central Asia; India; Nepal; Korea; and Cairo, Egypt. Has also been used in Uzbekistan to assess water resource capacity with regard to irrigation needs.

Contacts for Framework, Documentation, Technical Assistance

Jack Sieber

Senior Software Scientist, Stockholm Environment Institute (SEI), Boston; SEITellus Institute, 11 Arlington St., Boston, MA 02116-3411 USA; Tel: +1.617.266.5400; e-mail:; website:

Cost 2-year license fee ranges from US$1000-2500, depending on type of user. Free to developing countries. See for more information.

Arranz, R. and M. McCartney. 2007. Application of the Water Evaluation And Planning (WEAP) model to assess future water demands and resources in the Olifants catchment, South Africa. Colombo, Sri Lanka: International Water Management Institute. 103 pp. (IWMI Working Paper 116).

Hansen, E. 1994. WEAP — A system for tackling water resource problems. In Water Management Europe 1993/94: An Annual Review of the European Water and Wastewater Industry. Stockholm Environment Institute: Stockholm. U.S. Water News, Oct. 1992. Aral Sea is classic example of ecological suicide. No. V4, p. 12.

Huber-Lee, A., D. Yates, D. Purkey, W. Yu, and B. Runkle. 2003. Water, climate, food, and environment in the Sacramento Basin — contribution to ADAPT: Adaptation strategies to changing environment. Stockholm Environment Institute, Boston, MA, USA.

Raskin, P., E. Hansen, Z. Zhu, and D. Stavisky. 1992. Simulation of water supply and demand in the Aral Sea region. Water International 17(2):55-67.