Water Evaluation and Planning (WEAP) Tool

Submitted by Rosie Witton | published 7th Mar 2023 | last updated 10th May 2023

SEI Urban Toolbox for Liveable Cities Annotation

This tool is part of the SEI Urban Toolbox for Liveable Cities which has been developed by the SEI Initiative on City Health and Wellbeing. The Urban Toolbox is a collection of tools, developed within SEI or in coordination with SEI, aimed at supporting planning and decision-making for improving the health, well-being and resilience of city residents and urban systems more broadly.

WEAP Tutorial 01- Installation
WEAP letters with water, fields, a person collecting water, and birds on a lake, within the letters

About this tool

Freshwater management challenges are increasingly common. Allocation of limited water resources between agricultural, municipal and environmental uses now 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 is developed by the Stockholm Environment Institute's U.S. Center.

How does this tool work?

The Water Evaluation and Planning (WEAP) model is an integrated Decision Support System (DSS) designed to support water planning that balances water supplies and multiple water demands and environmental requirements characterized by spatially and temporally variable allocation priorities and supply preferences. WEAP employs a transparent set of model objects and procedures that can be used to analyze a full range of issues and uncertainties faced by water planners, including those related to climate, watershed condition, anticipated demand, ecosystem needs, regulatory climate, operational objectives and infrastructure. The model’s graphical user interface supports the construction of a watershed’s network representation and the water system contained within it, and facilitates multi-stakeholder water management dialogues organized around scenario development and evaluation. WEAP employs a priority based optimization algorithm, as an alternative to hierarchical rule-based logic that uses a concept of Equity Groups to allocate water in times of insufficient supply

Who might use this tool?

WEAP provides a comprehensive, flexible and user-friendly framework for planning and policy analysis. A growing number of water professionals are finding WEAP to be a useful addition to their toolbox of models, databases, spreadsheets and other software.

Which stakeholders are involved?

Planning and policy water professionals.

What stage of the process does this tool support?

As an integrated software tool, WEAP can be used at different stages of an assessment including:

  • defining the assessment
  • generating ideas
  • developing solutions
  • implementation
  • monitoring
  • evaluation

Tool overview

Over the last decade, an integrated approach to water development has emerged which places water supply projects in the context of demand-side management, and water quality and ecosystem preservation and protection. WEAP incorporates these values into a practical tool for water resources planning and policy analysis. WEAP places demand-side issues such as water use patterns, equipment efficiencies, re-use strategies, costs, and water allocation schemes on an equal footing with supply-side topics such as stream flow, groundwater resources, reservoirs, and water transfers. WEAP is also distinguished by its integrated approach to simulating both the natural (e.g., evapotranspirative demands, runoff, baseflow) and engineered components (e.g., reservoirs, groundwater pumping) of water systems. This allows the planner access to a more comprehensive view of the broad range of factors that must be considered in managing water resources for present and future use. The result is an effective tool for examining alternative water development and management options.

WEAP operates in many capacities:

  • Water balance database: WEAP provides a system for maintaining water demand and supply information.
  • Scenario generation tool: WEAP simulates water demand, supply, runoff, stream flows, storage, pollution generation, treatment and discharge and instream water quality.
  • Policy analysis tool: WEAP evaluates a full range of water development and management options, and takes account of multiple and competing uses of water systems.

The WEAP Approach

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 analyst represents the system in terms of its various supply sources (e.g., rivers, creeks, groundwater, reservoirs, and desalination plants); withdrawal, transmission and wastewater treatment facilities; water demands; pollution generation; and ecosystem requirements. The data structure and level of detail can be easily customized to meet the requirements and data availability for a particular system and analysis.

WEAP applications generally include several steps.

  • Study definition: The time frame, spatial boundaries, system components, and configuration of the problem are established.
  • Current accounts: A snapshot of actual water demand, pollution loads, resources and supplies for the system are developed. This can be viewed as a calibration step in the development of an application.
  • Scenarios: A set of alternative assumptions about future impacts of policies, costs, and climate, for example, on water demand, supply, hydrology, and pollution can be explored. (Possible scenario opportunities are presented in the next section.)
  • Evaluation: The scenarios are evaluated with regard to water sufficiency, costs and benefits, compatibility with environmental targets, and sensitivity to uncertainty in key variables.

Application: WEAP case study examples

WEAP has been applied in water assessments in dozens of countries, including : the United States, Mexico, Brazil, Germany, Ghana, Burkina Faso, Kenya, South Africa, Mozambique, Egypt, Israel, Oman, Central Asia, India, Sri Lanka, Nepal, China, South Korea, and Thailand.

WEAP can be used at any scale from household to city to watershed and larger. In most of WEAP's work, the urban sector is integrated as part of the larger scale and some case studies are linked below to highlight this:

Using WEAP to create integrated water resource management tools: this case study uses the Water Evaluation and Planning (WEAP) Tool to create integrated water resource management tools for three towns in the Lake Victoria region.

Using WEAP as a decision support system for sustainable water supply planning: this case study uses the Water Evaluation and Planning (WEAP) Tool as a decision support system for sustainable water supply planning in the United States.

Capabilities and resources required

The capabilities and resources required depends on how WEAP will be used and applied. The table below provides a few examples of how WEAP can be applied and the specific input date required. Further information can be found in the WEAP tutorials

Application Input Data Needs
Hydrology only Climate, stream flows, land-use, soil properties
Urban demand and supply Population, sectoral demand disaggregation, infrastructure, urban water plans, hydrology related inputs, supply infrastructure
Reservoirs and hydropower Reservoir physical and operating characteristics, rule curves
Water quality Stream flows, water quality data, land use
Climate change Climate change projections
Agriculture Cropping patterns, crop coefficients, irrigation infrastructure, irrigation scheduling
Groundwater Aquifer properties, groundwater levels

Implementation tips: key enablers and potential barriers


WEAP is available in English, Chinese, Farsi, and Spanish.


Online interactive and self-guided courses are available online.

The online, self-guided courses are free and are hosted in the Canvas education platform.  Separate courses are available for English and Spanish.  The courses guide the user through the basic and intermediate features of WEAP.  Coursework includes exercises from the WEAP Tutorial, accompanying WEAP YouTube channel videos, quizzes and homework assignments.  No previous experience is necessary.

Technical Support

Licensed users are entitled to free upgrades and limited technical support during the two year license term.

Potential Barriers

Limited access to the required hardware and software, as well as the cost of accessing WEAP may be potential barriers for some users. However, Non-profit, governmental or academic organization based in a developing country can apply for a free license online.

Potential integration with other tools

This tool could be integrated or used along side other tools to conduct scenario based generations, policy analysis, and create water balance databases.