WEAP (the Water Evaluation and Planning system) is a model-building tool for water resource planning and policy analysis that is distributed at no charge to non-profit, academic, and governmental organizations in developing countries.
WEAP can be used to create simulations of water demand, supply, runoff, evapotranspiration, water allocation, infiltration, crop irrigation requirements, instream flow requirements, ecosystem services, groundwater and surface storage, reservoir operations, pollution generation, treatment, discharge, and instream water quality. The simulations can be created under scenarios of varying policy, hydrology, climate, land use, technology, and socio-economic factors.  WEAP links to the USGS MODFLOW groundwater flow model and the US EPA QUAL2K surface water quality model.
WEAP was created in 1988 and continues to be developed and supported by the U.S. Center of the Stockholm Environment Institute, a non-profit research institute based at Tufts University in Somerville, Massachusetts. It is used for climate change vulnerability studies and adaptation planning and has been applied by researchers and planners in thousands of organizations worldwide.
Establishing the ‘current accounts’ and building scenarios and evaluating the scenarios about criteria are the main WEAP applications in Simulation problems.