Introduction to WEAP

The Water Evaluation and Planning (WEAP) model is a software tool for Integrated Water Resources Management (IWRM) developed by the Stockholm Environmental Institute (SEI). WEAP has a long history of development and use in the water planning area (Yates et al., 2005). Over the years, several organizations (e.g. the World Bank, EU Global Water Initiative and U.S. Army Corps of Engineers) have supported and funded the development of WEAP. WEAP integrates physical hydrologic processes with the management of demands and infrastructure, as well as environmental and economic aspects of water planning. Simulations in WEAP are constructed as scenarios. Scenarios can be constructed and analyzed based on different trends in hydrology, water use and demands, demography, technology, operation rules and water management policies. WEAP is developed with the purpose of being a flexible and transparent tool for aiding IWRM, and is not a tool for modeling detailed water operations, such as optimization of hydropower production.

WEAP features an intuitive graphic interface that provides a user-friendly working environment and a straightforward understanding of the system studied. Figure 3.x is a screenshot of the WEAP interface. The main menu consists of seven sub-menus: an area, an edit, a view, a schematic, a general, an advanced and a help menu. There are five basic views in WEAP that can be chosen from the view bar seen on the left of the figure. Each view determines the layout of the rest of the screen:

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1. Schematic view: This is the starting point for all activities in WEAP. It is a

spatial layout of the area studied. The view features an easy “drag and drop” graphical interface for physical visualization and configuration of the system studied. Objects such as catchments, reservoirs, demand nodes, aquifers etc., can be created and edited in this view. Geographic Information System (GIS) files can be added as background layers to provide clarity. Any information or result linked any object is easily accessed by right clicking on the object.

2. Data view: This is where the system is defined and built. All assumptions, data

and data structures, variables and relations, and documentation for “current accounts” and each scenario are entered here. Data view includes a hierarchical “tree” that organize the data structures under six categories: key assumptions, hydrology, demand sites, supply and resources, environment and other assumptions.

3. Results view: This view is the reporting tool of WEAP where the simulation

results are reviewed. The results can be viewed as tables, charts or on the schematic, either in monthly or yearly values for any period of time within the time horizon. Reports of the results are available either as graphs, tables or maps and can be saved as text, graphic or spreadsheets. Each report can be customized as preferred and favorites saved for later retrieval. In addition, the result view is an important tool for validating all assumptions, data and models, and make sure they are consistent.

4. Scenario explorer view: This view groups together the tables and charts

created in the results view into “overviews”. The overviews allows for simultaneous comparison of important aspects of the system. Effects of various assumptions across different scenarios can be studied by selecting what data to be displayed and which scenarios to compare. It is possible to change the inputs at the spot and WEAP will automatically recalculate to results.

5. The notes view: This is a simple notepad where assumptions, documentation

and references are entered for each object in the tree. Note may include formatting or windows objects, e.g. Word files or Excel spreadsheets. Note are good way of documenting the scenarios as you model.

One of the strengths with WEAP is the flexibility. It is possible to run WEAP at many different temporal resolutions, ranging from daily to annual time steps with a time horizon from just one day to more than 100 years. WEAP lets the user choose from many different methods for defining and calculating water demands related to irrigated agriculture, industry and . The structure of demand data can be adapted based on the availability of data and the type analysis to be conducted. All supply and resource calculations are driven by a linear program allocation algorithm that determines the amounts of water delivered to each demand site, based on their priority defined by the user. The priority is given as number between 1 and 99, where 1 represents the highest priority and 99 the lowest. WEAP includes four methods for simulating hydrologic processes. These are the Irrigation Demands Only Method, Rainfall Runoff Method, Soil Moisture Method and the MABIA Method. It is also possible to link WEAP to other models such as MODFLOW, QUAL2K, LEAP:

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 MODFLOW is a three-dimensional finite difference groundwater model developed by the USGS. Linking to MODFLOW is an alternative if the built- in WEAP groundwater model is not sufficiently complex. Data and results is transferred back and forth between WEAP and MODFLOW when they are properly linked.

 QUAL2K is a one-dimensional water quality model that can model chemical and biological constituents such as nitrate, pH, ammonia, algae, etc. The model can also calculate water temperatures based on the WEAP climate data. WEAP has a built-in water quality model but the QUAL2K is far more detailed and can model more types of constituents if necessary.

 The Long-range Energy Alternatives Planning System (LEAP) is a tool for analysis of energy policy and climate change mitigation. It is developed by SEI and can be linked to WEAP to track energy production, consumption, resource extraction and greenhouse gas emissions (GHG) from both energy and non- energy sectors.

In general, applications of WEAP can be divided into three main areas: As a database, WEAP provides a system for maintenance of water demand and supply information. As a forecasting tool, WEAP simulates water balance and allocation, or as a policy analysis tool that evaluates alternative policy and management options, and accounts for multiple and competing water users (Sieber and Purkey, 2011).

A WEAP analysis consists several of steps. The first step is to set up the time horizon, geographical boundaries, system components and configuration of the problem. Further is the baseline conditions of the area studied established in the “Current Accounts”. Current Accounts represent the present situation, and include the actual climatic conditions, existing water users, and their demands, supply requirements and pollution loads. Factors that affect demands, policies and costs may be built into the Current Accounts as key assumptions. The model is calibrated based on the Current Accounts before the scenarios are created and simulated. Scenarios build on the Current Accounts, and represent alternative sets of assumptions about the future development of the area studied. These assumptions can be related to a wide range of factors, such as change in climatic and hydrologic conditions, changing water management policies and operations, improved irrigation technology, etc. At last, the scenarios are explored and evaluated. The scenarios can be compared with each other or relative to the reference scenario.