Computer modeling in the field of water resources has a long history. Many sophisticated models have faltered by being mathematically obscure and overly ambitious in attempting to "optimize" solutions to real-life problems. Experience shows that the best approach is to build a straightforward and flexible tool to assist, but not substitute for, the user of the model. WEAP represents a new generation of water planning software that utilizes the powerful capability of today's personal computers to give water professionals everywhere access to appropriate tools.
The design of WEAP is guided by a number of methodological considerations: an integrated and comprehensive planning framework; use of scenario analyses in understanding the effects of different development choices; Demand-management capability; Environmental assessment capability; and Ease-of-use. These are discussed in turn below.
WEAP places the evaluation of specific water problems in a comprehensive framework. The integration is over several dimensions: between demand and supply, between water quantity and quality, and between economic development objectives and environmental constraints.
With WEAP, you first create a Current Accounts of the water system under study. Then, based on a variety of economic, demographic, hydrological, and technological trends, a "reference" or "business-as-usual" scenario projection is established, referred to as a Reference Scenario. You can then develop one or more policy scenarios with alternative assumptions about future developments.
The scenarios can address a broad range of "what if" questions, such as: What if population growth and economic development patterns change? What if reservoir operating rules are altered? What if groundwater is more fully exploited? What if water conservation is introduced? What if ecosystem requirements are tightened? What if new sources of water pollution are added? What if a water-recycling program is implemented? What if a more efficient irrigation technique is implemented? What if the mix of agricultural crops changes? What if climate change alters the hydrology? These scenarios may be viewed simultaneously in the results for easy comparison of their effects on the water system.
WEAP is unique in its capability of representing the effects of demand management on water systems. Water requirements may be derived from a detailed set of final uses, or "water services" in different economic sectors. For example, the agricultural sector could be broken down by crop types, irrigation districts and irrigation techniques. An urban sector could be organized by county, city, and water district. Industrial demand can be broken down by industrial subsector and further into process water and cooling water. This approach places development objectives--providing end-use goods and services--at the foundation of water analysis, and allows an evaluation of effects of improved technologies on these uses, as well as effects of changing prices on quantities of water demanded. In addition, priorities for allocating water for particular demands or from particular sources may be specified by the user.
WEAP scenario analyses can take into account the requirements for aquatic ecosystems. They also can provide a summary of the pollution pressure different water uses impose on the overall system. Pollution is tracked from generation through treatment and outflow into surface and underground bodies of water. Concentrations of water quality constituents are modeled in rivers.
An intuitive graphical interface provides a simple yet powerful means for constructing, viewing and modifying the system and its data. The main functions--loading data, calculating and reviewing results--are handled through an interactive screen structure that prompts the user, catches errors and provides on-screen guidance. The expandable and adaptable data structures of WEAP accommodate the evolving needs of water analysts as better information becomes available and planning issues change. In addition, WEAP allows users to develop their own set of variables and equations to further refine and/or adapt the analysis to local constraints and conditions.
One of the strengths of WEAP is that it is adaptable to whatever data is available to describe a water resources system. That is, it can use daily, weekly, monthly, or annual time-steps to characterize the system's water supplies and demands. This flexibility means that it can be applied across a range of spatial and temporal scales. Indeed, WEAP has been used throughout the world to analyze a diverse set of water management issues for small communities and large managed watersheds alike.
Historically, WEAP has been used primarily to assess the reliability of water deliveries and the sustainability of surface water and groundwater supplies under future development scenarios. This type of application of WEAP has focused on the water supply implications of proposed management and/or infrastructural changes, but has overlooked the impacts of these changes on the management of storm water and wastewater. Recent advancement of the model, however, has allowed for the holistic, comprehensive consideration of each of these facets of managing local water resources. The updated model can now be used to address questions surrounding the integration of storm water, waste water, and water supply. These include:
How will water supply and wastewater treatment facilities be affected by the retention and/or diversion of storm waters?
How will improvements in water collection systems affect water supply and wastewater treatment?
How will modifications of combined sewer overflow systems affect wastewater treatment?
The enhanced WEAP model includes updated features that allow the user to include the following:
Infiltration and Inflow from groundwater to sewage collection systems. These inflows can stress rivers and streams by removing clean water from watersheds and place additional burden on wastewater treatment by taking up valuable plant capacity and limiting future sewer connections.
Infiltration Basins & Retention Ponds as management practices. These can be used to offset the impacts of urbanization, where water demands increase and potentially threaten water supplies as more rainfall runs off of expanding impervious surfaces, rather than recharging local aquifers. They can also serve to attenuate non-point source pollution.
Display of User-Defined Performance Measures as Results. This will allow for the output of site-specific performance measures and criteria, which are commonly guided by the objectives of individual studies and systems configuration and local conditions.
Tiered Water Pricing policies as a means of promoting demand management.
Combined Sewer Overflows (CSOs) that pose potential risks to public health and aquatic life, because they discharge chemicals and disease-causing pathogens directly into waterways.