Water Evaluation And Planning (WEAP)
- a tool for showcasing the effect of NbS

WEAP as an Integrated Water Resource Management (IWRM) tool for showcasing the impact of NbS for water resource management

Improving the quality of freshwater environments is of high priority considering their importance for biodiversity management, human needs, socio-economic development, and ecosystem services. However, with rapid global changes (population growth, urbanization, land-use land-cover change, and climate change) this finite resource is under tremendous pressure in terms of both quality and quantity. Scientific tools can be used to forecast the future state of freshwater environments under different scenarios. These tools are useful in that their outputs provide science-based evidence for decision-making and local stakeholders are involved in the process. This can improve the modeling results (e.g., by better accounting for the local context) as well as build the capacities of the stakeholders involved in water management. Many models are growing in popularity, especially those which can incorporate different mitigation measures for improving water management plans, whether it’s hard infrastructure like wastewater treatment plants (WWTPs) or an NbS like check dams, wetlands, and reservoirs. WEAP is one such model.

WEAP is a computer numerical simulation tool that is capable of estimating both hydrological components and water quality components in an integrated and transdisciplinary manner (Figure 1). It has the following key characteristics that make it one of the most useful tools for water resource management. It can estimate the effectiveness of different adaptation and mitigation measures planned for water resource management. More specifically, it can serve several purposes for designing an NbS for water resource management. For example, it can be used to compare the effect of creating a less concretized watershed vs a check dam on water resource management at specific locations and times. WEAP is a decision support system tool that has been extensively used by both scientific communities and policy makers over the last few decades for the implementation of integrated water resource management plans. Along with its ability to build various plausible scenarios, the WEAP model can evaluate different environmental master planning functions to assess mitigation measures that are planned to improve water quality in the future. The WEAP model also includes a catchment module for rainfall-runoff simulation, stopping the need to find another hydrologic model for streamflow simulation, which is an important input parameter for water quality modeling. WEAP can provide a GIS-based interface to import-export spatial data and images, and incorporate information about different facilities, such as wastewater generation and treatment systems, at various spatial scales. WEAP can simulate several water quality variables, both conservative and non-conservative in nature, in addition to pollution generation and removal. Through building different scenarios, the WEAP model can provide sound scientific evidence to answer different “What-if” questions. This scientific evidence supports the formulation of different policy alternatives, hence ultimately results in robust water resource management planning. Because of its transdisciplinary nature, the WEAP model can support scenario building that incorporates various key drivers and pressures affecting water quality and quantity, including socio-economic dimensions, hydropower generation, population growth, climate change, industrial and commercial activities, land use/land cover, and different mitigation measures that are already in place for the improvement of water quality.

Another important point about WEAP is that it is not data intensive, which is a blessing for developing countries where data are scarce. Also, WEAP is freely available for developing nations, where the problem of water scarcity is sometimes severe.

The Santa Rosa watershed in the Philippines faces serious issues regarding water quality deterioration. WEAP was used to project how water quality in this watershed will change, between 2015 and 2030, under different plausible scenarios (Figure 2).

The results clearly indicated that water quality will further deteriorate from the current situation (2015) (blue bars) to the target year (2030) if no mitigation measures are taken (red bars). However, water quality would be improved if hard infrastructure like WWTPs were built and used (green bars). These results helped policy makers to make appropriate decisions in a timely manner. Although only hard infrastructures were considered here, because of data availability, WEAP is also robust enough to assess the efficiency of different NbS (wetland, retention ponds, reservoirs, check dams) on water quality improvement. It can also be successfully utilized to estimate the size and volume of NbS required, as well as expenditure, for water resource management at any particular location.

In summary, WEAP is one IWRM tool which can be used to assess the efficiency of different NbS for designing water management plans and has the ability to fill the gap between the science-policy interface.