Water Mapping Explained
by Christina Hughes, PE, CFM, ENV SP
When designing any development, one of the major considerations is water. As engineers, we think about this in the context of nuisance water (drainage, floodplain storage, condensate from buildings, etc.) and necessary water supply (irrigation, fire suppression systems, drinking water, and other domestic uses). These two elements of water-based design can be linked to form a more sustainable and partially closed loop system of on-site water reuse. The first step in understanding the feasibility of water reuse for site development is water mapping.
Water mapping is the process by which all available sources of water that can reasonably be made available at the site and the water demands of the project to identify the potential linkages of sources and demands are mapped to provide an understanding of the systems. An example to show how this works could be a proposed 10-acre site that is planned for a three-story office building, a half-acre parking lot, a manicured lawn, and a wet-bottom amenity/stormwater detention pond. The example development has the following options for sources of water that are available at the site:
- Purchased potable water from a municipal supply
- Direct rainwater runoff from an office building roof
- Site stormwater runoff
- Condensate and blowdown from the building air condition system
Other than capital and Operations and Maintenance (O&M) costs to maintain onsite water systems, the municipal supply is the only source of water that constitutes an ongoing cost. Collection and reuse of direct rainwater and stormwater runoff on-site can eliminate the cost of potable water that is typically purchased for non-potable uses. Potential non-potable water demands for this site include:
- Cooling tower makeup water
- Make-up water for evaporation and seepage losses in the amenity pond
- Flush water for toilets 1
- Fire suppression
A water map of the site including these non-potable sources and demands is shown below.
This map includes potable water demands and wastewater outflow to illustrate how only a portion of the water usage for this site is a closed loop. If on-site water treatment is provided to close the loop, there are potential additional sources for on-site water reuse including greywater (dishwashing, shower, faucets) and blackwater (sanitary waste), although costs and regulatory requirements typically increase significantly beyond non-potable reuse. This water mapping exercise illustrates how an emergency backup system, which must be considered to offset drought deficit and to address downtime for maintenance, can be integrated into the reuse collection, storage, and/or distribution system. Once the final layout of the system is set, implementation becomes a multidisciplinary effort involving the Civil engineer, landscape architect, MEP engineer, and other specialty consultants to address system-specific elements such as water treatment and permitting.
In addition to helping the design team visualize how water reuse and treatment system would work at the site scale during the planning phase, water mapping may also be an essential part of the final educational and public/user outreach component of a sustainable design project. Water mapping helps site users understand the purpose of the design and promote water conservation and reuse.
As water becomes a more limited resource and as the costs for municipal water treatment and distribution grows, water mapping during conceptual design can help identify the benefits of water reuse at the site scale.
1 Depending upon the local jurisdiction, additional treatment may be required for non-potable water reuse in these systems.