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Introduction
There is an ever-increasing awareness that water resources exist in limited quantities, and available supply varying considerably during the course of a year. There is an urgent need to find ways of saving, reusing and recycling water and to develop methodologies to improve water resource management. One long-term strategic goal advocated by several authorities is developing a stable supply of water through out the year. As a result, several government agencies have attempted to engineer rainwater catchment devices that reduce the demand during high peak seasons. While several local agencies throughout the world have implemented such systems, there is wide-spread concern that such strategies need simultaneously to target areas with highest potential for conservation, reduce the ecological impacts associated to high stormwater flows, and keep costs minimal. We propose a methodology that utilizes a Geographic Information System (GIS) for prioritizing regions and assess the effectiveness of conservation strategies for the City of Seattle, Washington (USA).

Water Demand Analysis for Seattle

Background
The city of Seattle is located in the Northwest of the United States, and serves drinking water to a population of 1.3 million people (SPU). Of these, there are over 350 thousand single family households with total water consumption of 145 million gallons per day (MGD) and annual water budget of 53 billion gallons (200 billion liters). Seattle Public Utilities (SPU), the primary water provider in the region, provides direct retail water service to about 595,000 people in the City of Seattle, parts of Shoreline and small areas just south of the city limits. SPU also sells water wholesale to 26 neighboring cities and water districts serving another 686,000 people. The average consumption per capita for the city of Seattle is 300 gallons per day and current forecasts predict inadequate supply of drinking water by 2020.

Although Seattle does not have a comprehensive water supply plan, Seattle Public Utility has been exploring conservation strategies aimed at curbing water demand. City officials have proposed two strategies. One that involves installation of rainwater capture systems for outdoor use, and another that utilizes water reuse technology for indoor purposes. Currently, there is only enough publicly accessible data to determine the potential for applications of the rainwater capture system. The strategy proposed is capturing rainwater from rooftops during the winter months, store until summer months, and use for irrigation purposes. While this technology is by no means new, the capital investment to install rainwater catchment devices across the city is considerable. A thorough, yet targeted, analysis of is necessary.

This paper evaluates the potential water savings associated to the installation and operation of rain capture devices in specific parcels in the city. While the proposed method is applicable to one city, namely, Seattle, we foresee application to any city that has the appropriate data. The raw data are publicly available, provided by the City of Seattle and the National Oceanic and Atmospheric Administration (NOAA).

Methods and Preliminary Findings
We propose a parcel (or plot) based analysis of water use by households living in Single Family houses (N= 217,562)) in the city of Seattle for the year 1999. The analysis utilizes a GIS software (ARC View) and is structured around three major steps: First we identify all parcels (users) with high irrigation rates. Irrigation rates are calculated by assuming no irrigation during winter months (base consumption), and subtracting total summer use from winter use. The marginal difference is the additional water consumption attributable to outdoor use or, in this case, irrigation. Second we combine the building footprint area with historic rainfall data to calculate potential for water capture for each parcel in the city. Using land-use maps of both total parcel area and building footprint area, it is possible to record total built area across the city. In addition, rainfall data averaged over the last 48 years for winter months, allow for the calculation of total runoff volume from built surfaces. A third step involves determining total overflow reduction across priority Combined Sewage Overflow (CSO) basins in the city. The CSOs were prioritized based on frequency of flooding and discharge rates by SPU staff. Total runoff volumes from built areas were calculated per CSO basin to estimate reduction in stormwater flow rate.

The analysis predicts rain capture of 300 million gallons across the city for one year. This is roughly proportional to the amount of water required for irrigation purposes. The correlation between irrigation and runoff potential is promising for installations of rainwater capture tanks in specific locations across the city. Over 1000 parcels have high irrigation volumes and runoff potential for implementation of rain capture systems. Across the priority CSO basins there is also significant reductions in overflow volumes. The benefits from the reduction in overflow are innumerable, but the primary ones being reduction in flood frequency, and reduced erosion from high discharge volumes in stream systems. This approach allows for targeted applications of rainwater capture systems for reduced water demand during summer months, and decreased runoff volumes during winter months.

Conclusion and General Comments
Among the important lessons learned in developing this study, we found that particular attention must be given in using publicly available data. In fact, currently, there is no standardization for GIS data in US, and the quality and nature of data range widely. During the development of this analysis, we realized how important meta-data and data checking are to produce reliable information. Important is also to establish procedures for checking data against familiar areas, and crosschecking with water users, (i.e. Seattle Public Utilities, Parks and Recreation, etc.). We found establishing consistency of data and adequate preparation and knowledge of data, prior to commencing research as important (if not more) to this process.

While it is essential to continue developing new GIS and remote sensing techniques for assessing the condition of global resources, it is equally important to address application to resource management at the municipal level. In other words, we need to identify ways in which the “enhanced computing power” we write and speak of so commonly can help us in our everyday lives. This approach is a first attempt to bridge the gap between the worlds of spatial analysis and resource management. Such information have several advantages and can assist resource managers in a number of ways, including, improving the operation of existing water supply systems, postponing investment in water facilities, reducing energy consumption, reduced disposal into sewerage systems, and adequately address public concerns.