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Study Area
The city of Seattle is located in the Northwest of the United States and is a rain-fed landscape. Average annual rainfall is 39 inches (99.06 cm) with dry summers. The City serves drinking water to a population of 1.3 million people, of which there are 350 thousand single-family households. Total water consumption of the single-family households is 145 million gallons per day (MGD) and annual water budget of 53 billion gallons (200 billion liters). SPU provides direct retail water service to about 595,000 people in the City of Seattle and small areas adjacent to 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 for the past 10 years is 114 gallons per day. Total water provided from SPU averages around 150 million gallons per day (MGD). The Cedar River provides about 70 percent of this supply, the Tolt River provides about 29 percent, and the remaining 1 percent comes from the Highline Wellfield.

Spurred in part by a yearlong drought Seattle reduced water demand by 39 percent from 1988 to 2002. SPU brought about these reductions through a combination of leak detection programs, domestic conservation programs featuring public education and retrofit low-flow fixtures, conservation services provided to industrial and commercial users, and rate increases for water supply and wastewater treatment. Seattle was also among the first major water utilities to treat conservation and demand management as a potential water resource, rather than simply as a management tool. This conservation program, calling upon the strong environmental ethic of the Pacific Northwest, enabled the SPU system to operate through the 1992 drought and to help reduce total water consumption levels despite population increases in the service area. Conservation continues to be the primary area for reductions in water demand.

Methods
We propose a parcel (or plot) based analysis of water use for Single Family Residents (SFR, N= 217,562) in the City of Seattle for the year 1999. Parcels are appropriate units for analysis because most land-use planning in the United States is done at this level, and the aim of this project entails installation of harvesting systems at a parcel scale. In addition, rather than calculating water consumption patterns for the whole city, we focus on 24 distinct 'priority' basins. The City defines priority basins according to information on the frequency and volume of overflows, proximity to public beaches, and other social and ecological criteria. Of the 24 priority basins we isolate four to test the applicability of this methodology, and report the findings here. The four priority basins are located next to each other, have ample area for assessment of water consumption patterns, and provide city officials with targeted information for application of the pilot project.

The analysis utilizes ARC View 3.2 GIS software and is structured around four major steps. First we recognize that seasonal variation in water consumption plays a very strong role in the Pacific Northwest with summer consumption orders of magnitude greater than winter-this is often one of the characteristics of a rain-fed landscape. Accordingly, we use Seattle land-use and seasonal water consumption data to identify all parcels (users) with high 'irrigation' rates. Irrigation rates are calculated by assuming no irrigation during winter months (base consumption), and subtracting total winter use from summer use. The marginal difference is the additional water consumption attributable to outdoor use or, in this case, non-potable use. We test the strength of this assumption prior to proceeding to the next step.

In the second step we perform a bivariate categorical comparison between the built area and nonbuilt area of each priority basin. The built area refers to the dwelling unit on the parcel, and the nonbuilt area encompasses the remaining non-dwelling area (i.e. property landscape). The built area and nonbuilt area are broken in sequences of square foot increments based on the capacity of the built area to capture and, thereby, irrigate nonbuilt area. In developing the increments we assume equal rainfall across all basins, complete rainfall capture on built area, requirement of irrigation to all of the nonbuilt property landscape, and equal irrigation rates. We also examine the correlation between summer use and nonbuilt area to test our assumption that the additional water use in summer is for irrigation. The second step is essential in identifying parcels within the priority basins that have adequate built area for capturing rainwater and, ultimately, for irrigation of nonbuilt areas. We develop a matrix based on the relationship between built and nonbuilt areas, and rank all parcels across the priority basins from highest to least conservation potential.

Third, we identify all parcels with the highest percentage of comparable built versus nonbuilt areas. We classify parcels based on specified built versus nonbuilt increments to allow city officials to assess highest conservation potential. In other words, when selecting which parcels to target within the basin for installation of rainwater harvesting systems, basins with the highest number of comparable parcel ratios are chosen. The City expects to target these dwellings for the pilot project. In the fourth and final step, we use only those parcels with the highest conservation potential with building area and historic rainfall data to calculate potential for water capture for the four priority basins. By averaging rainfall data over the last 57 (1945-2002) years during winter months we calculate total runoff volumes from all SFR buildings in the four selected priority basins. The rainwater capture from those parcels having highest conservation potential enables us to estimate the amount of water removed from the drainage system.

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