Results
Using standard statistical packages (i.e. SPSS and R) we diagnose the correlation between summer and winter consumption patterns to test the validity of our seasonal variation assumption. For the city of Seattle, there is a strong inverse relationship between season and consumption. In fact, during the summer months (June through October) water consumption is, on average, 30% greater. Comparisons with multiple years indicate average summer water consumption between 25-45% higher than winter months. While the relationship between summer use and nonbuilt area were not strong across the entire city, there were statistically significant correlations at the individual priority basin scale (adjusted R-Squared= 0.61, P-value < 0.001). As evidenced by data about summer and winter uses, we are assured that seasonal variation plays an important role in Seattle's water consumption. In addition, we are certain that the increase in summer consumption is attributable to non-potable uses within the priority basins.
Built versus Nonbuilt Areas
In determining the amount of water required for irrigating nonbuilt areas in each basin we rely on historic meteorological data. We use the world's largest meteorological database from NOAA, and estimate the average rainfall for winter months from 1945 to 2002 to be 15.63 inches (39.7 cm). This average amount of rain if captured is 9.743 gallons per square foot. Given these figures we estimate the approximate amount of built area required to irrigate specific areas of land within each priority basin. The bivariate comparison indicates that, in general, the largest number of parcels fall between built area of 1500 to 2000 square feet and nonbuilt area of 4000 to 5000 square feet. We perform this analysis on each priority basin, and develop a coding scheme to reflect the rainwater harvest potential. Those parcels with the highest percentage of comparable areas are identified as having the greatest rainwater harvesting potential, while those with fewer comparable areas are assigned to 'medium', and those with the least into a 'low' category. As a result, all parcels within the priority basins will fall into a high, medium or low category. The indication of high, medium and low refers to both the conservation potential and a strategy to minimize the costs associated with the installation of rainwater harvesting system.
Table 1: Total parcels and percentage in priority basin 14 falling within specific built area and non-built area categories. The shaded cells represent those parcels with high and medium conservation potential. All non-shaded cells represent those parcels with low conservation potential.
| |
|
|
Built Area (Sq. ft.) |
Total |
| |
|
|
0-1000 |
1001-1500 |
1501-2000 |
2001-2500 |
2501-3000 |
3001 or more |
| |
0-3000 |
33 |
1 |
13 |
7 |
7 |
3 |
2 |
33 |
| |
|
16% |
0.48% |
6.25% |
3.37% |
3.37% |
1.44% |
0.96% |
| |
3001-4000 |
56 |
2 |
18 |
22 |
12 |
1 |
1 |
56 |
| |
|
27% |
0.96% |
8.65% |
10.58% |
5.77% |
0.48% |
0.48% |
| Non-Built |
4001-8000 |
80 |
2 |
13 |
34 |
16 |
4 |
11 |
80 |
| Area |
|
38% |
0.96% |
6.25% |
16.35% |
7.69% |
1.92% |
5.29% |
| (Sq. ft) |
8001-10000 |
11 |
0 |
0 |
2 |
4 |
1 |
4 |
11 |
| |
|
5% |
0.00% |
0.00% |
0.96% |
1.92% |
0.48% |
1.92% |
| |
10001 or more |
28 |
3 |
0 |
3 |
3 |
5 |
14 |
28 |
| |
|
13% |
1.44% |
0.00% |
1.44% |
1.44% |
2.40% |
6.73% |
| |
Total |
208 |
8 |
44 |
68 |
42 |
14 |
32 |
208 |
| |
|
100% |
3.85% |
21.15% |
32.69% |
20.19% |
6.73% |
15.38% |
Table 1 illustrates the bivariate comparison between built and nonbuilt areas for priority basin number 14. In this example over 16.35 percent of parcels have
comparable built areas with non-built areas. That is, of the 208 parcels in this basin, 34 have a high potential for installation of rainwater capture systems, 56 (18+22+16) have a medium potential, and the remaining 118 have low potential. We use this procedure to select high, medium and low potential candidates across all the parcels within each priority basins.
The GIS is essential to investigate further the suitability of parcel selection. Across the four priority basins we examine here, 489 parcels are in the high category, 543 to the medium category, and 1807 to the low. Figure 2 illustrates all subcategories in a color map format. The yellow parcels indicate those areas with a high number of comparable built to nonbuilt areas, green indicates those areas with medium, navy blue with low, and gray are those parcels not in the four priority basins. These results indicate that only 5.8 percent (489) of the parcels within the four priority basins are likely to be considered for installation of the pilot rainwater harvesting system. The map also indicates geographic clustering of potentially high conservation areas. The next section describes the implications of these findings to urban drainage in specific areas of Seattle.