Supporting Information for Rainwater Catchment Rebate: When creating a cost-benefit analysis this can be very difficult because numerous parameters factor into water savings such as: demand, tank size, roof size, and the amount of precipitation. Few cost-benefit analyses have been published to date. The EPA suggests taking into consideration: capital cost, maintenance cost, water conservation benefit, storm water management benefit, public outreach and sustainability benefit, energy use and environmental benefit when conducting a cost-benefit analysis. It s understood that the return of investment for a rainwater catchment system can take some time, especially for smaller systems. The benefit of this program is public outreach and educating people that water can be reused. EPA states rain harvesting systems are recognized as a Low Impact Development technique for storm water management in reducing the runoff volumes of pollutant loads entering the storm water collection system. Many state and local governments encourage the use of rainwater harvesting as a storm water Best Management Practice. Rainwater harvesting decreases impacts of urban surface runoff and drainage systems during storm events Rainwater harvesting also decreases dependability on potable water resources Soquel Water District states, larger rain-catchment systems to the tune of thousands of gallons of storage when used in conjunction with water-wise landscaping can significantly reduce or even eliminate a household s need to use potable water outdoors during the dry summer months. Some of our customers with large tanks (3000 gallons or more) and water-wise landscaping (and perhaps a small veggie bed) can use the rainwater in their tank for irrigation all summer if their tank fills in the winter, which usually takes several winter storms. We have a groundwater overdraft problem and our Board is willing to go beyond cost-effectiveness to encourage conservation where we can. Potential estimate of how much rain a home can catch (From Saving Water Partnership Seattle and Participating Utilities : o Rain caught (in gallons) = (inches of rain) x 0.6* x (portion of building footprint in square feet) * 1 inch of falling rain per square foot = 0.6 gallons of water o Example: 1000 Square foot Roof with an average annual precipitation of 16 inches could collect 9,600 gallons of rainwater per year to irrigate their landscape and/or use for other non-potable sources (60% of 16,000 gallons = 9,600 gallons). If we had an estimated thirty 1,000 square foot homes install rainwater catchment systems they could potentially save 288,000 gallons per year or 0.88 acre feet per year.
The table below shows the catchment area needed to maximize barrel use throughout the year. It also shows the maximum size an irrigated area should be, in square feet, in order to off-set 50% of the annual landscape water demand with rainwater. Size of Barrel (Gallons) Annual Average Precipitation in a Normal Year for Marina is 16 inches Square Feet Needed to Maximize Barrel Use Irrigated Area Should not Exceed the Square Feet Listed Below in Order to Provide an Estimated 50% Annual Landscape Water Demand 50 350.00 130 100 350.00 150 150 400.00 175 200 450.00 225 250 500.00 250 300 550.00 300 350 850.00 400 400 850.00 425 450 850.00 450 500 850.00 500 550 850.00 500 600 850.00 500 650 900.00 525 700 950.00 600 750 950.00 600 800 1000.00 650 850 1050.00 650 900 1100.00 700 950 1150.00 700 1000 1200.00 750 1250 1450.00 900 1500 1650.00 1050 1750 1800.00 1175 2000 2000.00 1325 2250 2100.00 1425 2500 2300.00 1575 We expect the majority of our customers to install smaller systems, between fifty to two hundred gallons. It is understood that the water-savings from customers using the popular 50-100 gallon rain barrels is not significant, particularly in California s climate where we get all of our rain at once in the winter and then it s dry for six months. The small rain barrels fill quickly, often within minutes, and 50-100 gallons of rainwater doesn t last long as a source of irrigation water in the summer. That said, the benefit of incentivizing people to install the smaller barrels is that it meets the people s interest in offsetting potable water use where nonpotable water can be used. It also serves as an educational opportunity allowing people to engage hands on in conservation. Smaller barrels can be a baby step or gateway into other water conservation actions that can show greater results.
For those customers that are more avid landscapers (assuming 1,000 square feet of landscape), that install a larger system (500+ gallons), and have an assumed catchment area of 500 square feet, the total annual landscape water use could potentially be offset from 31% to 50%. The storage capacity is key to making a larger impact. The table and chart below illustrate that a house with a catchment area of 500 square feet, a 500 gallon rain barrel, and a landscape that is 1,000 square feet would offset 31% of the total annual landscape water use for: part of, October, all of, December,, February,, April, part of, and June. Sixty nine percent of the total annual landscape water use would have to come from supplemental, potable water, during, June,, August,, and October. Input Values Catchment area (ft 2 ): 500 Collection efficiency (%): 95 Initial tank volume (gal): 500 Tank size (gal): 500 Plant water use coeff: 0.5 Irrigated area (ft 2 ): 1000 Monthly indoor demand (gal): 0 Avg. monthly rainfall (in) Avg. PET (in) AC Condensate (gal) : 3.1 1.44 0 February: 3.1 1.71 0 : 2.7 2.96 0 April: 1 4.19 0 : 0.5 4.63 0 June: 0.1 4.81 0 : 0 4.03 0 August: 0 3.81 0 : 0.2 2.98 0 October: 0.6 2.63 0 : 1.8 1.62 0 December: 2.4 1.39 0 Total: 15.5 36.2 0 Yearly Percent Average Rainfall (%) Year 1: 80% Year 2: 80% Year 3: 80% Month Supply Demand Tank Volume Supplemental Water 734 449 500 0 February 734 533 500 0 639 922 217 0 April 237 1305 0 851 118 1442 0 1324 0 1255 0 1255 47 928 0 881 426 505 0 78 Year 2 December 568 433 135 0 734 449 421 0 February 734 533 500 0 639 922 217 0 April 237 1305 0 851 118 1442 0 1324 0 1255 0 1255 47 928 0 881 426 505 0 78 Year 3 December 568 433 135 0 734 449 421 0 February 734 533 500 0 639 922 217 0 April 237 1305 0 851 118 1442 0 1324 0 1255 0 1255 47 928 0 881 426 505 0 78 December 568 433 135 0 Total Landscaping water Demand Evapotranspiration (inches) Plant water use Coefficient Plant water needs in Inches Gallons per square foot Square Footage of Landscape in gallons 1.44 0.5 0.72 0.45 1000 449 February 1.71 0.5 0.86 0.53 1000 533 2.96 0.5 1.48 0.92 1000 922 April 4.19 0.5 2.10 1.31 1000 1305 4.63 0.5 2.32 1.44 1000 1442 June 4.81 0.5 2.41 1.50 1000 1498 4.03 0.5 2.02 1.26 1000 1255 August 3.81 0.5 1.91 1.19 1000 1187 2.98 0.5 1.49 0.93 1000 928 October 2.63 0.5 1.32 0.82 1000 819 1.62 0.5 0.81 0.50 1000 505 December 1.39 0.5 0.70 0.43 1000 433
Harvested Rainwater Volume (gal) 1200 Tank Volume and Supplemental Water Needs for 3 years 1000 800 600 400 200 0 Year 1 Year 2 Year 3 Tank Volume Supplemental Water
The table and chart below illustrate that a house with a catchment area of 500 square feet, a 1,500 gallon rain barrel, and a landscape that is 1,000 square feet would offset 40% of the total annual landscape water use for: part of, October, all of, December,, February,, April, part of, and June. Sixty percent of the total annual landscape water use would have to come from supplemental, potable water, during, June,, August,, and October. Month Supply Demand Tank Volume Supplemental Water Input Values Catchment area (ft 2 ): 500 Collection efficiency (%): 95 Initial tank volume (gal): 1500 Tank size (gal): 1500 Plant water use coeff: 0.5 Irrigated area (ft 2 ): 1000 Monthly indoor demand (gal): 0 Avg. monthly rainfall (in) Avg. PET (in) AC Condensate (gal) : 3.1 1.44 0 February: 3.1 1.71 0 : 2.7 2.96 0 April: 1 4.19 0 : 0.5 4.63 0 June: 0.1 4.81 0 : 0 4.03 0 August: 0 3.81 0 : 0.2 2.98 0 October: 0.6 2.63 0 : 1.8 1.62 0 December: 2.4 1.39 0 Total: 15.5 36.2 0 Yearly Percent Average Rainfall (%) Year 1: 80% Year 2: 80% Year 3: 80% 734 449 1500 0 February 734 533 1500 0 639 922 1217 0 April 237 1305 149 0 118 1442 0 1175 0 1255 0 1255 47 928 0 881 426 505 0 78 Year 2 December 568 433 135 0 734 449 421 0 February 734 533 622 0 639 922 339 0 April 237 1305 0 730 118 1442 0 1324 0 1255 0 1255 47 928 0 881 426 505 0 78 Year 3 December 568 433 135 0 734 449 421 0 February 734 533 622 0 639 922 339 0 April 237 1305 0 730 118 1442 0 1324 0 1255 0 1255 47 928 0 881 426 505 0 78 December 568 433 135 0 Evapotranspiration (inches) Plant water use Coefficient Plant water needs in Inches Gallons per square foot Square Footage of Landscape Total Landscaping water Demand in gallons 1.44 0.5 0.72 0.45 1000 449 February 1.71 0.5 0.86 0.53 1000 533 2.96 0.5 1.48 0.92 1000 922 April 4.19 0.5 2.10 1.31 1000 1305 4.63 0.5 2.32 1.44 1000 1442 June 4.81 0.5 2.41 1.50 1000 1498 4.03 0.5 2.02 1.26 1000 1255 August 3.81 0.5 1.91 1.19 1000 1187 2.98 0.5 1.49 0.93 1000 928 October 2.63 0.5 1.32 0.82 1000 819 1.62 0.5 0.81 0.50 1000 505 December 1.39 0.5 0.70 0.43 1000 433
Harvested Rainwater Volume (gal) 1800 1600 1400 1200 1000 800 600 400 200 0 Tank Volume and Supplemental Water Needs for 3 years Year 1 Year 2 Year 3 Tank Volume Supplemental Water
The table and chart below illustrate that a house with a catchment area of 1,000 square feet, a 2,500 gallon rain barrel, and a landscape that is 1,000 square feet would offset 47% of the total annual landscape water use for: part of, October, all of, December,, February,, April, part of, and June. Fifty percent of the total annual landscape water use would have to come from supplemental, potable water, during, June,, August,, and October. Input Values Catchment area (ft 2 ): 1000 Collection efficiency (%): 95 Initial tank volume (gal): 2500 Tank size (gal): 2500 Plant water use coeff: 0.5 Irrigated area (ft 2 ): 1000 Monthly indoor demand (gal): 0 Avg. monthly rainfall (in) Avg. PET (in) AC Condensate (gal) : 3.1 1.34 0 February: 3.1 1.75 0 : 2.7 3.31 0 April: 1 4.86 0 : 0.5 5.6 0 June: 0.1 6.35 0 : 0 6.23 0 August: 0 5.76 0 : 0.2 4.24 0 October: 0.6 3.34 0 : 1.8 1.72 0 December: 2.4 1.35 0 Total: 15.5 45.85 0 Yearly Percent Average Rainfall (%) Year 1: 80% Year 2: 80% Year 3: 80% Month Supply Demand Tank Volume Supplemental Water 1468 417 2500 0 February 1468 545 2500 0 1278 1031 2500 0 April 473 1514 1460 0 237 1744 0 48 June 47 1978 0 1931 0 1941 0 1941 August 0 1794 0 1794 95 1321 0 1226 October 284 1040 0 756 852 536 316 0 Year 2 December 1136 421 1032 0 1468 417 2083 0 February 1468 545 2500 0 1278 1031 2500 0 April 473 1514 1460 0 237 1744 0 48 June 47 1978 0 1931 0 1941 0 1941 August 0 1794 0 1794 95 1321 0 1226 October 284 1040 0 756 852 536 316 0 Year 3 December 1136 421 1032 0 1468 417 2083 0 February 1468 545 2500 0 1278 1031 2500 0 April 473 1514 1460 0 237 1744 0 48 June 47 1978 0 1931 0 1941 0 1941 August 0 1794 0 1794 95 1321 0 1226 October 284 1040 0 756 852 536 316 0 December 1136 421 1032 0 Month Evapotranspiration (inches) Plant water use Coefficient Plant water needs in Inches Gallons per square foot Square Footage of Landscape Total Landscaping water Demand in gallons 1.34 0.5 0.67 0.42 1000 417 February 1.75 0.5 0.88 0.55 1000 545 3.31 0.5 1.66 1.03 1000 1031 April 4.86 0.5 2.43 1.51 1000 1514 5.60 0.5 2.80 1.74 1000 1744 June 6.35 0.5 3.18 1.98 1000 1978 6.23 0.5 3.12 1.94 1000 1941 August 5.76 0.5 2.88 1.79 1000 1794 4.24 0.5 2.12 1.32 1000 1321 October 3.34 0.5 1.67 1.04 1000 1040 1.72 0.5 0.86 0.54 1000 536 December 1.35 0.5 0.68 0.42 1000 421
Harvested Rainwater Volume (gal) 3000 Tank Volume and Supplemental Water Needs for 3 years 2500 2000 1500 1000 500 0 Year 1 Year 2 Year 3 Tank Volume Supplemental Water