Supporting Information for Rainwater Catchment Rebate:

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1 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.

2 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 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 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 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.

3 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) : February: : April: : June: : August: : October: : December: Total: Yearly Percent Average Rainfall (%) Year 1: 80% Year 2: 80% Year 3: 80% Month Supply Demand Tank Volume Supplemental Water February April Year 2 December February April Year 3 December February April December 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 February April June August October December

4 Harvested Rainwater Volume (gal) 1200 Tank Volume and Supplemental Water Needs for 3 years Year 1 Year 2 Year 3 Tank Volume Supplemental Water

5 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) : February: : April: : June: : August: : October: : December: Total: Yearly Percent Average Rainfall (%) Year 1: 80% Year 2: 80% Year 3: 80% February April Year 2 December February April Year 3 December February April December 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 February April June August October December

6 Harvested Rainwater Volume (gal) Tank Volume and Supplemental Water Needs for 3 years Year 1 Year 2 Year 3 Tank Volume Supplemental Water

7 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) : February: : April: : June: : August: : October: : December: Total: Yearly Percent Average Rainfall (%) Year 1: 80% Year 2: 80% Year 3: 80% Month Supply Demand Tank Volume Supplemental Water February April June August October Year 2 December February April June August October Year 3 December February April June August October December 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 February April June August October December

8 Harvested Rainwater Volume (gal) 3000 Tank Volume and Supplemental Water Needs for 3 years Year 1 Year 2 Year 3 Tank Volume Supplemental Water