Design of Rainwater Capture System

The rainwater cistern needs to capture as much rainwater as possible for toilet flushing, laundry washing, and landscape irrigation in a cost effective way. The area assumed to be turfgrass is the entire lot area minus house area, porch area, garage area, driveway, and area irrigated by the greywater irrigation system. The turfgrass areas for Seattle, Scottsdale, Omaha, and Tampa are 4415, 4905, 4055, and 4405 square feet, respectively. The collection area (i.e., area of roof) for the design house is 2171 square feet. However, not all of the water that falls on the roof will be captured. Some water will be captured in the first flush diverters, some will overflow the gutters, and some will evaporate. For this study, the capture efficiency is assumed to be 90%. The Texas

Rainwater Harvesting Manual (Brown et al. 2005) recommends a value between 85-90%.

69 90% was assumed for this project because pipes and gutters are adequately sized. The indoor usage rates (i.e., for toilet flushing and laundry washing) were calculated in the Section 3.2. The excess rainwater will be used to irrigate the turfgrass in the lawn. In order to insure that there is always water available for toilet flushing and landscape irrigation, each cistern has a back-up potable water connection. In this study, the back-up begins to run when the water level falls below the 5% full mark and continues to run until the cistern is 10% full.

The optimum cistern size was determined by trading off the percentage of rainwater captured and the payback period. The Rainwater Harvester program (North Carolina State University, 2010) was used to determine the percent of rainwater captured in a given year for various cistern sizes. Historic daily rainfall data were found for each of the sites to run the model. The model calculates the water level or storage in the tank at the end of each day. The payback period was calculated from the tank cost and the

savings from reduced water use.

The optimum cistern size was determined from a weighted average of the two goals (criteria) of maximizing the percent water captured (PWC) and minimizing the pay-back period (PBP). The values of the criteria (PWC and PBP) for various sizes of tanks were first normalized using the following relationship:

_  |_   !/_#$   !| (Eq. 3.9)

where: ci = normalized value of criterion i (either PWC or PBP); Vi = the actual criterion value for tank size i; Vworst = the worst criterion value over all tank sizes; and Vbest = the best criterion value over all tank sizes. The normalized criterion values are then traded-off using the following relationship to determine the optimum tank size:

70

%_  &_'()* _'(), , &_'-'* _'-', (Eq. 3.10)

where: Zi is the trade-off value for tank size i; wPWC and wPBP are relative weights for the two criteria (equal values of 0.5 were used); and cPWC and cPBP are the normalized values of the criteria for tank size i. The tank size that produced the highest value for Zi is the optimum-sized tank. The analysis resulted in 2500 gal tanks for all four sites.

3.3.2.1 Sizing the Rainwater Cistern in Seattle

In Seattle, historic daily rainfall was retrieved from the Agweather website from Washington State University (www.weather.wsu.edu). Using this historic rainfall and water and sewer prices from Mayer’s study (1999), the Rainwater Harvester model was used to find the optimum tank size (North Carolina State University, 2010). The tanks simulated had volumes of 500, 1000, 2000, 2500, 5000, and 10000 gallons. Using Equation 3.9 and the results of the simulations, a 2500 gallon cistern was chosen in Seattle.

The Rainwater Harvester model calculates the cistern storage volume at the end of each day. The rainfall from historic rainfall is the only water input. However, there are several uses or outputs. In Seattle, the indoor usage rate for toilets and laundry that was calculated in Section 3.2 is 28.8 gallons/day, so 28.8 gallons are withdrawn each day for toilet flushing and laundry washing. The irrigation needs for turfgrass were calculated using the model. The evapotranspiration data used was the same data from Section 3.1.

An average silt loam soil was assumed for the Seattle area. Impact sprinklers (which have an irrigation efficiency of 75%) were assumed to be the irrigation application method.

Figure B.1 in Appendix B displays the water level in the cistern over the modeled year.

71 Figure B.1 shows that the need for supplementing irrigation with potable water is

minimal.

3.3.2.2 Sizing the Rainwater Cistern in Scottsdale

In Scottsdale, historic daily rainfall was retrieved for the Scottsdale Municipal Airport from the Natural Resources Conservation Service (NRCS, 2010). Using this historic rainfall and water and sewer prices from Mayer et al. (1999), the Rainwater Harvester model was used to determine the tank size. The tanks simulated had volumes of 500, 1000, 2000, 2500, and 5000 gallons. Using Equation 3.9 and the results of the simulations, a 2500 gallon cistern was chosen in Scottsdale.

In Scottsdale, the indoor usage rate for toilets and laundry washing that was calculated in Section 3.2 is 27.6 gallons/day, so 27.6 gallons are withdrawn each day for toilet flushing and laundry washing. The irrigation needs for turfgrass were calculated using the model. The evapotranspiration data used was the same data from Section 3.1.

An average silt loam soil was assumed for the Scottsdale area. Impact sprinklers (which have an irrigation efficiency of 75%) were assumed to be the irrigation application method. Figure B.2 in Appendix B displays the water level in the cistern over the modeled year. Figure B.2 shows that the need for supplemental irrigation with potable water is substantial throughout the year.

3.3.2.3 Sizing the Rainwater Cistern in Omaha

In Omaha, historic daily rainfall was retrieved from the High Plains Regional Climate Center (http://www.hprcc.unl.edu). Using this historic daily rainfall and water and sewer prices from Mayer et al. (1999), the Rainwater Harvester model was used to determine the tank size. The tanks simulated had volumes of 500, 1000, 2000, 2500, and

72 5000 gallons. Using Equation 3.9 and the results of the simulations, a 2500 gallon cistern was chosen in Omaha.

In Omaha, the indoor usage rate for toilets and laundry washing that was

calculated in Section 3.2 is 33.8 gallons/day, so 33.8 gallons are withdrawn each day for toilet flushing and laundry washing. The irrigation needs for turfgrass were calculated using the model. The evapotranspiration data used was the same data from Section 3.1.

An average silt loam soil was assumed for the Omaha area. Impact sprinklers (which have an irrigation efficiency of 75%) were assumed to be the irrigation application method. Figure B.3 in Appendix B displays the water level in the cistern over the modeled year. Figure B.3 shows that little supplemental potable water is needed for irrigation purposes.

3.3.2.4 Sizing the Rainwater Cistern in Tampa

In Tampa, historic daily rainfall was retrieved from the Florida Climate Center.

Using this historic daily rainfall and water and sewer prices from Mayer et al. (1999), the Rainwater Harvester model was used to determine the tank size. The tanks simulated had volumes of 500, 1000, 2000, 2500, 5000, and 10000 gallons. Using Equation 3.9 and the results of the simulations, a 2500 gallon cistern was chosen in Tampa.

In Tampa, the indoor usage rate for toilet flushing and laundry washing that was calculated in Section 3.2 is 28.0 gallons/day, so 28.0 gallons are withdrawn each day for toilet flushing and laundry washing. The irrigation needs for turgrass were calculated using the model. The evapotranspiration data used was the same data from Section 3.1.

An average silt loam soil was assumed for the Tampa area. Impact sprinklers (which have an irrigation efficiency of 75%) were assumed to be the irrigation application method.

73 Figure B.4 in Appendix B displays the water level in the cistern over the modeled year.

Figure B.4 shows that minimal supplemental potable water is needed for landscape irrigation.