Sun Valley Water & Wastewater Project
Rainwater Harvesting
To provide an additional source of water, a rainwater harvesting system will be incorporated into the water supply design. This design will be a simple collection system with a gutter system that allows the rain to drain to a barrel next to the dome house. This system will have one side of the gutter elevated compared to the other so that it will be gravity fed. The barrel will have a connecting outlet where a hose attachment will be simple and easy. The client will be able to easily fill up a container with water and use it for vegetation.
To obtain an estimation of the effectiveness of the rainwater harvesting system, the follow tables, Tables 4, 5, and 6, were constructed estimating the supply, demand, and storage [1].
Table 4 shows the estimated values for the total monthly yield of the rainwater harvesting system. The monthly rainfall in inches was based on the number collected for the city of Holbrook. The conversion column is simply a conversion factor used to convert the inches of rainfall and square footage of area to gallons. The runoff coefficient was selected for soil that is flat and bare that will be similar to the surface of the dome house structure. The average of the high and low estimations for this type of surface was used. The catchment area was determined estimating the circular area of a 30’ diameter central house and 4 smaller dome houses with a 16’ diameter each. The values presented in each of the columns in Table 4 were multiplied in excel to determine the total monthly yield in gallons.Equation 1 was used to calculate the total monthly yield. Equation 2 provides the equation and calculation performed to determine the catchment area.
Equation 1: Rainwater Monthly Yield: R(in)* C * A(ft^2) *CR =Y(gal)
Where R= rainfall (inches)
C = 0.623, conversion factor to convert inches and squared feet to gallons
A = catchment area (feet2)
CR= 0.75, runoff coefficient selected for flat and bare soil
Y= total monthly yield of rainwater (gallons)
Equation 2: Catchment Area: A = pi(r)^2 =706.85ft^2
Table 4: Rainwater Harvesting Supply
Table 5 shows the rainwater harvesting demand for the plants that will be included on the site.
Equation 3 shows the factors used for calculating total monthly demand.
Equation 3: Total Monthly Landscaping Demand: ET(in) * C * A(ft^2) = D(gal)
Where ET= evapotranspiration rate (inches)
C = 0.623, conversion factor to convert inches and squared feet to gallons
A = area of landscaping that the plants will cover (feet2)
Y= total monthly landscaping demand (gallons)
An evapotranspiration rate for Holbrook for each month is shown in the first column of Table 5 as ET. The area of landscaping was estimated with the assumption that a few plants native to Arizona will be planted near the dome house structures and on the path leading to the dome house. Equation 4 shows the equation that was used to determine this area. It was assumed that one desert broom shrub and two agave plants will also be placed on each side of the central dome house [2].
Equation 4: Landscaping Area:W * h = A (ft^2)
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Where w= width of plant
h= height of plant
Table 5: Rainwater Harvesting Demand
Table 6 shows the estimation of the amount of water that will be in storage for the rainwater harvesting system. This table is used to determine how effectively the rainwater system will be providing the water needed for the plants. It does not represent the actual amount of water that will be placed in a storage container because the water from this system will be distributed to the plants. It assumes that for the first year in January, the system will start off with no water. The yields from Table 3 and the demand values from Table 4 were incorporated. The third column shows the demand subtracted from the yield. As Table 6 shows, the system may require water from an additional source in some of the summer months. To give a clear indication of how well the rainwater harvesting system is providing the water needed by the plants on the site, Equation 5 was used and implemented to form Table 6. This table shows that this system will provide more than enough water to provide water form several additional plants or other activities.
Equation 5 Cumulative Storage = Yield - Demand
Table 6: Rainwater Harvesting Storage
For a better representation of the general pattern that the rainwater harvesting system will exhibit throughout a year, Figure 3 was created. Once again, it is clear that the rainwater harvesting system will supply enough water for the client to plant several bushes and shrubs near the main dome house as well as plant additional plants around the site.
Figure 3: Rainwater harvesting cumulative storage over one year
Barrel Capacities
To determine the appropriate barrel capacities for each dome house, the information provided in Tables 4 through 6 were used and modified to separate the central dome house’s capacity, demand, and storage from those of the smaller surrounding dome houses. This helped determine the required capacity of the barrels next to each of the dome houses. For the central dome house, the maximum volume in the barrel that will be capturing the rain will be about 500 gallons.
Operation & Maintenance
To maintain a rainwater harvesting system guarantee that there is no debris in the gutter or tank after each rainy season and check and clean the filter on a weekly basis [3]. If there is a large amount of debris within the rainwater harvesting barrel, it should be flushed out. During these small operation and maintenance tasks, the system should be evaluated to determine whether any components, mainly the filter, needs to be repaired.
Regulations
ADEQ, ADWR and the Arizona Pollutant Discharge Elimination System (APDES) do not require a permit and have no specific requirements for the rainwater harvesting [4].
With the determination of the requirements and calculations for the sizing of the imported water supply and rainwater harvesting systems, these systems were verified as reliable sources for the Sun Valley Ranch water supply to sustain the population of approximately 10 people. Also, the rainwater harvesting system will provide a small irrigation system for a few native plants placed near each of the dome houses.
References:
[1] City of Holbrook. (2013). 2013 Holbrook Annual Drinking Water Quality Report. [Online].
Available FTP:http://www.ci.holbrook.az.us/index.asp?SEC=D155A727-B339-4751-B896-%20156F36372EB0&DE=B84E9068-836E-4442-8BF2-2B70256767BC&Type=B_BASIC
[2] ] Arizona Cooperative Extension. (2014). Yavapai County Native & Naturalized Plants.
[Online]. Available FTP: http://cals.arizona.edu/yavapaiplants/index.php
[3] P. Waterfall. (2004). Rainwater Harvesting for Landscape Use. [Online]. Available FTP: http://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1344.pdf
[4] Arizona Department of Environmental Quality. (2010). Blue Ribbon Panel on Water
Sustainability. [Online]. Available FTP:http://www.azwater.gov/AzDWR/waterManagement/documents/BRP_Final_Report-12-1-%2010.pdf
