Rainwater Conservation System
Town of Cary
Senior Center Facility
Senior Center Facility


17,000 sq. ft
$2.5 Million
Water Use
Courtyard Irrigation
Lake System
3 Ballfields 3.5 ac
Bond Lake 50 ac
Senior Center Layout
Garden Plots
& Lawn
10,000 sq. ft.
15 GPM
Pump
Tank 1200 gal
90 GPM
Modifications
1. Direct rain gutter
downspouts into tanks.
2. Increase tank size to allow
for rain storage.
Collection & Storage
Collection Gutter
system
Storage 2 Tanks,
6000 gals
total
Distribution
Pump
system
&
Piping
system
Averaged Rain Supply and
‘Rule of Thumb’ Irrigation Demand

Average Weekly Rain Supply:
Roof = 3250 sq.ft.; Rain = 0.9 in.; Efficiency = 75%
Yield = 1,400 gals./week

Average Weekly Water Demand:
Garden = 10,000 sq.ft.; Irrigation = 1 in./wk.“rule of thumb”
Demand = 6,400 gals./week
- Supply = 22% of Use. Other 78% requires other sources.
- Note an average 0.9 inches of rain also fell on garden.This
exceeds the needed 78%.
- So in theory, Conserved Rain satisfies the Demand deficit.
Supply & Demand: No Rain Scenarios

Design for periods of no rain / no collection:
Provide storage/supply capacity for 3 weeks no rain

3 weeks Demand = 19,200 gals (@ 1 inch/week)

3 weeks Supply = 25,600 gals (19,200 gals / 0.75 efficiency).

Current system Storage: 2 tanks - 6,000 gals
- Supply capacity is inadequate.
- Alternate water source, increased collection surface
and/or increased storage is required.
Potable Water Demand Reduction

Current system inadequate to meet full demand.

At 75% efficiency, system still reduces potable water
demand by an average
1,400 gals./week or 200 gals per day.

Average annual irrigation weeks = 34 weeks

Average reduced potable water demand
= 47,600 gals / yr (1,400 x 34 wks)
Cost Recoup: Reduced Potable
Water Purchase

Net cost for potable water = $4.51 / Kgal
(Typical potable water rate + O&M = $4.46 + $0.05)

Average potable water saving = 47,600 gals/year

Reduced potable water costs = $215 / yr
($4.51 x 47.6 Kgals)

Rainwater system cost $7,250.
Recoup of system costs in 7,250 / 215 = 34 years
Cost Recoup: Potable Water
Purchase Eliminated


Net cost = $4.51 /Kgal
Full season irrigation demand = 218,000 gals/year
(10,000 sq.ft. @ 1 in/wk for 34 weeks)

Reduced potable water costs = $983 /yr
($4.51 x 192 Kgals)

Expanded system cost = $31,175
– 4.3 factor size increase to meet 3 week drought.
– Economies of scale ignored
Recoup of system costs in 31,175 / 983 = 32 years
Lake System Cost Recoup: Potable
Water Purchase Eliminated




The lake supplies irrigation water for 8.5 acres
(ballfields).
Irrigation averages 1 inch or 231,000 gals per week for
27 weeks.
Yearly water demand = 6,231,000 gals per year
Potable water costs savings = $28,100 year
(6,231 Kgals x $4.51)

Lake system costs $163,600 to construct.
Recoup of system costs in 6 years (163,600 / 28,100 per yr.)
Cost Recoup: Deferment Basis

Estimated yearly deferment value of reduction in
town peak day potable water demand
= $0.25 x gals of daily reduction.
• Basis: estimate of deferment value for Town of Cary water
treatment facility expansion and O&M.
• Value does not include certain external benefits. Example, value
of reduced stormwater management.

Rain system reduces peak demand 200 gals/day.
 Yearly deferment value = $0.25 x 200 = $ 50
 Construction of the rainwater system cost $7,250.
Recoup of system cost in 145 years ($7,250 / $50)
Cost Recoup: Meeting Full Demand

Full system demand = 218,000 gals/year (238 days
demand)



System reduces town peak demand = 916 gals/day
Yearly deferment value = $0.25 x 916 = $229
Expanded system cost = $31,175
– 4.3 factor size increase to meet 3 week drought.
– Economies of scale ignored
Recoup of system cost in 140 years ($32,250 / $229 per yr.)
- Economies of scale should decrease recoup time.
Lake System Cost Recoup:
Deferment Basis

Yearly water demand = 6,231 Kgals per year

Irrigation season is 190 days (7x27)

Deferred potable water production = 32,800 gal/day.
(6,231 Kgals / 190 days)

Yearly deferment value = $8,200 (32,800 x 0.25)

Lake system costs $163,600 to construct.
Recovery of system cost = 20 years (163,600 / 8,200 )
Alternative to Storage Tanks
Create water
storage ponds.
Or
Combine irrigation
water storage
with stormwater
retention.
May lower costs,
improve payback &
add amenity
value
Other Benefits of
Rain Conservation Systems

Conservation of potable water supply
source
 Contributes to stormwater management
– Reduced runoff peak volume
– Increased land treatment of runoff

Value of irrigation of landscaping when
strict potable water conservation
measures are in effect
 Value as a public education tool