Holistic impact assessment and cost savings of rainwater harvesting at the watershed scale

We evaluated the impacts of domestic and agricultural rainwater harvesting (RWH) systems in three watersheds within the Albemarle-Pamlico river basin (southeastern U.S.) using life cycle assessment (LCA) and life cycle cost assessment. Life cycle impact assessment (LCIA) categories included energy demand, fossil fuel, metals, ozone depletion, global warming, acidification, smog, blue and green water use, ecotoxicity, eutrophication, and human health effects. Building upon previous LCAs of near-optimal domestic and agricultural RWH systems in the region, we scaled functional unit LCIA scores for adoption rates of 25%, 50%, 75%, and 100% and compared these to conventional municipal water and well water systems. In addition to investigating watershed-scale impacts of RWH adoption, which few studies have addressed, potential life cycle cost savings due to reduced cumulative energy demand were scaled in each watershed for a more comprehensive analysis. The importance of managing the holistic water balance, including blue water (surface/ground water), green water (rainwater) use, and annual precipitation and their relationship to RWH are also addressed. RWH contributes to water resource sustainability by offsetting surface and ground water consumption and by reducing environmental and human health impacts compared to conventional sources. A watershed-wide RWH adoption rate of 25% has a number of ecological and human health benefits including blue water use reduction ranging from 2–39 Mm3, cumulative energy savings of 12–210 TJ, and reduced global warming potential of 600–10,100 Mg CO2 eq. Potential maximum lifetime energy cost savings were estimated at $5M and $24M corresponding to domestic RWH in Greens Mill and agricultural RWH in Back Creek watersheds.


SM 2. Description of Costs
We computed the life cycle cost of an agricultural RWH system by estimating initial investment, as well as the present value of future costs of components incurred during the service life (Table S1). All initial investment costs occurring in the base date (Year 2014) were not discounted since they were in present value. Future costs of component replacements occurring at the end of service, one-time, and at non-annual intervals were discounted from the end of service life to calculate present value. Residual values of the capital investments were estimated using straight-line depreciation. For annually recurring costs such as operation and maintenance, we considered real discounting rate addressing the time value of money. Below, we describe the computation procedures of life cycle costs (summarized in Table S1) consistent with engineering economics (Revelle et al. 2004) and life cycle cost analysis guidelines of the National Institute of Standards and Technology (Fuller and Petersen 1996). We describe initial capital investments costs (e.g., costs of sediment chamber (pond), pipe, pivot center, and valves); replacement costs (e.g., pivot center replacement at the end of 20 and 40 years); residual value; and costs that recur annually such as operation and maintenance (O&M) and sediment dredging and disposal.

Sediment Chamber (Pond) Cost
The cost of a sediment chamber (pond) is calculated based on cost per unit volume, V p ($3.8/m 3 ) of a pond, as suggested by the Virginia FY14 Environmental Quality Incentive Program (EQIP) Payment Schedule (USDA 2014) (Equation S1).
Polyethylene Tank Cost The cost of polyethylene tank (C tank ) ($) is based on tank volume, V t (m 3 ), as provided by the State of Michigan (SOM 2003): Pivot Center Cost The cost of a typical pivot center, C pivot ($/ha), is derived from cost per unit of irrigated farm area, a (Bliesner and Spare 2001): Replacement Cost Replacement costs are one-time costs at non-annual intervals. Replacement occurred at the end of service life of a component: pivot center and valves were replaced at 20 and 7.5 years, respectively. The present value of each replacement cost (R PV ), $, was estimated using the corresponding single present value (SPV) discount factor: where, Technology (NIST 2013). We also performed sensitivity analysis of i from 3% to 10% (Register 1999).
F t = Future price of an item such as agricultural RWH pivot center, $, estimated as the replacement cost at replacement times (multiple replacements of the component's service life less than or equal to 50). We estimated future price, F t , as identical to the base date cost, assuming zero real price escalation rates (the rate of price change @ general price inflation rate), as suggested by the NIST guideline (Fuller and Petersen 1996).
Using Equation S4 We note that actual future replacement cost of an item, with a price escalation rate deviating from general inflation rates, may also be estimated using a nominal price escalation rate such as The present value of this residual value was then estimated at $4,368, using Equation S4.

Annual Costs
The present value of annual operation and maintenance costs were estimated using the uniform present value (UPV) factor. Water prices vary with location, block price and year, and energy price varies by fuel type, price escalation rate and census region (Fuller and Petersen 1996), but neither were included in our analysis. For all annual costs, we accounted for future discounting by estimating present value (PV) of annually recurring uniform amounts, as defined by: where, We used 1.7% of total investment costs as the O&M costs of an agricultural RWH system (Hogan et al. 2007). Sediment removal is performed once every 2-15 years depending on pond type (Commission 2007), but we estimated costs of annual removal. The volume of sediment dredging was based on 0.02 m depth by 7108 m 2 surface area per year, at $20/m³ (Marsalek and Marsalek 1997;USDA 1997;Commission 2007). The surface area assumes average water depth of 1.8 m (6 ft) in a sedimentation chamber of 13000 m 3 volume (Ghimire et al. 2014). Sediment disposal costs were estimated at $7/m 3 (See Table S1).