Supplemental Information Text

Supplemental Information Text

The Water Short List: The Most Effective Actions U.S. Households Can Take to Curb Water Use

Benjamin D. Inskeep and Shahzeen Z. Attari

Actions Included

For our recommendations, we selected efficiency and curtailment actions commonly found on lists of water conservation tips provided by government agencies, by leading water conservation organizations, from a comprehensive book on the subject by Vickers, and developed new recommendations based on disaggregated water usage data.1 Since many recommendations we found on conserving residential water use were quite specific, leading to unwieldy and lengthy lists, it was necessary to group them into broader categories to create a useful water short list. For example, “turning the faucet off while brushing teeth and shaving” and “washing produce in a bowl instead of under a running faucet” was transformed into “Reduce the amount of time the faucet is left running by 2 minutes per person per day.”

Actions Not Included

An exhaustive list calculating the savings associated with every possible action that could save residential water would be both prohibitively time-consuming to generate and defeat the purpose of having a short list of effective actions. We therefore excluded actions relating to uncommon uses of water that would not apply to average U.S. households (e.g., actions related to conserving water in evaporative coolers, which are not common throughout the U.S.). Some actions also had to be omitted if the savings could not be calculated due to a lack of sources (e.g., repairing underground leaks). Finally, we focused on direct household water uses and did not include actions designed to reduce ‘virtual water’ use, or water that is used in the making of an agricultural or industrial product (e.g., reducing meat consumption).

Outdoor actions unrelated to lawn and garden (i.e., non-turfgrass plants) irrigation were excluded. This includes water-saving tips for pool owners (e.g., water-efficient filters or using a pool cover to minimize evaporation) and for hose use related to cleaning (e.g., power-washing decks, sidewalks, and driveways and washing cars). These outdoor uses appear to account for a small amount of water use for the average household, and savings related to pools depends heavily on the evaporation rate, which varies substantially by climate.2

Calculating Savings for Indoor Water Conservation Actions

All unit abbreviations used in calculations are presented in Table 1. Estimated water savings, W, arising from efficiency and curtailment actions indoors are calculated by estimating water used by the appliance or fixture after the conservation action was implemented and subtracting it from an estimated baseline:

W=O*FO-(N*FN) (1)

where O is the volume of water the old appliance or fixture uses (gallons per household per use), N is the volume of water the new appliance or fixture uses (gallons per household per use), and FO is the frequency of daily household use of the appliance or fixture before the water conservation action is implemented, and FN is the frequency of daily household use after the water conservation action is implemented. The frequency of appliance and fixture use was assumed to remain the same for efficiency actions (FO=FN=F). Therefore, the net daily household water conserved (in gphd) for efficiency actions, Wefficiency, can also be calculated as

Wefficiency=(O-N)*F. (2)

Curtailment actions involve either reducing the frequency of a water-use activity (e.g., taking fewer showers per week) or changing the duration or type of activity that delivers the same ‘water service’ (e.g., taking a shorter shower each time you shower). When the household reduces the frequency of a water-using activity (FOFN), the water use of the activity itself is not changing (O=N), so Equation 1 can be simplified for curtailment actions:

Wcurtailment=O*FO-FN. (3)

Finally, estimated W for each action can be divided by the estimated household indoor water use to get the percent indoor water saved. Indoor water use (including all household leaks) averaged 69.3 gpcd in the REUWS, of which leaks were responsible for 9.5 gpcd.3 Household indoor water use is thus estimated at ((69.3 gpcd) – (9.5 gpcd)) x (2.6 capita per household) = 155.5 gphd.4

Indoor Efficiency Actions

Clothes Washing

We calculate the average household savings from replacing existing clothes washing machines, which use an average of 40.9, with Energy Star-labeled clothes washing machines, which use no more than 15 gpl.5 We assume the frequency of use is one load per household per day (lphd) ((0.37 lpcd) x (2.6 cph)).6 Thus the average household savings from replacing all standard washing machines with Energy Star-labeled washing machines is (40.9 gpl - 15 gpl) x (1 lphd) = 25.9 gphd, or 16.7%of household indoor water use ((25.9 gphd) / (155.5 gphd)).

Showering

We calculate the average household savings from shifting average showerhead flow rates from the current population average, 2.2 gpm, to the maximum flow rate allowed under WaterSense-labeling criteria, 2 gpm.7 We assume a frequency of use of (0.7 spcd) x (2.6 cph) = 1.8 sphd with an average shower duration of 8.2 minutes.8 The savings are therefore estimated at (2.2 gpm – 2 gpm) x (1.8 sphd) x (8.2 mpsh) = 3 gphd, or 1.9% of household indoor water use ((3 gphd) / (155.5 gphd)).

Toilet flushing

We calculate the savings of replacing all standard toilets, which use 3.5 gpf, with WaterSense-labeled toilets, which use a maximum of 1.28 gpf.9 We assume a frequency of 13 fphd ((5 fpcd) x (2.6 cph)).10 The savings are therefore estimated at (3.5 gpf – 1.28 gpf) x (13 fphd) = 28.9 gphd, or 18.6% of household indoor water use ((28.9 gphd) / (155.5 gphd)).

We calculate the savings of installing toilet tank water-saving inserts in standard toilets, which is assumed to reduce flush volume by 0.7 gpf to 2.8 gpf (3.5 gpf – 0.7 gpf).11 As described above, we assume a frequency of 13 fphd. The savings are therefore estimated at (3.5 gpf - 2.8 gpf) x (13 fpd) = 9.1 gphd, or 5.9% of household indoor water use ((9.1 gphd) / (155.5 gphd)).

Dishwashing

Estimating the savings available from dishwashing requires a slightly different calculation method because the REUWS only provides data on dishwashers and does not identify water used from hand-washing dishes. To calculate savings from changes to dishwashing practices, we assume that 24.1% ((27.2 million households with Energy Star dishwasher) / (113.6 million households total)) of American households have an Energy Star-labeled dishwasher, 35.4% (((67.4 million households use a dishwasher) – (27.2 million households with Energy Star dishwashers))/(113.6 million households total)) use a standard dishwasher, and 40.7% (((113.6 million households total) – (67.4 million households with dishwashers)) / (113.6 million households total)) do not have a dishwasher.12

We assume 70% of households that own dishwashers pre-treat their dishes with water before putting them in the dishwasher, such as by pre-rinsing, scrubbing, or soaking dishes.13 To estimate the volume of water used in this process, we assume a pre-treatment of running the faucet at 1.3 gpm for 5 seconds per dish.14 A standard dishwasher has a capacity of at least 6 serving dishes and 8 place settings, where each place setting is 5 dishes (silverware are excluded), for an average dishwasher capacity of 46 dishes ((5 dishes/place settings) x (8 place settings) + (6 serving dishes)).15 The pretreatment process therefore uses an estimated 5 gpl ((1.3 gpm) x (5 seconds/dish) x (1 minute / 60 seconds) x (46 dishes per load)).

We assume that an Energy Star dishwasher uses 4.25 gpl, a standard dishwasher uses 10 gpl, and hand-washing uses 37 gpl (139.9 liters).16 We estimate a population baseline water usage for dishwashing (including pretreatment) through the following calculation:

[(4.25 gpl) + (5 gpl) x (0.7)] x (0.241) Energy Star dishwasher

+ [(10 gpl) + (5 gpl) x (0.7)] x (0.354) standard dishwasher

+ (37 gpl) x (0.407) hand-washing

= 21.7 gpl.

This total is then multiplied by (0.1 lpcd) x (2.6 cph) to get the daily population-weighted estimate of 5.7 gphd used on dishwashing. Calculated savings reflect changes to this population-based baseline, so when households incorporate these actions they will experience greater savings. For example, a household that hand-washes their dishes will experience greater savings from switching to an Energy Star-labeled dishwasher than the average household savings arising from shifting the entire population to Energy Star-labeled dishwashers, as some proportion of the population already has this technology.

(a) Improving average dishwasher water efficiency to Energy Star-labeled dishwasher water efficiency

We calculate the average savings of replacing standard dishwashers with Energy Star-labeled dishwashers by assuming all households with standard dishwashers replace them with Energy Star-labeled dishwashers. The savings are estimated by first calculating theoretical water use after the action has been implemented:

[(4.25 gpl) + (5 gpl) x (0.7)] x (0.241 + 0.354) Energy Star dishwasher

+ (37 gpl) x (0.407)) hand-washing

= 19.7 gpl.

(b) Using an Energy Star-labeled dishwasher instead of hand-washing dishes

We calculate the average savings of switching from hand-washing dishes to washing dishes in an Energy Star-labeled dishwasher by assuming all households without dishwashers purchase and use an Energy Star-labeled dishwasher to wash dishes. The savings are estimated by first calculating theoretical water use after the action has been implemented:

[(4.25 gpl) + (5 gpl) x (0.7)] x (0.241 + 0.407) Energy Star dishwasher

+ [(10 gpl) + (5 gpl) x (0.7)] x (0.354)) standard dishwashing

= 9.8 gpl.

This total is then multiplied by (0.1 lpcd) x (2.6 cph) to get the daily population-weighted estimate of 2.5 gphd, which is then subtracted from the baseline water use, 5.7 gphd, to get an estimated savings of 3.2 gphd, or 2.1% of indoor water use ((3.2 gphd) / (155.5 gphd)).

Faucet use

We calculate the savings of replacing all standard faucets, which have a maximum flow rate of 2.2 gpm, with WaterSense-labeled faucets (or equivalent flow-reducing faucet aerators), which have a maximum flow rate of 1.5 gpm.17 We assume that faucets are turned on at an average of 60% of their maximum flow rate, and that they are used for (8.1 minutes per capita per day) x (2.6 cph) = 21.1 mphd.18 The savings are therefore estimated at (1.3 gpm – ((1.5 gpm) x (0.6)) x (21.1 mphd) = 8.4 gphd, or 5.4% of household indoor water use ((8.4 gphd) / (155.5 gphd)).

Indoor curtailment actions

Clothes washing

We calculate the savings of only washing full loads of clothes in a standard clothes washing machine (or manually adjusting the settings to match a load’s size would accomplish similar savings), which uses 40.9 gpl. We assume that frequency of use is one lphd ((0.37 lpcd) x (2.6 cph)). The average load of clothes for an 8-pound wash load capacity clothes washing machine is assumed to be 5.9 ppl.19 If one lphd are needed to wash 5.9 ppl, then only 0.7 loads are needed to wash the same amount of clothes if the load size increases to 8 ppl ((1 lphd) x (5.9 lbs per load) = 5.9 ppl = (0.7 lphd) x (8 lbs per load)). The savings are therefore estimated at (40.9 gpl) x (1 lphd – 0.7 lphd) = 12.3 gphd, or 7.9% of household indoor water use ((12.3 gphd) / (155.5 gphd)).

Showering

We calculate the average household savings from reducing shower duration from 8.2 mpsh to 5 mpsh. We assume a frequency of use of (0.7 spcd) x (2.6 cph) = 1.82 sphd and an average showerhead flow rate of 2.2 gpm. The savings are therefore estimated at ((8.2 mpsh x 2.2 gpm) – (5 mpsh x 2.2 gpm)) x (1.8 sphd) = 12.7 gphd, or 8.2% of household indoor water use ((12.7 gphd) / (155.5 gphd)).

Toilet flushing

We calculate the savings of reducing toilet flushes by 3.3 fphd (25%) by not using toilets as a trash can and by flushing liquid waste less frequently. We assume the average toilet flush uses 3.5 gpf and that households flush 13 times per day ((5 fpcd) x (2.6 cph)) in the baseline. The savings are therefore estimated at (3.5 gpf) x (13 fphd) x (0.25) = 11.4 gphd, or 7.3% of household indoor water use ((11.4 gphd) / (155.5 gphd)).

Dishwashing

We calculate the average savings of not pre-treating dishes with water by assuming all households with dishwashers switch to scrapping off dishes without using water before putting the dishes in the dishwasher. The savings are estimated by first calculating theoretical water use after the action has been implemented:

(4.25 gpl ) x (0.241) Energy Star dishwasher

+ (10 gpl) x (0.354) standard dishwasher

+ (37 gpl) x (0.407) hand-washing

= 19.6 gpl.

This total is then multiplied by (0.1 lpcd) x (2.6 cph) to get the daily population-weighted estimate of 5.1 gphd, which is then subtracted from the baseline water use, 5.7 gphd, to get an estimated savings of 0.6 gphd, or 0.4% of indoor water use ((0.6 gphd) / (155.5 gphd). (See Dishwashing subsection under efficiency actions for more information about the assumptions we used to calculate savings related to dishwashing.)

Faucet use

We calculate the savings of eliminating food disposer use, which we assume uses (1 gpcd) x (2.6 cph) = 2.6 gphd or 1.7% of household indoor water use ((2.6 gphd) / (155.5 gphd)). 20