Calculate water savings by subtracting post-conversion irrigation use from a comparable baseline, using the same units and time period. Then account for weather, establishment watering and other changes. A published saving from another city is not a measurement of your yard.
1. Define what you are measuring
Record the converted area and dates. Decide whether the comparison covers one bed, all outdoor irrigation or the entire property’s water use. A separate irrigation meter gives a clearer outdoor measure than a household bill that includes showers, laundry and other uses.
Keep the source of each number: meter reading, utility bill, controller estimate or calculated flow. If you must estimate outdoor use from seasonal household bills, label it as an estimate. Indoor use can change with occupancy, visitors and leaks.
2. Build a comparable baseline
Collect a full year’s readings where available, with billing dates and number of days. Compare the same seasons after conversion. Record rainfall, unusual heat, irrigation restrictions and repairs. A wet summer after a dry one can reduce water use even without a planting change.
Do not compare a short winter bill with a long summer bill. Daily averages help compare unequal periods, but do not correct weather or seasonal plant demand by themselves. Ask the utility or an irrigation professional about a weather-adjusted comparison if you need a defensible project evaluation.
3. Keep units consistent
Check whether the bill uses gallons, cubic feet, hundred cubic feet or cubic meters. Use the utility’s stated conversion. Write the unit beside every quantity, and do not mix an annual total with a monthly rate.
If estimating from irrigation flow, total gallons equal gallons per minute multiplied by actual minutes of operation. Add the zones and watering events. Account for changed emitters, manual watering and seasonal settings. This estimates delivered volume; it does not prove that all water reached the intended roots.
4. Calculate volume and percentage
Water saved = baseline use minus later use. Percentage saved = water saved divided by baseline use, multiplied by 100. The percentage is undefined when the baseline is zero.
| Illustrative annual irrigation record | Gallons |
|---|---|
| Comparable baseline | 60,000 |
| After establishment | 36,000 |
| Difference | 24,000 |
In this invented example, 24,000 divided by 60,000 gives a 40% reduction. It is an arithmetic demonstration, not a prediction. For a 1,000-square-foot converted area, that difference would be 24 gallons per square foot per year, provided the measured change genuinely belongs to that area.
5. Convert water savings to bill savings carefully
Use the actual rate structure. Fixed service charges may remain unchanged, and tiered prices mean not every gallon has the same cost. Check whether sewer charges depend on the same consumption measure before including them.
At a hypothetical flat variable rate of $5 per 1,000 gallons, 24,000 gallons avoided would reduce that charge by $120. This excludes fixed fees and assumes the rate remains unchanged. Comparing both periods at the same tariff helps separate consumption changes from price changes.
6. Separate design estimates from measured results
EPA’s Landscape Water Budget Tool supports site-specific landscape planning. A design budget can help evaluate a proposal, but it is not a meter reading or proof of later savings. Keep forecast, actual use and assumptions in separate columns.
Track the establishment period independently. A new landscape’s early watering may differ from its mature use. Do not discard an inconvenient first year or present a mature projection as already achieved.
7. Assess costs without overstating payback
Keep installation costs, confirmed rebates and annual maintenance costs. Simple payback divides net upfront cost by annual net savings only when those savings are positive and reasonably supported. It does not account for every future replacement, rate change or financing cost.
Confirm current incentive requirements directly with the responsible utility before starting work. Remove unsupported city-comparison tables from your planning assumptions: another program’s gallons saved, total acre-feet or rebate rate may refer to a different population and period.
Finish with a short record stating the area, dates, units, baseline, later use, adjustment method and limitations. That is more useful than claiming that all xeriscapes save the same percentage or cents per square foot.
