Landscape water budget calculator

The state Model Water Efficient Landscape Ordinance (MWELO) applies to new construction with a landscape area of 500 square feet or more that needs a building or landscape permit, plan check or design review, and to rehabilitated landscapes of 2,500 square feet or more that need the same. Projects of 2,500 square feet or less can instead follow the prescriptive checklist in section 492. Cities can adopt stricter local versions, so the permit desk has the final word.

The one in the ordinance: each zone contributes its plant rating divided by irrigation efficiency, times area, times annual reference evapotranspiration, times 0.62, in gallons per year. The budget cap multiplies reference evapotranspiration by 0.62 and by 0.55 of the regular landscape area for homes (0.45 for non-residential), plus areas permanently and solely dedicated to edible plants counted in full (recycled-water areas and park-type recreational areas count the same way, but private single-family residential landscapes are never recreational areas under the definition). The comparison is estimated use against the cap.

From WUCOLS, the state plant water use classification: very low is under 0.10, low is 0.10 to 0.30, moderate is 0.40 to 0.60 and high is 0.70 to 0.90. This calculator uses the midpoint of each range. For a permit submission, look up your exact plant list in WUCOLS and use zone-specific factors.

Because the ordinance sets them, not the equipment vendor: 0.75 for spray heads and rotors, 0.81 for drip. Using drip raises the efficiency divisor, which lowers the computed water use for the same plants and area. That is why switching zones to drip often brings a project under the cap.

It gives the same arithmetic reviewers check first, so it is the right place to test a design before drawing anything. A full submission also needs the worksheet in the required format, plant lists, an irrigation design and other documents your city lists. Treat this page as the fast check, not the paperwork. The state worksheet itself, Appendix A filled in from your zones and plant factors, is a $39 PDF on this site. Get the filled worksheet

Plant choice is the strongest lever in the formula. Moving a zone from high water (0.8) to low water (0.2) cuts that zone to a quarter of its use before any irrigation change. Area is the second lever, irrigation type the third.