MWELO Water Budget Calculator

Understanding MWELO

What triggers compliance: MWELO applies to all property types for new construction landscapes over 500 sq ft and renovated landscapes over 2,500 sq ft. This includes residential, commercial, HOA common areas, public parks, and institutional grounds.

Regulatory timeline: California adopted the original MWELO in 2009 and significantly updated it in 2015, lowering the ETAF cap for non-residential landscapes from 0.55 to 0.45. The January 2025 amendment further updated the formula and requirements. The state reviews the ordinance periodically.

Other states: Colorado, Nevada, Arizona, and several other western states have adopted or are developing landscape water efficiency frameworks modeled on California's MWELO approach.

Getting your site's official ETo: The California Irrigation Management Information System (CIMIS) maintains a network of weather stations that provide reference evapotranspiration data. Your local water agency can also provide the official ETo value for your zip code.

How Droughtless Can Help With MWELO

Droughtless includes water budget tracking that can be configured to work toward your MWELO goals, but it is not a purpose-built MWELO compliance tool.

Water budget enforcement: Set your MAWA as a budget constraint and the optimizer will schedule irrigation to stay within it.

Irrigation event logging: The system logs all irrigation events for your records, which you can use when documenting water use for your utility.

Weather-aware scheduling: The optimizer adjusts irrigation based on weather data, which naturally helps keep usage within budget during cooler months.

What is MWELO?

The Model Water Efficient Landscape Ordinance is a California state regulation that requires new and renovated landscapes to meet calculated water budgets based on plant types, irrigation efficiency, and local climate. It was originally adopted in 2009, significantly strengthened in 2015, and further amended in January 2025. It applies to all property types.

What is the Maximum Applied Water Allowance (MAWA)?

MAWA is the upper limit on annual irrigation water use for a landscape, calculated using the MWELO formula based on your landscape area, plant factors, irrigation efficiency, and local reference evapotranspiration. It represents the maximum amount of water you are allowed to apply to your landscape each year. Going over your MAWA can trigger penalties from your local water agency.

Does MWELO apply to existing landscapes?

MWELO applies to all property types for new construction landscapes over 500 sq ft and renovated landscapes over 2,500 sq ft. Some local jurisdictions have adopted stricter rules that may also apply to existing landscapes. For example, several water districts in Southern California require water budgets for all accounts regardless of when the landscape was installed. Check with your local water agency for specific requirements.

Methodology

This calculator implements the MWELO formula per CCR Title 23 and the January 2025 MWELO amendment.

MAWA = ETo × 0.62 × [(ETAF × LA) + (1.0 × SLA)]

ETAF = Plant Factor (Kp) / Irrigation Efficiency (IE)

This is a simplified per-hydrozone ETAF calculation. The full MWELO worksheet computes ETAF as a weighted average across all hydrozones. Irrigation efficiency values follow ASABE/ICC 802-2014: drip/micro at 0.81 and sprinkler heads at 0.75.

The 0.62 conversion factor converts inches of water per square foot to gallons (1 inch of water over 1 sq ft = 0.6234 gallons). ETo zones are approximate; for compliance purposes, use the official ETo value from your local water agency or CIMIS.

Related: ET Calculator · Crop Water Needs Calculator · Water Savings Calculator