What Does ET-Based Scheduling Mean for California Lawns?

Introduction: the concept and why it matters in California

Evapotranspiration (ET)-based scheduling is a science-driven approach to landscape irrigation that calculates water needs from the actual water loss occurring in a landscape: evaporation from the soil plus transpiration from the plants. For California lawns, where climate variability, drought risk, and regulatory pressure are constant realities, ET-based scheduling is rapidly becoming the baseline for responsible, effective watering.
This article explains what ET-based scheduling is, how it is applied to turfgrass in California, why regulators and water agencies favor it, and exactly how homeowners, property managers, and landscape professionals can implement it to save water while maintaining healthy lawns.

What is evapotranspiration (ET)?

Evapotranspiration (ET) is the combined process of:

• evaporation: water loss from soil and surfaces, and
• transpiration: water loss through plant leaves.

ET is expressed as a depth of water (for example, inches or millimeters) over a period of time (daily or weekly). Reference evapotranspiration (ETo) is a standardized value that describes the evaporative demand of the atmosphere at a given location using a reference surface (usually well-watered grass). To estimate actual landscape water needs you multiply ETo by a crop coefficient (Kc) that represents the specific water use characteristics of the plant type — turfgrass has its own Kc value depending on species, growth stage, and season.

Why ET-based scheduling is especially important in California

California has highly variable climate zones, frequent drought periods, and strong policy incentives to use water efficiently. ET-based scheduling matters for several reasons:

• Precision: ET reflects real-time weather conditions (temperature, humidity, wind, solar radiation) so irrigation responds to actual demand rather than a fixed calendar.
• Water savings: ET-based schedules typically reduce overwatering by accounting for lower plant need in cool, cloudy, or humid periods and higher need in hot, dry spells.
• Turf health: Watering based on ET promotes deep, infrequent irrigation that encourages deeper roots and better drought resilience.
• Regulatory alignment: California’s landscape water-efficiency programs, local water agency requirements, and modern ordinances favor or require weather-based or sensor-based irrigation controllers and ET budgeting.
• Incentives and compliance: Many local water agencies offer rebates for smart controllers and require ET-based adjustments for large landscapes. Adopting ET scheduling can help avoid fines or restrictions during declared shortages.

How ET-based scheduling works in practice for lawns

The basic calculation

At a high level, an ET-based irrigation schedule is derived by:

1. Obtaining the reference ET (ETo) for your location and period (daily or weekly).
2. Multiplying ETo by the crop coefficient (Kc) for your specific turf species to get the turf evapotranspiration (ETc).
3. Subtracting effective precipitation and applying any permitted reductions (such as maintenance or conservation factors).
4. Converting the net irrigation depth into run time for each irrigation zone using the sprinkler application rate and system efficiency.
5. Programming a controller (or letting a smart controller) apply that runtime on an appropriate frequency that supports deep watering (multiple cycles or single long cycle depending on runoff risk).

Sources for ETo and required inputs

In California, common sources and inputs include:

• Local weather station networks, most notably CIMIS (California Irrigation Management Information System), which provides ETo data across many locations.
• Onsite weather sensors or smart controller data if you have a connected system.
• Turf Kc values: typical peak-season Kc ranges for turfgrass (approximate) are 0.6 to 1.0 depending on species and region — warm-season grasses often have lower peak Kc than cool-season grasses. Use local guidance for exact numbers.
• Soil type, slope, and sprinkler precipitation rate, which determine how to split runtimes and avoid runoff.
• Effective rainfall or reclaimed water credits where applicable.

Example (conceptual) calculation

• Weekly ETo from CIMIS = 1.4 inches.
• Turf Kc (cool-season peak) = 0.95 – ETc = 1.4 x 0.95 = 1.33 inches per week.
• Effective rainfall this week = 0.2 inches – Net irrigation need = 1.13 inches.
• Sprinkler application rate for a zone = 0.6 inches/hour – Required runtime = 1.13 / 0.6 = 1.88 hours per week ( 113 minutes).
• If you split into two watering days, program about 56 minutes per cycle, with appropriate soak cycles to prevent runoff on slopes or heavy soils.

Types of controllers and technologies for ET scheduling

• Weather-based (ET) controllers: these receive local ETo data (via station, internet, or onsite sensor) and calculate required runtimes automatically. Many controllers let you input plant factors and soil type.
• Soil moisture sensors: measure actual water in the root zone and can suspend irrigation when soil is wet enough. Best when paired with ET scheduling for redundancy.
• Smart controllers: combine weather forecasts, on-site sensors, and historical data to optimize watering and respond to rain events or watering restrictions.
• Basic controllers with manual ET-based programming: even a non-smart controller can follow ET schedules if the homeowner calculates required runtimes periodically (weekly or monthly) and updates programming.

Practical considerations and adjustments for California lawns

Soil, slope, and irrigation efficiency

Soil texture affects infiltration and the recommended cycle times:

• Sandy soils: high infiltration; longer run times but less risk of runoff.
• Loamy soils: moderate infiltration; typical run times.
• Clay soils: slow infiltration; shorter cycles and multiple soak intervals to prevent runoff.

Considerirrigation uniformity and system efficiency (most residential systems range 60-80%). Factor efficiency into runtime calculations by dividing irrigation depth by the efficiency coefficient.
Always measure sprinkler output with catch cups before finalizing a schedule — application rate directly affects run time calculations and water distribution uniformity.

Turf species and crop coefficient adjustments

Identify whether your lawn is cool-season (fescue, rye, bluegrass) or warm-season (bermudagrass, buffalograss, zoysia). Cool-season grasses tend to peak in water use earlier in spring and fall, while warm-season peak in the hottest months. Use the appropriate Kc seasonality:

• Establishment: newly seeded or sodded turf has higher water needs and needs a different schedule until roots establish.
• Dormancy or low-growth periods: reduce or suspend irrigation when turf is dormant, subject to local regulations.
• Recovery or stress: after disease, compaction, or renovation, adjust schedules to support recovery.

Seasonality and rainfall credits

California has strong seasonal variation. ET-based systems must be adjusted for winter rains and spring transitions. Most ET controllers have a rain sensor or a rain delay function and permit manual shutoff during significant rainfall. Keep records of effective rainfall credits if your water accounting or rebate program requires it.

Regulatory and policy context in California (what to expect)

California policy has moved toward quantifying outdoor water budgets and encouraging weather-based irrigation control. Local water agencies and municipalities may require or incentivize:

• ET-based or sensor-based controllers for new or retrofit installations.
• Water budgets based on ETo and landscape area for larger properties.
• Rebates for smart controllers, irrigation audits, and water-efficient upgrades.
• Enforcement measures during droughts, including mandatory restrictions and fines for wasteful irrigation such as runoff or excessive watering.

If you manage a commercial or multi-family property, expect more formal reporting and possible inspections. Residential customers should check local water agency rules for specific requirements and rebate opportunities.

Common mistakes and how to avoid them

• Blindly following a calendar schedule: calendar-only programming ignores weather changes and leads to over- or under-watering.
• Using average Kc values without local adjustment: microclimates, shade, and irrigation distribution can change actual needs.
• Ignoring system uniformity: poor sprinkler maintenance or clogged nozzles can produce turf stress even when total applied depth is technically adequate.
• Not accounting for runoff or soil infiltration: long continuous runs on clay soils can lead to runoff and wasted water; use cycle-and-soak.
• Failing to monitor and adjust: ET should inform schedules, but periodic observation (soil probe, visual turf health checks) must confirm that the schedule is delivering the intended results.

Step-by-step checklist to implement ET-based scheduling for your lawn

• Determine your local ETo source (CIMIS station closest to your site or your smart controller source).
• Identify turf species and estimate an appropriate crop coefficient (Kc) and seasonal adjustments.
• Measure sprinkler application rates and evaluate system uniformity with a catch-cup test.
• Calculate weekly irrigation depth from ET x Kc, subtract effective rainfall, and convert to controller runtimes accounting for system efficiency.
• Program an ET-based or smart controller to apply the calculated runtimes, using cycle-and-soak when needed.
• Install a rain sensor or soil moisture sensors for redundancy.
• Monitor turf health, watch for runoff, and adjust Kc or runtimes for shade, microclimates, and seasonal changes.
• Keep records of adjustments and any rain credits if required by your water provider.

Practical takeaways and final advice

• ET-based scheduling is not experimental or optional in California: it is the most practical and increasingly expected method to manage turf water use.
• The goal is to match supply to demand precisely: water enough to maintain healthy turf without waste.
• Invest time in measuring your system (application rate and uniformity) — accurate measurements are the single most important factor in an effective ET program.
• Use technology where it makes sense: smart controllers and soil sensors pay dividends in water savings and reduced manual adjustments, and may qualify for rebates.
• Make seasonal and situational adjustments: establishment, stress recovery, and drought restrictions require temporary deviations from normal schedules, but base decisions on ET-informed calculations.
• If in doubt, hire a certified irrigation auditor or landscape professional familiar with California conditions; many local water districts provide audits or lists of qualified contractors.

Adopting ET-based scheduling will help California homeowners and landscape managers conserve water, comply with evolving regulations, and maintain healthier lawns that tolerate heat and drought better. The effort to measure, calculate, and monitor is rewarded by predictable performance, lower bills, and reduced environmental impact.