What Does Evapotranspiration Mean For Michigan Irrigation Schedules

Evapotranspiration is the combined loss of water from the soil surface by evaporation and from plants by transpiration. For Michigan growers, landscapers, and turf managers, understanding evapotranspiration (ET) is the cornerstone of efficient irrigation scheduling. This article explains ET in practical detail, describes how to convert weather-based ET into irrigation decisions, and gives concrete examples and rules of thumb tailored to Michigan climate zones, soils, and crop types.

What evapotranspiration actually measures

Evapotranspiration is expressed as a depth of water per unit time, typically inches per day or millimeters per day. It is a demand-driven measure: how much water the atmosphere and plants together remove from the soil-plant system given current weather and plant conditions.
Evapotranspiration has two conceptual parts:

• Evaporation: water lost directly from exposed soil, mulch, and wet surfaces.
• Transpiration: water vapor emitted by plants as they exchange gases during photosynthesis and cooling.

Reference evapotranspiration, ETo, represents the ET from a well-watered reference surface (usually short grass) under given weather. Crop evapotranspiration, ETc, is the actual ET for a specific crop and is computed as ETc = ETo * Kc, where Kc is the crop coefficient that depends on crop type and growth stage.

Why ET matters for Michigan irrigation scheduling

Michigan is a humid continental state with strong seasonal swings. Spring and fall have lower ET; summer months have the highest. Precipitation is not distributed evenly, and soil texture varies widely across the state, from sandy soils in western Lower Peninsula to loams and clays elsewhere. ET-based scheduling allows irrigation to match plant water demand rather than fixed calendar schedules, yielding water savings, healthier plants, and reduced runoff and nutrient loss.
Key reasons to use ET for scheduling:

• It quantifies actual crop water need based on weather rather than guesswork.
• It allows you to calculate when to irrigate (interval) and how much to apply (depth).
• It integrates rainfall, soil storage, and crop growth stage to avoid under- or overwatering.

Core components of an ET-based schedule

To translate ET into an irrigation schedule you need:

• ETo data from a local weather station or regional service appropriate to your site.
• An appropriate crop coefficient, Kc, for the crop or turf and its growth stage.
• Soil information: root zone depth, available water capacity (AWC or PAW, inches of available water per inch soil).
• Management parameters: allowable depletion fraction (how much of the available water you will let plants use before irrigating), and system application efficiency or distribution uniformity.
• Recent effective rainfall (the portion of rainfall that actually replenishes plant root zone).

Practical calculation steps

1. Obtain local ETo in in/day for the period of interest (daily or weekly average).
2. Choose Kc for the crop and growth stage and compute ETc:
   ETc (in/day) = ETo (in/day) * Kc.
3. Determine root zone depth and available water per inch of soil. Total available water (TAW) = root zone depth (in) * PAW (in water per in soil).
4. Decide allowable depletion (fraction of TAW). Readily available water (RAW) = TAW * allowable depletion.
5. Irrigation trigger interval (days) = RAW / ETc.
6. Gross irrigation depth required when irrigating = RAW / irrigation efficiency.
7. Convert gross depth to run time using sprinkler or drip application rate.

Example: A turf with root zone 12 in, PAW 0.12 in/in, allowable depletion 50%, ETo = 0.20 in/day, Kc = 0.85, irrigation efficiency 75%:

• ETc = 0.20 * 0.85 = 0.17 in/day.
• TAW = 12 * 0.12 = 1.44 in.
• RAW = 1.44 * 0.50 = 0.72 in.
• Interval = 0.72 / 0.17 = 4.2 days.
• Gross irrigation = 0.72 / 0.75 = 0.96 in.
• If sprinklers apply 0.48 in/hr, run time = 0.96 / 0.48 = 2 hours.

Michigan-specific considerations

Regional ETo and Kc behavior:

• Peak ETo in Michigan typically occurs in June-July and then drops in August-September. Daily ETo at peak often ranges from about 0.12 to 0.25 in/day depending on region and cloud cover; monthly averages are more useful for planning.
• Kc values vary by crop: turf and vegetables tend to have Kc in the 0.7-1.0 range at peak; corn and annual crops can reach Kc above 1.0 at mid-season (0.9-1.15 typical); orchards, vineyards, and perennials have lower Kc early and moderate at full canopy.

Soil texture and rooting depth:

• Sandy soils have low PAW per inch and shallow TAW. They require smaller, more frequent irrigations and a lower allowable depletion (for sensitive crops) to avoid stress.
• Loam and clay loam soils hold more water per inch and allow longer intervals between irrigations but have slower infiltration rates.
• Rooting depth varies with crop: turf typically 6-12 in, field crops 12-24 in or deeper, trees 24-40 in effective zone. Use realistic root zone depth to compute TAW.

Seasonal timing:

• Spring recharge from rainfall often satisfies early-season needs; irrigations typically become necessary in late May through September as ET outpaces rainfall.
• Critical growth stages (grain fill for corn, fruit set for orchards, high-quality turf season) require stricter allowable depletion thresholds.

Tools and data sources to use in Michigan

• Local weather station networks and university extension services provide ETo estimates and crop coefficients calibrated for regional conditions. Use stations with similar elevation and proximity when possible.
• Soil moisture sensors, tensiometers, and capacitance probes allow on-site verification of soil water status and can replace or validate ET-based triggers.
• For managed turf and landscape, automated controllers that accept ETo or ETc inputs can automate schedule adjustments throughout the season.
• Remote-sensing and evapotranspiration maps can show spatial variation across a farm, highlighting areas that need targeted irrigation or soil amendments.

Irrigation system considerations

Scheduling must be tied to what the system can deliver:

• Application rate: Know how many inches per hour your system applies. Run time = required gross depth / application rate.
• Distribution uniformity (DU): If DU is low, some areas receive less water. Either increase run time to meet the driest areas or improve system uniformity.
• Efficiency: Drip and microsprays often have higher efficiency than overhead sprinklers. Account for efficiency when calculating gross depths.
• Phased watering: For long run times that would cause runoff, break the irrigation into multiple shorter cycles separated by soak-in periods.
• Timing: Irrigate early morning when evaporation losses are lowest and to reduce disease risk compared with late evening irrigation.

Sample irrigation schedules and examples

Example 1: Home lawn in southern Lower Peninsula in July

• ETo = 0.22 in/day, Kc for well-established cool-season turf = 0.85.
• ETc = 0.187 in/day. For a 6 in root zone with PAW 0.10 in/in, TAW = 0.6 in, RAW at 40% allowable depletion = 0.24 in.
• Interval = 0.24 / 0.187 = 1.3 days. This means short, frequent irrigations every other day or so if you allow only 40% depletion, or stretch to 3-4 days if you permit 50% depletion and have deeper roots.

Example 2: Corn at mid-season in central Michigan

• ETo = 0.20 in/day, Kc = 1.05.
• ETc = 0.21 in/day. For a 24 in root zone with PAW 0.12 in/in, TAW = 2.88 in, RAW at 50% = 1.44 in.
• Interval = 1.44 / 0.21 = 6.9 days. So irrigate about once a week, applying gross depth = 1.44 / efficiency (say 80%) = 1.8 in.

These examples illustrate how crop, root depth, and management choices change how frequently and how much you should irrigate.

Practical takeaways and recommendations

• Start with local ETo data and an appropriate Kc for your crop. If local weather data are unavailable, use a nearby station but verify with soil sensors.
• Know your soil. Measure or look up PAW and set realistic root zone depth. Small errors in root zone depth can change intervals substantially.
• Choose an allowable depletion based on crop sensitivity: 30-40% for high-value, stress-sensitive crops; 40-60% for turf and many field crops; lower during critical growth stages.
• Factor system efficiency and distribution uniformity into gross irrigation depth calculations. Fix low DU rather than overwatering the entire field.
• Use a combination of methods: ET-based schedules for planning, validated by soil moisture sensors for on-the-ground decisions.
• Adjust for rainfall: subtract effective rainfall from RAW before irrigating. Not all rainfall is effective; light storms or high-intensity storms with runoff may not fully replenish the root zone.
• Maintain records: daily ETc, rainfall, irrigation events, and observed crop response. Over one season, records help refine Kc and management decisions for the following year.
• For landscapes, consider grouping zones by crop type, soil, and sun exposure so ET-based controller adjustments are meaningful and efficient.
• Late-season irrigation: reduce or stop irrigation to allow plants to harden off before frost, except where autumn moisture is needed for root growth or where shallow-frozen soils could cause desiccation in evergreens.

By translating evapotranspiration into an operational schedule that accounts for crop coefficients, soil water capacity, system performance, and local weather, Michigan irrigators can save water, reduce costs, and maintain plant health. ET-based scheduling is not a single formula but a framework: collect good local data, apply the calculations shown here, validate with sensors, and adjust management thresholds by crop and risk tolerance.