Evapotranspiration (ET) is the combined process of water evaporating from the soil and transpiring through grass and plant leaves. In simple terms, it measures how much moisture your lawn naturally loses to the atmosphere due to temperature, sunlight, wind, and humidity. The U.S. Geological Survey (USGS) explains that evapotranspiration is a critical part of the water cycle and directly influences soil moisture levels (https://www.usgs.gov/special-topics/water-science-school/science/evapotranspiration-and-water-cycle).
In Minnesota, evapotranspiration rates vary significantly throughout the growing season. According to the University of Minnesota Extension, cool-season turfgrasses (like Kentucky bluegrass, common throughout Minnesota lawns) typically require about 1 to 1.5 inches of water per week during peak summer growth, depending on weather conditions (https://extension.umn.edu/lawn-care/watering-established-lawns).
The Minnesota Department of Natural Resources (DNR) also publishes statewide climate data showing that evapotranspiration rates increase during hot, dry, and windy periods — particularly in July and August when lawns experience the highest moisture demand (https://www.dnr.state.mn.us/climate/index.html).
When temperatures rise and humidity drops, ET increases — meaning your lawn loses water faster and may require longer irrigation runtimes. During cooler or rainy periods, ET decreases, and watering needs drop.
Modern EPA WaterSense-labeled weather-based irrigation controllers use local ET data and weather patterns to automatically adjust watering schedules, preventing overwatering and conserving water (https://www.epa.gov/watersense/weather-based-irrigation-controllers).
Why This Matters for Minnesota Lawns
Because Minnesota primarily has cool-season turfgrass, improper watering can quickly lead to shallow root systems, disease pressure, or drought stress. Watering based on evapotranspiration allows you to:
- Deliver the correct amount of water based on real weather conditions
- Promote deeper, healthier root growth
- Reduce runoff and nutrient leaching
- Lower water costs
- Protect your landscape during peak summer stress
At West Irrigation, we design and program smart irrigation systems that adjust to Minnesota’s seasonal evapotranspiration rates — ensuring your lawn gets exactly what it needs throughout the growing season.