Frequently Asked Questions

What is Evapotranspiration?

Evapotranspiration (ET) is a measure of how much water is lost from the soil and plants into the atmosphere. It combines two processes:

1. Evaporation: Water evaporating directly from soil, grass, and other surfaces
2. Transpiration: Water vapor released by plants through their leaves

Think of ET as "water demand" - on a hot, sunny, windy day, more water evaporates and plants need more water. On a cool, cloudy, calm day, less water is lost.

Why does ET matter?

ET helps you make smart watering decisions:

• Irrigation management: Know exactly how much water your lawn or crops lost today
• Water conservation: Avoid overwatering by replacing only what was lost
• Plant health: Understand when plants are experiencing water stress
• Cost savings: Reduce water bills by watering efficiently

For example, if ET shows 0.25 inches today, that's how much water your lawn needs to replace through rainfall or irrigation.

How do we calculate ET?

We use the FAO-56 Penman-Monteith method, which is the international gold standard for ET calculation. This method was developed by the United Nations Food and Agriculture Organization (FAO) and is trusted by agricultural researchers, irrigation managers, and water agencies worldwide.

What weather data do we use?

Our calculation uses real-time data from your weather station:

• Temperature (high and low) - Drives evaporation rate
• Humidity (high and low) - Lower humidity = more evaporation
• Solar radiation - More sun = more energy for evaporation
• Wind speed - Wind carries away moisture, increasing evaporation
• Your location (latitude and elevation) - Affects solar intensity and air pressure

The calculation process

The Penman-Monteith method considers how energy and moisture move through the atmosphere:

1. Solar energy: We calculate how much sun energy is available to evaporate water, accounting for:

   • Time of year (affects sun angle)
   • Your latitude (affects day length and intensity)
   • Cloud cover (estimated from measured vs. expected solar radiation)
   • How much sunlight grass reflects (about 23%)

2. Atmospheric conditions: We determine how "thirsty" the air is by calculating:

   • How much moisture the air can hold at current temperatures
   • How much moisture is actually in the air (from humidity readings)
   • Atmospheric pressure (varies with elevation)

3. Wind effect: We account for how wind speed helps move moisture away from surfaces

4. Energy balance: We combine all these factors using the Penman-Monteith equation to determine the daily water loss in millimeters or inches per day

What assumptions does the calculation make?

To provide a standardized measurement, we make several scientifically-validated assumptions:

Reference crop (grass)

• We calculate ET for a standard grass reference crop - well-watered, 12cm (5 inch) tall grass
• This provides a baseline that works for lawns and can be adjusted for other crops
• Different plants (trees, vegetables, etc.) may use 50-150% of reference ET depending on the crop type

Daily timeframe

• ET is calculated as a daily total (inches or mm per day) that resets at midnight
• The displayed value shows the current day's accumulation so far - it builds throughout the day as temperatures rise, solar radiation accumulates, and conditions change
• ET values are most accurate late in the day (evening/night) when the full daily temperature range and solar radiation have been captured
• Early morning ET values will appear lower because the day's high temperature and peak solar radiation haven't occurred yet
• Soil heat storage is assumed to be negligible over a full day (balances out)

Standard height

• Wind speed is assumed to be measured at or adjusted to 2 meters (6.5 feet) height
• This is the meteorological standard height for wind measurements

Well-watered conditions

• The calculation assumes no water stress - that water is available for plants to use
• Actual evaporation may be lower in drought conditions when soil is very dry

Minimum wind speed

• We use a minimum wind speed of 0.5 m/s (1.1 mph) even if measured wind is lower
• This prevents calculation errors in completely calm conditions and accounts for local air movement

What do the ET values mean?

ET is reported in both millimeters per day (mm/day) and inches per day (in/day).

Important: The ET value you see represents today's total so far since midnight. For the most accurate daily total, check ET in the late evening when the full day's weather conditions have been captured.

Typical ranges (complete 24-hour period):

• Cool/cloudy days: 1-3 mm/day (0.04-0.12 in/day)
• Moderate conditions: 5-7 mm/day (0.20-0.28 in/day)
• Hot summer days: 8-12 mm/day (0.31-0.47 in/day)
• Very hot/windy/dry: Over 12 mm/day (>0.47 in/day)

What if I don't have a lawn?

The reference ET (grass) is still useful! Multiply by a crop coefficient to adjust for your specific plants:

• Turf grass: 0.8-1.0
• Vegetables (mature): 0.9-1.2
• Trees/shrubs: 0.5-0.9
• Flowers: 0.6-1.0

For example, if ET shows 0.30 inches and you have mature tomatoes (coefficient ~1.1), your plants need about 0.33 inches of water.

How accurate is the calculation?

Our implementation has been validated against official FAO-56 test cases with 0.8% accuracy. This means our calculations match the international reference standard to within 1%.

The accuracy of your ET values depends on:

• Sensor quality: Accurate temperature, humidity, solar radiation, and wind readings are essential
• Sensor placement: Sensors should be properly positioned per manufacturer guidelines
• Location settings: Correct latitude and elevation in your station settings

When might ET be unavailable?

ET calculation requires multiple weather parameters. You won't see ET values if your station is missing:

• Solar radiation sensor (required)
• Temperature readings (high and low)
• Humidity readings (high and low)
• Wind speed data
• Location coordinates (latitude) or elevation

Additional resources

For more technical details about the Penman-Monteith method:

• FAO Irrigation and Drainage Paper 56 - Official methodology
• FAO Chapter 2: Penman-Monteith equation - Equation details
• University of Florida: ET Calculation Guide - Step-by-step methodology

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Need help? If you have questions about your ET readings or want to learn more about using ET for irrigation scheduling, contact our support team.