Lysimeter ET & Evapotranspiration by Water Balance
Measures rainfall
Enter rainfall, irrigation, the lysimeter weight change and its surface area to get evapotranspiration: ET = rainfall + irrigation − (weight change ÷ area), plus the ET per day.
Enter your lysimeter readings
Next: use 4.7 mm/day as your reference crop water use — multiply by your field area to schedule replacement irrigation, and recheck the lysimeter each growth stage as ET climbs toward peak canopy.
Water balance: ET = (rain + irrigation) − Δstorage; Δstorage = weight change ÷ area; 1 kg/m² = 1 mm.
Lysimeter ET — key facts
- ET
- rain + irrigation − (Δweight ÷ area)
- ET per day
- ET ÷ days in the period
- Inputs
- rainfall + irrigation (mm)
- Weight change
- water lost from the block
- Area
- converts mass to depth
- Measures
- actual evapotranspiration
- Reference method
- most accurate field ET
- Privacy
- Runs in your browser; nothing uploaded
Weigh the soil block and the water it lost is the ET
A weighing lysimeter is the gold standard for measuring how much water a crop actually uses. Everything that enters — rainfall and irrigation — minus the change in stored water leaves as evapotranspiration. The trick is the weight change: dividing the mass of water lost by the lysimeter's area turns it into a depth in millimetres, so the whole balance lands in the same units and the answer is the real water demand of the crop.
This tool gives the ET in mm, the ET per day and a clear read on your inputs from rainfall, irrigation, weight change and area. Use it to schedule irrigation, derive crop coefficients and verify weather-based ET estimates. Pair it with the Groundwater Balance, Drip Zone Scheduling and Irrigation Set Volume tools for a full water plan.
Measure real ET
Actual crop water use, not an estimate.
Get mm per day
The daily rate for scheduling irrigation.
Derive crop coefficients
Compare against reference ET for the period.
Verify weather models
Check field ET against estimates.
Frequently Asked Questions
How is ET calculated from a lysimeter?+
By a water balance on the soil block: everything that comes in minus the change in stored water equals what left as evapotranspiration. ET = rainfall + irrigation − (weight change ÷ area), where the weight change is the loss in stored water converted to a depth by dividing by the lysimeter surface area. Drainage, if collected, is also subtracted; the result is the actual water the crop and soil used over the period.
What is a weighing lysimeter?+
It is a large container of soil and growing crop set into the field on a sensitive scale, isolated so all water flows in and out can be accounted for. Because it sits on a balance, the change in its mass over a period is a direct measure of water gained or lost. It is the reference method against which other ET estimates are checked.
Why divide the weight change by the area?+
Weight change is a mass of water, but ET is expressed as a depth (mm). Dividing the mass of water lost by the lysimeter's surface area converts that volume into an equivalent depth spread over the area. A 1 mm depth over 1 m² is 1 litre, so the area is what turns kilograms of water into millimetres of ET.
What is ET per day?+
It is the evapotranspiration divided by the number of days the period covers, giving a daily rate in mm/day. A weekly total of 35 mm is 5 mm/day. The daily rate is the most useful figure for scheduling irrigation and comparing crop water demand across seasons and weather.
What units should I use?+
Enter rainfall and irrigation as depths in mm, the weight change as a mass (kg) and the area in m². The tool converts the weight change to mm using the area, then sums everything to ET in mm. Keep depth inputs in mm and the result is in mm; divide by days for mm/day.
Does this measure actual or reference ET?+
A lysimeter measures actual evapotranspiration — the real water used by the specific crop and conditions inside it. That differs from reference ET (a standard grass surface) and from potential ET. To get a crop coefficient you would compare this measured actual ET against a reference ET for the same period.
What about drainage and runoff?+
A well-designed lysimeter captures any water that drains through the bottom and prevents runoff, so the balance closes cleanly. If your setup collects drainage, subtract it as an outflow along with the weight change. For a sealed weighing lysimeter with no drainage over the period, the weight change alone captures the stored-water change.
How often should I read the lysimeter?+
Daily readings give the cleanest daily ET, but readings every few days or weekly still work — you just divide by the number of days in the interval. Read at the same time of day each time to keep dew and timing effects consistent, and pair each reading with the rainfall and irrigation applied in that interval.
Can negative ET happen?+
ET should be positive over a day, but a short interval with heavy rain or dew and little evaporation can show the soil block gaining water, which the balance reports as near-zero or slightly negative ET. That usually means the interval was too short or a reading was off; lengthen the period or check the inputs and the figure settles to a sensible positive rate.
Are the figures precise?+
They are exact for the numbers you enter, and a weighing lysimeter is the most accurate field method there is. Real accuracy depends on a clean water balance — capturing all inflows, preventing leaks and runoff, and weighing precisely. Treat the result as a high-quality measurement when the lysimeter is well maintained.