Lysimeter ET & Evapotranspiration by Water Balance

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.