A lysimeter is a device used to measure evapotranspiration, the rate at which plants and soil release moisture into the atmosphere. Strictly speaking, a lysimeter only collects or removes water or soil samples, rather than performing any calculations. There are a range of designs used in lysimeters, though there is some debate about exactly which designs should be given the name.
Evapotranspiration is part of the water cycle and is a combination of evaporation and plant transpiration. Evaporation is the same process as the way sweat disappears from our skin on a hot day. It refers specifically to liquid which evaporates from the soil, plus rainfall which lands on leaves and then evaporates before falling to the ground. Transpiration refers to moisture which is absorbed by a plant, usually through its roots, and then released into the air, mainly through leaves but also through branches or stalks.
One way of measuring evapotranspiration is through a pan lysimeter. This is simply a bucket-style container which is placed in the soil with its rim level with the ground surface. Ideally the container will be filled with the soil which was displaced as part of its installation. Knowing how much soil is in the container, a researcher can measure the water content of the soil, compare it with the rainfall levels, and calculate the rate of evapotranspiration.
A modern version of the lysimeter combines a porous ceramic cup and a tube to which allows samples to be collected from beneath the surface. This version of the lysimeter uses a vacuum to create the suction needed to withdraw the water. The pores on the cup will be small enough that only the water is removed, rather than any soil itself.
One interpretation of the word lysimeter, which is common in Europe, only refers to models which hold soil flush to the ground surface. Somebody using this strict definition would therefore not consider a suction cap-based model to be a lysimeter. This means there may be some circumstances when users need to clarify exactly what they mean by the term.
There are many uses for the data gathered through the use of a lysimeter. On a larger scale it can be used to measure changes in the water cycle, particularly where human activity may have disrupted it. On a smaller scale, it can provide valuable information to farmers about how well their soil is likely to retain moisture and to what degree it needs to be hydrated beyond natural rainfall.