Lysimeters are still a state-of-the art-device for determining Evapotranspiration or the fate of chemicals in soil/plant systems. Whereas the lower boundary condition of the simplest and most common type of lysimeters – the drainage lysimeter – allows no inflow and outflow only under atmospheric pressure (wet soil), the pressure head at the bottom of tension lysimeters is adjusted to that of the surrounding field at the same depth. This requires on the one hand accurate measurements of the water tension in the field and on the other hand the ability to maintain this value at the bottom of the Lysimeter.
Measurements of water tensions at some locations with agricultural use in Austria show that the values are well within tensiometer range in regions with humid climate, even at a depth of 1 m. This applies also for the more arid region with favorable soils, except for very dry years. However, in less favorable soils (A-horizon < 1 m, gravelly C-horizon) matric potential < -0.85×105 Pa was observed at a depth of 1.4 m for several months in many years.
Based on the in-situ-measurements of matric potential and water content on such a location and of knowledge of some soil hydraulic parameters from lab experiments, we performed numerical simulations in order to estimate the error in bottom flux of a drainage Lysimeter due to improper lower boundary condition and of a tension Lysimeter due to keeping the matric potential at -0.85×105 Pa during periods with lower values.
A tension Lysimeter with a depth of 2 m is very likely to yield accurate bottom fluxes at this site, but proper matric potential measurements in the surrounding field are no matter of course.