The Control of the Lower Boundary in Weighable Lysimeters Under Different Climatic Conditions.

Lysimeters are used as a representative and accurate tool for studying the water balance of ecosystems they are large enough to embody a representative volume for many process studies. However, while the upper boundary of a lysimeter is well in accordance with the natural conditions, the lower boundary forms always a rupture and modify both the soil water and temperature gradients compared to the surrounding natural soil and may lead to important bias errors. In most cases the lower boundary is formed by an open percolating gravel layer, which implies that downward flows only occur under saturated conditions, and upward flows are inhibited. Advanced lysimeter designs allow minimizing such disturbances by a lower boundary control (LBC) system at the base of the lysimeter. The water tension can be held to a reference state, which is recorded in the surrounding undisturbed soil, and the temperature at the lower boundary can be kept in equilibrium with the surrounding by a tube system.

A thorough comparison test with both controlled and uncontrolled (without LBC system) lysimeters carried out under humid conditions reveals only little differences between the different soil tension regimes. Under moderate climate conditions and with lysimieters of 2m length the dominant flow is downward, and the pressure difference in the lysimeters can only be observed at late summer, and limited to the lower parts. As a result, significant differences in evapotranspiration were found. The situation is however rather different under arid climates, and with reduced lysimeter depths. Under such conditions, the LBC system is inevitable to maintain a representative water gradient at the base of the lysimeter. Here, we further discuss the performance of controlled lysimeters installed in a Meditteranean savanna ecosystem (Majadas, Spain) and present first results of a comparison of methods for determining evapotranspiration (lysimiters, eddy covariance, canopy chambers, sap-flows).