Drainage flux measurement and errors associated with automatic tension-controlled suction plates

Automatic control of high tension soil solution samplers (suction lysimeters) in response to the surrounding soil matric potential has been proposed as a new method to reduce the convergence and divergence fluxes around the lysimeters. It is important to evaluate the instruments' performance with automatically regulated suction because the efficiency of the control and its effects on the flux volumes could vary with the surrounding soil matric potential. An automatic equilibrium tension lysimeter (AETL) system was developed composed of 16 small-area (572 cm(2)) ceramic suction plates that were continuously kept in equilibrium by an automated control device. The automated control device consisted of a datalogger connected to electronic tensiometers and to an electric vacuum pump that was activated to regulate the ceramic plate suction. The system was evaluated by simulating its performance with the HYDRUS-2D finite-element model. The AETL system continuously kept the lysimeters' suction in equilibrium with the surrounding soil when the soil water matric potential head ranged from -306 to 0 cm H2O. Suction control was less effective during low drainage conditions (soil matric potential head less than -50 cm H2O), when it took more time to re-equilibrate the pressure heads. However analysis of the variability of drainage and model simulations showed that incorrect suction control in drier conditions had a limited effect on the collected volumes. Suction was much better controlled when drainage flux was high, allowing a correct estimate of drainage. Automatic control of suction plates is a valuable tool for estimating drainage fluxes. Fine control of suction is needed when drainage flux is high because even the slight overapplication of suction, which is typically done during field experiments to overcome porous plate resistance and ensure that a sample is collected, could produce marked overestimation of water drainage (> 30%).