Controlled‐Suction Period Lysimeter for Measuring Vertical Water Flux and Convective Chemical Fluxes

Unsaturated infiltration of rain and irrigation water and convective chemical transport processes in the vadose zone are poorly understood. This is partly due to the traditional techniques used to sample unsaturated soil water. This study evaluated the performance of a recently developed controlled‐suction period lysimeter in the field for more than 400 d. The lysimeter consists of two tensiometers and a porous plate connected to a suction system. The soil matric pressures immediately above the horizontally buried porous plate, and at the same depth in the natural soil profile, are measured at 3‐s intervals. The water extraction period is controlled so that the readings of the two tensiometers match. The lysimeter was installed at a depth of 30 cm in a sparse forest. The soil was sandy loam classified as Cambisol. The lysimeter maintained the soil moisture condition in the sampling profile similar to that in the natural soil profile. The mean absolute difference between the two tensiometers was 4.1 cm, with the mean relative difference of 3.5%. Water extraction by the lysimeter did not cause appreciable convergence or divergence in the soil water flow. The water loss due to evapotranspiration (ET) estimated from the water‐sampling rate was similar to the evaporation rates measured using water‐filled pans and the ET rate estimated by the Thornthwaite method, considering the reduction because of drought. In addition to the water flux, convective fluxes of dissolved silica, nitrate, and ammonium were quantified. As a result, the controlled‐suction period lysimeter has great potential to provide quantitative information on water and solute transport in the vadose zone.