Variable-suction multicompartment samplers to measure spatiotemporal unsaturated water and solute fluxes

To better understand the movement of water and solutes in soils, and the risk of groundwater contamination, we need water and solute flux observations distributed in space and time. We designed a new variable-suction multicompartment percolation sampler that can be buried below an undisturbed soil volume in the field. The instrument collects percolate from 100 cells within a 32.5- by 32.5-cm area. Drop counters record the number of drops falling into the sample collectors of all cells. The cells are covered by a porous material to which suction is applied in accordance with nearby tensiometer readings. The collected water can be repeatedly extracted in situ, allowing the breakthrough curve of each cell to be measured. This temporal and spatial resolution greatly helps to quantify heterogeneous flow at scales between that of the individual sampling cell and the entire sampler. Three prototypes were tested in different laboratory (under a soil monolith) and field experiments (buried in situ) in Australia and the Netherlands. One sampler was covered with a nylon mesh, another with sintered porous stainless steel plates, and a third with a polyamide membrane. Water was uniformly applied at the soil surface by irrigation (laboratory) or natural rainfall (field). Suction could be controlled once the covers were saturated, blocking the inflow of air. The instruments operated continuously for several months. The membrane and metal covers worked best. The wealth of data obtained from a single experiment provides considerable insight in water and solute transport processes in undisturbed soils