The composition and quantity of leachate as it moves down through the vadose zone is seldom measured directly because sampling in this unsaturated zone at the depths required has proven to be extremely difficult. A promising technique is the use of large porous plates known as equilibrium tension plate lysimeters (ETPLs), which have a controlled suction exerted on them that mimics the soil matric potential measured in the surrounding undisturbed soil profile. In the design phase for the installation of 15 ETPLs at five different depths (three replicates) around a central access chamber in the vadose zone in the Lake Taupo catchment of New Zealand, questions arose regarding the effects of the design and installation layout of the ETPLs on the measured leachate fluxes. To investigate the important design criteria and the spacing of the ETPLs, a numerical investigation using the HYDRUS-2D software was conducted. This analysis showed that the distance between the tension plate and the base (lower boundary) of the ETPL needs to be large enough to ensure that the effect of the dry zone (rain shadow effect) created below the ETPL on the soil zone being sampled above the plate is minimized. The boundary condition between the ETPL and the central access chamber was also shown to be critical, requiring a free-drainage condition in this location. The sampling efficiency of the ETPLs increases when the horizontal distance between adjacent ETPLs increases. Less obvious was the result that the sampling efficiency decreases with increasing vertical distance. This work demonstrates that the design and installation configuration of the ETPLs can significantly impact sampling efficiency. The relationships demonstrated in this work must be recognized and implemented in the design process, particularly when multiple ETPLs are installed at various depths through the vadose zone at other locations.

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