An Automated Vacuum Extraction Control System for Soil Water Percolation Samplers

Duke et al. (1970) developed a mercury manometer and electrode-based controller for a soil water sampler A vacuum applied to soil water percolation samplers permits collecsystem. When coupled with a soil tensiometer, the detion of both macroand matrix-pore liquids. Performance of these field samplers is improved when the extraction vacuum is adjusted in vice automatically adjusted vacuum levels supplied to accordance with the tension in the surrounding soil. This is particularly the sampler tension plate in relation to in situ soil water important when monitoring a network of spatially distributed samplers potential. In field tests, the manometer system proved and for samplers installed in medium to fine textured soils. We deto be impractical owing to its excessive maintenance signed a vacuum extraction system to more efficiently collect vadoserequirements, and a manual system was substituted zone soil solution samples. A single vacuum pump, vacuum tank, and (Duke and Haise, 1973). More recently, Brye et al. air dryer provided a vacuum supply for 12 soil water sampling sites via a branching polyethylene pipe network. A vacuum controller con(1999) deployed a rectangular equilibrium-tension samtaining two inexpensive pressure transducers, a voltage regulator, pler with 2.5-cm-high sidewalls. The researchers manurelay, and solenoid valve was developed and tested for field installaally adjusted sampler suction to match the soil water tion. Data loggers operated the controllers, monitored extraction vacpotential of the surrounding bulk soil, which they meauum and ambient soil water potential, and adjusted relative vacuum sured with two heat-dissipation sensors. Brye et al. at each percolation sampling site. The automated vacuum controllers (1999) concluded that, while an automated samplersuccessfully maintained sampler extraction pressures at levels proportional to ambient soil water potential and provided the added benefit suction control would have been an added refinement, of recording the pressure values for use in subsequent data interprethe manual suction adjustment procedure was adequate tation. for their field conditions because soil water potentials at the 1.4-m sampler depth tended to vary slowly. Natural rainfall supplied their field water inputs. I designed appropriately, tension samplers collect We installed vacuum extraction soil water percolation macropore and matrix-pore soil water from a known samplers in a medium-textured soil at the 1.2-m depth cross-sectional area and provide a means of measuring to intercept and sample both macroand matrix-pore pore-water solute concentration and estimating downliquids in furrow-irrigated soils. Furrow irrigations supward water flux (Duke and Haise, 1973; Wilson et al., plied from 2 to 5 cm of water in a single 12to 24-h 1995). For a tension sampler to collect water moving irrigation and were repeated on 10to 14-d intervals downward in response to a soil-water tension gradient, during the growing season. These inputs produced greater the sampler’s collecting surface is oriented horizontally and more rapid changes in soil water content than may and the applied tension is adjusted to match in situ soil occur under natural rainfall. We wished to automatically conditions. If the in situ soil water tension is different adjust sampler extraction vacuum to in situ soil water from the suction applied to fixed-tension sampler, it can cause water-flow convergence or divergence near the tension conditions at each sampling site. intake membrane, which produces errors in measured Initially, a mercury manometer switch was used to water flux (Cochran et al., 1970; Haines et al., 1982). control extraction vacuum, with furrow inflow-end samThis problem may be lessened if tension samplers are plers maintained at a lower extraction vacuum than designed with side walls. The soil volume confined in outflow-end samplers. However, operating the manomsamplers with sidewalls is thought to reduce the influeter was inconvenient because pressure switch settings ence of varying soil water potential on water flow were difficult to adjust and lacked precision, water conthrough the sampler opening (Corey et al., 1982). If taminated the mercury and compromised electrode tension sampler sidewalls extend upward through the function, and the power supply transformer failed. Like soil profile too far, however, they may interfere with Duke and Haise (1973), we decided that the mercury subsurface lateral flow processes in some soils and bias manometer-base system required excessive maintenance measured downward fluxes and leachate solute concenand was unreliable. trations. Annual percolation fluxes measured with Our objective was to design and field test an autowalled fixed-tension samplers differed from tile-drainmated vacuum controller that was capable of tracking flow values by 0 to 33% (Montgomery et al., 1987). ambient soil water tension near the soil water sampler Hergert (1986) reported that average annual percolaopening, and adjusting sampler vacuum to a user-specition fluxes measured from fixed-tension samplers differed from water-balance values by 3 to 72%. fied target value. This new system needed to be more reliable and require less maintenance than the mercury manometer systems. USDA, ARS, Northwest Irrigation and Soils Research Laboratory, 3793 N 3600 E, Kimberly, ID 83341. Received 5 Dec. 2001. *Corresponding author ([email protected]). Abbreviations: PVC, polyvinyl chloride; SDF, sampler drainage fraction. Published in Soil Sci. Soc. Am. J. 67:100–106 (2003).