Abstracts

Cobos, D.R., Salinity Effects on Drainage Measurement with the Gee Passive Capillary Lysimeter Application Note, Decagon pp 1-4

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Corwin, D.L. 2000, 'Evaluation of a Simple Lysimeter-Design Modification to Minimize Sidewall Flow', Journal of Contaminant Hydrology, vol. 42, pp. 35-49.

A common criticism of many soil lysimeter designs has been the existance of artificial flowpaths alon the soil-wall interface. This artificial flow is referred to as sidewall flow. A simple lysimeter-design modification was evalutated that utilizes annular rings to divert sidewall flow near the soil surface into the soil column to minimize the occurrence of sidewall flow along the remainder of the column's length. A chloride-tracer experiment was used to evaluate the effectiveness of annular rings in minimizing sidewall flow in a mesoscale soil lysimeter (0.6 m in diameter and 1.83 m in height). The tracer-experiment data showed that even though sidewall flow may not have been completely eliminated it was reduced to an undetectable level based on chloride distributions and time domain reflectometry measurements. However, a delicate balance exists between minimizing sidewall flow and significantly altering the natural water-flow dynamics when using annular rings. The simple design modification provides a means of using a disturbed column of soil to evaluate models of solute transport, and to study preferential flow and contaminant mobility without concern for spurious data due to artificial flow along the soil-wall interface of the lysimeter.

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Gee, G.W., Ward, A.L., et al. 2002, 'A Vadose Zone Water Fluxmeter with Divergence Control', Water Resources Research, vol. 38, no. 8, pp. 10.1029/2001WR000816.

Unsaturated water flux densities are needed to quantify water and contaminant transfer within the vadose zone. However, water flux densities are seldom measured directly and often are predicted with uncertainties of an order or magnitude or more. A water fluxmeter was designed, constructed, and tested to directly measure drainage fluxes in field soils. The fluxmeter was designed to minimize divergence. It concentrates flow into a narrow sensing region filled with a fiberglass wick. The wick applies suction, proportional to its length, and passively drains the meter. The meter can be installed in an augured borehole at almost any depth below the root zone. Water flux through the meter is measured with a self-calibrating tipping bucket, with a sensitivity of ~4 mL tip^-1. For our meter this is equivalent to detection limit of ~0.1 mm. Passive-wick devices previously have not properly corrected for flow divergence. Laboratory measurements supported predictions of a two-dimensional (2-D) numerical model, which showed that control of the collector height H and knowledge of soil hydraulic properties are required for improving divergence control, particularly at fluxes below 1000 mm yr^-1. The water fluxmeter is simple in concept, is inexpensive, and has the capability of providing continuous and reliable monitoring of unsaturated water fluxes ranging from less than 1 mm yr^-1 to more than 1000 mm yr^-1.

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Gee, G.W., Zhang, Z.F., et al. 2004, Passive-wick Water Fluxmeters: Theory and Practice, SuperSoil 2004: Third Australian New Zealand Soils Conference, University of Sydney, Australia. pp. 1-9.

Improvements in vadose-zone water-flux measurements are needed for a variety of reasons, including better water-use management for agriculture, for turf-grass (e.g., golf course) operations, and for monitoring the ground disposal of wastes from mining and other industries. For such purposes, we have developed and tested passive-wick water fluxmeters under a wide range of conditions, from nonvegetated desert settings in the USA to irrigated tea plantations in Sri Lanka and rain-fed squash plantations in the South Pacific. In desert settings, the drainage was found to depend upon the precipitation distribution, the surface soil and the type and amount of vegetation. In Washington State, USA, bare sands and gravels drained up to 60% of the annual precipitation while fine soils did not drain. In wetter environments, drainage was found to be closely linked to the rate and duration of precipitation events. Design calculations with a 2-D model show how divergence can be minimized for a wide range of soil conditions under expected transient fluxes. Model results show that for sands, the operational range of the water fluxmeter is from a few mm/yr to well above 10,000 mm/yr, for both steady state and transient conditions, while for silts and clays, the range is more limited and best operates in the range above a few hundred mm/yr. Passive-wick water fluxmeters provide a reliable, robust, and relatively inexpensive method to assess the quantity and quality of drainage waters over a wide range of conditions.

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Guber, A.K., Pachepsky, Y.A., et al. 2006, 'Field-Scale Water Flow Simulations Using Ensembles of Pedotransfer Functions for Soil Water Retention', Vadose Zone Journal, vol. 5, pp. 234-247.

Using pedotransfer functions (PTF) to estimate soil hydraulic properties may be necessary in soil water flow simulations for large-scale projects or in pilot studies. The accuracy of a PTF outside of its development dataset is generally unknown. The existence of multiple models that are developed and tested in one region, but may perform relatively poorly in other regions, is also common in meteorology, where multimodel ensemble prediction techniques have been developed (i.e., those using an averaged prediction from several models) to address this problem. The objective of this work was to estimate the applicability of an ensemble of PTFs for water regime simulations. Measured soil water contents and pressure heads of 60 points at five depths in a 6-m transect of a layered loamy soil were collected during an extremely wet year in Belgium. Soil water fluxes were measured with passive capillary lysimeters at two depths. Water retention was measured in the laboratory on samples taken at 60 locations at three depths. Contents of soil textural fractions, organic matter content, and bulk density were averaged across the transect and used as input in the ensemble of 22 published PTFs developed from large datasets in different regions. The HYDRUS-1D software was used to simulate water content time series with (i) each of the PTFs from the ensemble and (ii) the laboratory-measured water retention data of each of the 60 locations. Simulations with the PTF ensemble had, on average, two times smaller errors those from using laboratory data. A possible explanation for this is that the PTF estimation gave substantially better approximations of field water retention than the laboratory data. The ensemble prediction appears to be a promising source of soil hydraulic properties to simulate soil water dynamics.

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Louie, M.J., Shelby, P.M., et al. 2000, 'Field evaluation of passive capillary samplers for estimating groundwater recharge', Water Resources Research, vol. 36, no. 9, pp. 2407-2416.

Passive capillary samplers (PCAPS), which sample water from the vadose zone via a hanging water column in a fiberglass wick, have shown potential to provide superior estimates of soil water flux compared to alternate methods. The objectives of this study were to evaluate the performace of PCAPS under natural rain-fed conditions concerning (1) their operational characteristics and (2) their ability to estimate soil water flux. Forty-two PCAPS were installed in 21 commercial agricultural fields in Lane County, Oregon. Monthly measurements of soil water flux and precipitation were recorded at each sity for the 4-year project duration. Of the 42 installed PCAPS, 12 samplers at six sites were inoperable or did not operate effieciently: 10 samplers were consistantly below the water table, which overflowed the collection vessels rendereing the samplers inoperable. Only two of the PCAPS exhibited technical failure resulting in unusually low collection efficiencies, thought to be due to a collapse of the collection vessel from oversuction during sample retrieval. On average, the 30 remaining PCAPS measured soil water flux 25% greater than that obtained from a water balance estimate. This discrepancy represents ~8% of the total annual precipitation and irrigation each site received. PCAPS collection efficiency was found to be significantly correlated (R² = 0.75) to the water balance yearly estimated recharge. The difference between PCAPS measured and water balance estimated percolation could be the result of inaccuracy in water balance evapotranspiration estimates and/or oversampling in the presence of high water tables. To estimate the main yearly recharge at each site with a 30% bound on the mean at at 0.05 confidence level, eight PCAPS are required. This number corresponds closely to the results of Brani-Dohrn et al. [1996a] and is thought to be due to intrinsic variability of percolation.

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Mertens, J., Diels, J., et al. 2007, 'Numerical Analysis of Passive Capillary Wick Samplers prior to Field Installation', Soil Science Society of America Journal, vol. 71, no. 1, pp. 35-42.

Accurately measuring water fluxes and associated nutrient or contaminant concentrations through the vadose zone is difficult because an appropriate suction needs to be exerted on the soil to sample water under unsaturated conditions. Passive capillary wick sampling systems are cheap and reliable instruments resulting in acceptable measurements of water fluxes in the vadose zone; however, their success in measuring realistic fluxes depends on their compatibility with the soil and climatic conditions in which they are installed. This study was developed in the preplanning phase of a field experiment with its main objective the monitoring of dissolved organic matter and the associated transfer of Cu²⁺ and pesticides through the vadose zone. We studied a combination of two-dimensional and axisymmetrical three-dimensional numerical analyses using the HYDRUS-2D software to identify what sampler geometry, wick type, wick length, and number of wicks are most suitable for the soil conditions at the experimental site. An AM3/8HI wick with seasonally varying wick length (40 cm in winter and 100 cm in summer) was found to be most appropriate for the soil and climatic conditions of the experimental field. The numerical analysis indicated that well-designed wick samplers had a negligible effect on the soil moisture content close to the sampler. A double-ring wick sampler is proposed to minimize the effect of the area between the installation pit or trench and the sampler. This approach is easily applicable and transferable to other soil and wick types and climatic conditions. The study emphasizes the suitability of numerical modeling to optimize experimental design before installation.

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