A drive point application of the Guelph Permeameter method for coarse-textured soils W. Daniel Reynolds a, ⁎, Jeffrey K. Lewis b a Agriculture and Agri-Food Canada, Greenhouse and Processing Crops Research Centre, 2585 County Road 20, Harrow, Ontario, N0R 1G0, Canada b Swedish Defence Research Agency, Cementvägen 20 SE–901 82 Umeå, Sweden abstract article info Article history: Received 16 December 2011 Received in revised form 3 April 2012 Accepted 14 April 2012 Available online 23 May 2012 Keywords: Guelph Permeameter Drive point Hydraulic conductivity Sorptive number Vadose zone HYDRUS-2D The Guelph Permeameter (GP) is a widely used well/auger-hole method for in-situ determination of field- saturated hydraulic conductivity (K fs ) and sorptive number (α*) in the vadose or unsaturated zone. Its appli- cation can be difficult, however, in gravelly materials which are problematic to auger, and in un-cohesive sandy materials which collapse during the augering process or upon wetting of the unlined well. We circum- vented these issues, by replacing the GP well with a conically tipped well screen or “drive point” which is pushed, driven, rotated or vibrated to the desired depth. We also developed a steady flow analysis to account for the fact that water discharge through the drive point screen is solely radial, rather than both radial and vertical as in the original GP method. The HYDRUS-2D numerical simulation model was used to determine appropriate shape values (C DP ) for the drive point analysis; and an empirical regression equation was devel- oped to convert the discrete C DP values into continuous shape functions for porous materials with negligible- moderate, strong and very strong capillarity. In a well-sorted medium-coarse sand, the drive point method produced K fs and α* values which were plausible and consistent with other data from the field site. The orig- inal GP method, on the other hand, under-estimated the drive point results by factors of 2.1–3.1, which may have been caused by progressive collapse of the unlined GP well, and/or gradual sinking of the GP outflow tip into the unprotected well base. It was concluded that the drive point GP method can provide convenient and accurate estimates of K fs and α* in materials where hydraulic properties are not substantially altered by the disturbance of drive point installation. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. 1. Introduction The Guelph Permeameter, or “GP” (Reynolds, 2008; Reynolds and Elrick, 2002; Soilmoisture Equipment Corp., 2008), is a popular well/ auger-hole method for in-situ measurement of soil hydraulic parame- ters. In essence, the technique involves augering an unlined “well” into the vadose or unsaturated zone, ponding one or more constant heads of water in the well, and measuring the steady three-dimensional dis- charge of water out of the well into the surrounding unsaturated soil. From the ponded head and discharge data, estimates of field-saturated soil hydraulic conductivity, K fs , and sorptive number, α*, can be obtained. The GP theory is applicable to all types of unsaturated porous media, however, practical difficulties can arise in gravelly materials where augering can be problematic (e.g. coarse glacial tills, outwash mate- rials), and in un-cohesive materials where the well collapses during the augering process or upon wetting (e.g. clean, well sorted sands). Al- though well collapse can be avoided by inserting a removable “well screen” or by back-filling pea-gravel or coarse sand around the GP outflow tube (Reynolds, 2008), these procedures are both time- consuming and impractical for wells deeper than about 1–2 m. One convenient and rapid way to circumvent the above limitations is to form the well using a “drive point” casing rather than an auger. Here, a stout pipe with a screened section and a solid basal “drive point” is pushed, driven, rotated or vibrated into the gravelly and/or un- cohesive material to the desired depth (Fig. 1). The GP outflow tube is then simply inserted into the drive point casing; or with some equip- ment modifications, the drive point casing itself can serve as the outflow tube to produce a “drive point” GP. Similar installation procedures are often used in the “porous probe” (e.g. Daniel, 1989), “gravel per- meameter” (Miller et al., 2011), and “cone permeameter” methods (Gribb et al., 2004) for measuring K fs and other hydraulic properties. The porous probe and gravel permeameter methods are of questionable accuracy, however, because they employ saturated flow analyses which are inappropriate for the vadose zone. Although the cone permeameter applies a rigorous and appropriate unsaturated flow analysis, the fragil- ity of the apparatus often dictates careful installation through the base of a pre-drilled hole (e.g. Homma et al., 2010). In addition, cone per- meameter measurements can be complicated and time-consuming, and data analysis requires sophisticated numerical inversion procedures (Gribb et al., 2004) which can produce ambiguous results (Homma et al., 2010). The proposed drive point GP method avoids the above issues by combining unsaturated flow theory with simple data analysis Geoderma 187-188 (2012) 59–66 ⁎ Corresponding author. Tel.: + 1 519 738 1265; fax: + 1 519 738 2929. E-mail address: dan.reynolds@agr.gc.ca (W.D. Reynolds). 0016-7061/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2012.04.004 Contents lists available at SciVerse ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma