Epppcr oF Coxrecr MATERIAL ox TpNSIoN IxpnrRoMETgn MSAS uREMENTS V. Bagarello, M. Iovino, G. Tusa ABsrRAcr. A layer of contact materinl is often used to establish and maintain hyd.raulic contact between a tension Wltromeîer ruembrane and the soil surface. The aims of this study were to d.eterminethe change in hydraulic properties of two _types oÍ contact mnterinls afler repeated use oÍ the moteríals and to eyaluate the eJfectof contact -ateiol oi ttiody-"ní" infiltration rates. Re-using a natural sand contact materíal resulted in increasing vllues àf sqtiated hydrqulic condictivity, K,t (Kt = 246 to 311 mm h 1), due to a lrogressive loss of fine-textured partiiles- Howevea Spheriglass No. 2227 glais spheres,previously proposed as a suitable contact mqterial, nwintained stàble hydraulic properties afier repeated use"both in the,laborafory and in the rteH (K"t = 264 to 267 mm h:1, wqter enÍry pressure hzadhÍl = 40A tu J6d w6 air entry pressure headh^,= 440 to 450 mm). The steady-state îensíon inrthrometer infiltratíon rate, i0, aî d pressure headho = -50 mm w 's reduc-ed by about 30Voby removing the contact maÍerial from the smòothed and. Ievelid surjace of a sandy loam soil- It was concluded that Spheriglass No. 2227 glass spheres are adequate for both use and reuse'as teision infihrometer contact mateial, and thaî contacî material is required. to mainmin good hydraulic connection with the soil surface even when the surface has been smoothed and leveled.. Keywords. Hydraulíc Conductivity, Tension Infiltrometer, Contact Makrtal. he tension infiltrometer method (Perroux and White, 1988; Ankeny, 1992; Reynolds, 1993) is widely applied for in-situ determination of soil hydraulic properties. The reliability of tension infiltrometer measurements depends heavily on the establishment of a good, stable hydraulic contact between the infiltrometer membrane and the infiltration surface (perroux and White, 1988;Reynolds andZebchuk, 1996).To establish a good hydraulic contact,a smooth,level infiltration surface is often prepared by placing a layer of contact material over the undisturbedsoil surface.According to perroux and white (1988), the characteristics of the contact material for steady infiltrometer flow should include a hydraulic conductivity higher than that of the soil, a water entry pressure headlower (more negative)than the minimum pressure head applied to the infiltrometer membrane, and as small a thickness as possibleto minimize the material'seffect on flow. According to Reynoldsand Zebchuk (1996), the requiredcharacteristics of contact material should also include low variability of hydraulic properties, chemical inertness, and preferably, the ability to retrieve and re-use the material. chemical inertness is particularly important in solute transport studies. Re- Article was submitted for review in December 2000; approved for publication by the Soil & Water Division of ASAE in May 2001. Mention of a product does not constitute endorsement by the universirv of Palermo. The authors are Vincenzo Bagarello, Associate professor, Massimo fovino, AssociateProfessor, and Giuseppa Thsa, Engineer,Dipartimento di Ingegneriae Tecnologie Agro-Forestali, SezioneIdraurica, università degli Studi, Viale delle Scienze,Palermo, Italy. Corresponding author: Massimo Iovino, Dept. [IAE, Università di palermo, Fac. Di Agraria, Viale delle Scienze, 90128 Palermo, Italy; phone: +39-091-7029110; fax: +39-091-484035; e-mail: iovinom@unioa.it. usability means that variations in contact material hydraulic propertiesdue to repeated field useareeffectively negligible. Nevertheless,these specific criteria are often neglected, and contact material consistingof fine sand,coarsesand,or sieved soil is often used without regard to uniformity, hydraulic properties, or chemical characteristics. Reynolds andZebchuk (1996) showedthat the pressure head set on the tension infiltrometer membraneand the pressure head at the soil surface can differ substantially depending on the thickness, satiated hydraulic conductivity, and water entry value of the contact material, and on the flow rate out of the infiltrometer. These authors also tested in the laboratory a highly uniform glasssphere medium, which they proposed as a suitable contact material for practical field use. According to Logsdonand Jaynes (1993) and Wang et al. (1998), contact material may not be required for relatively smooth soil surfaces. Placing the tension infiltrometer membrane directly on the soil surface simplifies both the method and the data analysis, especially when early-time infiltration data must be used. Reynolds andZebchuk(1996), on the other hand, suggested that a layer of contact material be used in tension infiltrometer measurements regardless of whether the soil surface has been smoothed and leveled or left in its nafural state. Non-uniform wetting of the contact material was observed at the membrane interface in controlled laboratoryconditions(Closeet a1.,1998),where good hydraulic contact between the membrane and the contact material should be easy to achieve. The non-uniformity of wetting was more evident at low (more negative)pressure headvalues.Bagarelloet al. (2000) found that contactmaterial having a satiated hydraulic conductivity greater than the field-saturatedhydraulic conductivity of the soil did not appreciably affect infiltration rates taken in the field at a positive pressure head. Transactions of the ASAE O 2001 American Society of Agricultural EngineersISSN 0001-2351 YoL. M(4):91r-916 911