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TECHNICAL NOTES

Tension Infiltrometer Measurements

Implications of Pressure Head Offset due to Contact Sand

Agriculture and Agri-Food Canada, Greenhouse and Processing Crops Research Centre, 2585 County Road 20, Harrow, ON, Canada N0R 1G0
^* Corresponding author (reynoldsd{at}agr.gc.ca)

Received 11 July 2006.

The use of contact sand to achieve good hydraulic connection between the tension infiltrometer (TI) membrane and the soil is known to introduce an offset between the pressure head set on the bubble tower (h₀) and the pressure head applied to the soil surface (h_s). The nature and importance of the offset are poorly understood, however. Hence, the objectives of this study were to characterize the offset and to demonstrate its impacts on TI determinations of near-saturated hydraulic conductivity, K(h), sorptive number, *(h), flow-weighted mean pore diameter, D(h), and number of flow-weighted mean pores per unit area, N(h). The offset, h = h_s – h₀, consists of a constant elevation component and a variable head-loss component. The elevation component increases h_s relative to h₀, and comprises most of the offset for low TI flux density, q(h₀), and large contact sand hydraulic conductivity, K_cs. The head-loss component decreases h_s relative to h₀, and becomes more important as q(h₀) increases or K_cs decreases. The offset has little effect on the accuracy of K(h), *(h), D(h), and N(h) when these relationships are insensitive to changes in h₀. When the relationships are sensitive to changing h₀, the offset can change the shapes of the relationships; cause systematic overestimates of the K(h), *(h), and D(h) values; and cause systematic underestimates of the N(h) values. The amount of overestimate and underestimate increases with increasing offset and should be corrected using a form of Darcy's law to prevent the introduction of systematic biases in TI results.