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RESEARCH PRODUCT

Comparison of unconfined and confined unsaturated hydraulic conductivity

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Abstract The field tension infiltrometer (TI) and the laboratory unit hydraulic gradient (UHG) methods are widely applied to determine the near-saturated soil hydraulic conductivity, K . Comparison between the two methods is relevant given that they differ in the explored soil volume (undetached or detached) and in the flow process (unconfined or confined). The objective of this investigation was to compare unconfined and confined measurements of unsaturated hydraulic conductivity. Twenty TI experiments were conducted in a relatively coarse-textured soil having an appreciable hysteretic behavior by using two different dry-to-wet-to-dry (DTWTD) sequences of pressure head, h 0 , values that differed by the highest h 0 value imposed within the sequence (i.e. h 0  = − 150, − 75, − 30, + 5, − 30, − 75, − 150 in site A or h 0  = − 150, − 75, − 30, − 10, − 30, − 75, − 150 in site B). The same pressure head sequences were applied on twenty undisturbed soil cores, collected at the exact location of the TI measurements, to perform the laboratory UHG measurements. Regardless of the type of experiment (i.e. unconfined or confined) and the applied pressure head sequence (i.e. site A or B), higher K 0 values were obtained with a drying sequence of h 0 values ( K 0,d ) than with a wetting one ( K 0,w )and the discrepancies between K 0,w and K 0,d decreased as the imposed h 0 value increased, as it was expected due to hysteresis. A tendency of the UHG method to overestimate the K 0 values was detected (ratios of mean K 0,1D to mean K 0,3D values ranging from 0.93 to 4.35), but the statistical significance of the observed differences varied with the considered sequence of pressure head values. It was concluded that both the TI and the UHG methods were effective in detecting hysteresis effects on K 0 , but the laboratory method resulted in K 0 values that were higher and more variable probably as a consequence of a more substantial effect of macropore flow on the measured flow rates.