NEGATIVE HEAD HYDRAULIC PROPERTIES OF THE SOIL SURFACE AT DIFFERENT TIMES

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Abstract

Tension infiltrometers have become popular for determining unsaturated hydraulic conductivity (K) and associated measures in situ. The objective of this study was to use tension infiltrometers to determine K, sorptivity (S), macroscopic capillary length (Λc—a characteristic negative head), and gravity time (tgrav) several times over a growing season for different negative heads and infiltrometer base sizes. Infiltration was measured on seven dates, within 20 m. on a Webster clay loam (Fine, mixed, mesic, Typic Haplaquoll). The soil had been disked before planting corn (Zea mays L.) and cultivated twice after planting. All measurements were made in nonwheel-tracked interrows. Infiltration was measured for ascending heads (-150, −60, and −30 mm) at the same location. Both K and S fluctuated over the growing season with no relation to initial soil water content, which varied from 0.04 to 0.36 m/m. Over the growing season, large-base infiltrometers consistently had smaller calculated Λc (80 mm) than small-base infiltrometers (182 mm), which indicated more influence of gravity for large-base infiltrometers and more influence of capillarity for small base. Pooled across dates, there was no significant difference in base size for S or K. Gravity time is a crude estimate of the time necessary to reach steady-state, and calculated tgrav was usually less than 1 h. Tension infiltration indirectly indicates pore arrangement, suggesting that pore arrangements varied over a growing season.

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