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Dep. of Agronomy, Iowa State Univ., Ames, IA 50011
USDA-ARS, National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011
*Corresponding author(fxcasey{at}iastate.edu).
ABSTRACT
Agricultural chemical presence in groundwater has drawn attention toward transport processes occurring in soil. Hydraulic conductivity (K) and water-holding capacity of a soil have great influence on water flow and solute transport. However, much of the chemical transport to groundwater can occur through preferential flow pathways. The simplified, preferential flow, mobile-immobile model partitions the water content (
) into mobile (
m) and immobile (
im) domains, with solute exchange between the domains characterized by the mass-exchange coefficient (
). In this study a sequential tracer application technique was used and K,
,
im, and
were estimated for a series of pressure heads (H = 10, –30, –60, and –150 mm). This method uses a tension infiltrometer to measure both hydraulic and solute transport parameters in situ. The study took place in a no-till corn (Zea mays L.) field mapped as a Harps series soil (fine-loamy, mixed, mesic Typic Calciaquoll). Unsaturated values of
and K were distinct from the saturated values. Similarly, though less clear cut, distinctions between saturated and unsaturated values of
im, immobile water fraction (
im/
), and
were observed. The medians of
for the sequence of decreasing H values were 0.40, 0.34, 0.34, and 0.33 m3 m-3. The median K values for the same sequence of H were 108, 1.69, 1.51, and 0.72 µm s-1. The median
im/
values for the H sequence were 0.40, 0.28, 0.25, and 0.39. The median values of
for the H sequence were 0.59, 0.015, 0.0028, and 0.0029 h-1. A strong correlation between
and H suggests a velocity dependence of
.
Journal Paper no. J-17246 of Iowa Agriculture and Home Economics Exp. Stn., Ames, IA, Project no. 3287, and supported by Hatch Act and State of Iowa funds. This material is based on work supported by the Cooperative State Research, Education, and Extension Service, USDA, under Agreement no. (94-37102-0916).
Received for publication April 28, 1997.
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