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Research article2008

Effects of heterogeneous flow on discharge generation and solute transport

Lennartz B, Jarvis N, Stagnitti F

Abstract

After dye tracer tests that confirmed significant finger flow in a sandy soil, an undisturbed soil monolith with a volume of 0.78 m(3) (r = 0.5 m) was extracted and mounted on a lysimeter stand. The aim was to experimentally quantify finger flow effects on the flux field as characterized by measurements of soil water pressure head (psi) and volumetric soil water content (theta), and on discharge generation and solute transport. The experimental setup allowed discharge registration from six triangular center compartments and one side-wall segment. Time domain reflectometry and tensiometer measurements taken during an 80-day infiltration experiment at several locations within the lysimeter confirmed the nonhomogeneity of the flux pattern at early stages of the experiment. However, calculations of the hydraulic gradient suggested that the hydraulic conditions within the soil profile had nearly equilibrated after 7 days of continuous infiltration at 0.26 mm h(-1). Nevertheless, the onset of discharge varied between lysimeter segments by up to 33 days, demonstrating the profound effect of finger flow on water flux variabuity. Final flux rates varied consistently between spatial segments by a factor of 5 over 13 days, confirming the persistence of established flow regions. Initially, dry soil areas delivered less discharge, although the possibility of three-dimensional flux patterns with water exchange between segments did not allow us to draw definite conclusions for all lysimeter segments. Although discharge was generated from all segments, tracer leaching data indicated that large fractions of the soil monolith were almost nonoperational in solute transport. It is concluded that two- or three-dimensional flux field patterns obtained from psi and theta measurements at the onset of infiltration events are helpful in quantitatively deducing discharge variability. At later stages of infiltration, however, spatial theta data may be insufficient to explain discharge variability over a given flux plane. Especially in coarse-textured single-grained substrates that possess material functions that are characterized by a steep increase in hydraulic conductivity in the near-saturation range, the resolution of measurements might be insufficient to detect minor variations in theta and psi that cause large differences in flow and transport

Published in

Soil Science
2008, Volume: 173, number: 5, pages: 306-320 Publisher: LIPPINCOTT WILLIAMS & WILKINS