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Transport of a Conservative Tracer in the Field Under Continuous Flood Irrigation

U.S. Water Conservation Lab., 4331 E. Broadway, Phoenix, AZ 85040

R. S. Bowman

Dep. of Geoscience, New Mexico Inst. of Mining and Technology, Socorro, NM 87801

*Corresponding author.

ABSTRACT

A 5-cm deep pulse of bromide-tagged water was applied to four small field subplots and leached under continuously flooded conditions for 7 d. During leaching, solution samples were periodically withdrawn through suction samplers from seven depths within each subplot. Pore water velocities, v_s, and dispersion coefficients, D, were calculated by fitting the one-dimensional solution of the advection-dispersion equation to the concentration vs. time curves from each sampler. Both v_s and D were best described by a log-normal distribution rather than a normal distribution. D values were very large compared to values reported for laboratory experiments, but similar to other field values measured under similar conditions. Neither v_s nor D showed any significant correlation with depth or time, but the dispersivity (ratio of D to v_s) did show a weak positive correlation with depth. The relationship between ln D and ln v_s was linear with a slope near 1.0. However, when v_s and D data measured in an earlier study under an intermittently dosed irrigation regime at the same site were included, ln D was no longer a simple linear function of ln v_s. The ratio between the calculated pore water velocities and the velocities calculated from the surface flux divided by the average water content was equal to or slightly <1.0 for all depths >0.6 m. At depths <0.6 m the ratio was considerably >1.0, indicating that a fraction of the soil water was being bypassed or not participating in the leaching process. This result is in contrast to an earlier study conducted on this site when irrigated under a dosed irrigation scheme in which the ratio was consistently >1.0 at all depths.

Contribution of the USDA-ARS, U.S. Conservation Lab.

Received for publication July 20, 1987.

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