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Independent Calibration of a Mechanistic-Stochastic Model for Field-scale Solute Transport Under Flood Irrigation

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

R. S. Bowman

*Corresponding author.

ABSTRACT

Solute leaching at the field scale can show considerable variability because of variations in the hydraulic properties of the soil. A simple mechanistic-stochastic model for predicting solute transport on this scale during intermittent flood irrigation is described and a method of calibrating the model independent of any leaching measurements is presented. The model assumes that the pore water velocity varies horizontally across the field but is uniform in the vertical direction. Explicit expressions for chemical diffusion and hydrodynamic dispersion are not included in the model. Instead, horizontal variations in the velocity serve to disperse the solute when measured on a field wide basis. The model requires knowledge of the probability density function for the velocity and estimates of the function parameters. These parameters were estimated from variations in infiltration rates measured within 63 ring infiltrometers and from water retention data. Model simulations of Br^– leaching agreed well with most measured data for both position of the tracer peak and the degree of solute spreading, indicating that at least under flood irrigation, variations in infiltration as controlled by soil hydraulic properties explain most of the observed variations in Br^– leaching over a field.

Third author is now at Dep. Geoscience, New Mexico Inst. of Mining and Technology, Socorro, NM 87801. Contribution of the USDA ARS, U.S. Water Conservation Lab., Phoenix, AZ 85040.

Received for publication July 30, 1988.

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