Observed and Simulated Transport of a Conservative Tracer under Line-Source Irrigation

Although a number of solute transport models are currently available to predict the transport of agrichemicals in the vadose zone, validation of these models under field conditions has been limited. This study monitored the transport of a conservative tracer (Br⁻) under three water regimes (high, medium, and low) imposed by a line-source irrigation system and tested the validity of the simulation model, LEACHM, to predict Br⁻ transport. In July 1990, RbBr was surface applied to 12 uncropped columns at 56 kg Br⁻ ha⁻¹. Four columns (0.203-m diam., 1.2-m length) were positioned in each water regime and equipped with soil solution samplers at 0.36-, 0.66-, and 0.96-m depths. Soil solution samples were taken 20 times over 80 d to monitor Br⁻ transport. Cumulative water applied to each water regime, by irrigation and precipitation, ranged between 251 and 458 mm. Results indicated dramatic differences in Br⁻ transport among water regimes. Complete Br⁻ breakthrough curves (with apex concentrations of approximately 60 mg Br⁻ L⁻¹) were observed under high water regime at all depths whereas the majority of Br⁻ applied to the low water regime remained at or near the 0.36-m depth. Inputs used to predict Br⁻ transport in LEACHM simulations were either measured directly or estimated from experimental conditions. To simulate Br⁻ transport for each water regime, we used three LEACHM input files, which reflected the variability of the soil profile water release retention coefficients. Results indicated that LEACHM's predictions of Br⁻ concentration (at all depths and under all water regimes) differed from observed means by an average (n = 180) of 9.0 to 9.4 mg Br⁻ L⁻¹ (approximately 15–16% of the average apex concentrations). These results indicate that under the experimental system studied, LEACHM adequately predicted the transport of a conservative tracer in the vadose zone. Contribution of the Montana Agric. Exp. Stn., Journal no. J-2837 and the Nebraska Agric. Exp. Stn., Journal no. 10287.