OBSERVED AND SIMULATED TRANSPORT OF A CONSERVATIVE TRACER UNDER LINE-SOURCE IRRIGATION

Although a number of solute transport models are currently available t o predict the transport of agrichemicals in the vadose zone, validatio n of these models under field conditions has been limited. This study monitored the transport of a conservative tracer (Br-) under three wat er regimes (high, medium, and low) imposed by a line-source irrigation system and tested the validity of the simulation model, LEACHM, to pr edict Br transport. In July 1990, RbBr was surface applied to 12 uncro pped columns at 56 kg Br ha-1. Four columns (0.203-m diam., 1.2-m leng th) were positioned in each water regime and equipped with soil soluti on 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 wat er applied to each water regime, by irrigation and precipitation, rang ed between 251 and 458 mm. Results indicated dramatic differences in B r transport among water regimes. Complete Br breakthrough curves (with apex concentrations of approximately 60 mg Br L-1) were observed unde r 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 u sed to predict Br- transport in LEACHM simulations were either measure d directly or estimated from experimental conditions. To simulate Br- transport for each water regime, we used three LEACHM input files, whi ch reflected the variability of the soil profile water release retenti on coefficients. Results indicated that LEACHM's predictions of Br con centration (at all depths and under all water regimes) differed from o bserved means by an average (n = 180) of 9.0 to 9.4 mg Br- L-1 (approx imately 15-16% of the average apex concentrations). These results indi cate that under the experimental system studied, LEACHM adequately pre dicted the transport of a conservative tracer in the vadose zone.