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Observed and Simulated Solute Transport Under Varying Water Regimes: I. Bromide and Pentafluorobenzoic Acid

Dep. of Plant, Soil and Environmental Science, Montana State Univ., Bozeman, MT 59717-0312;
Dep. of Agronomy, Univ. of Nebraska, Lincoln, NE 68583-0915.

^* Corresponding author (usswi{at}msu.oscs.montana.edu).

ABSTRACT

Comparison of observed and predicted solute transport under field conditions is a necessary component for validating various modeling approaches. The objectives of this study were to compare transport of Br^– and pentafluorobenzoic acid (PFBA) under cropped and fallow conditions for a range of soil water contents and evapotranspirative demands, and to evaluate the predictive capability of a computer simulation model (LEACHM) using independent estimates of model input parameters. A field study was conducted (1991) using in situ columns (0.2 m diam., 1.22 m depth) on a Brocko silt loam (mixed, mesic Borollic Calciorthid) in Gallatin Co., MT. The transport of two tracers (Br^– and PFBA) was measured at three depths (0.36, 0.66, and 0.96 m) for three water regimes under cropped (barley, Hordeum vulgare L.) and fallow conditions for 60 d following chemical application. Observed breakthrough curves (BTCs) demonstrated increased travel times with increasing depth and decreasing water application. Solute transport in cropped columns was significantly slower than fallow as a result of higher evapotranspiration. Comparison of Br^– and PFBA BTCs under cropped conditions suggested significantly greater plant uptake of Br^– compared to PFBA. Predicted volumetric water contents, evaporation (under fallow conditions), and solute BTCs using LEACHM agreed favorably with observed results. Under cropped conditions, agreement between predicted and observed soil water contents was less favorable, most likely a result of inaccurate assumptions concerning depth-dependent root water uptake. LEACHM-predicted mean travel times (d) agreed within 22% of observed PFBA values for fallow conditions and within 9% of observed PFBA values for cropped conditions. Results indicated that adequate predictions of the transport of PFBA were obtained using LEACHM when coupled with independent estimates of model input parameters.

Contribution of the Montana Agric. Exp. Stn., Journal no. J-5002.

Received for publication April 10, 1995.

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