OBSERVED AND SIMULATED SOLUTE TRANSPORT UNDER VARYING WATER REGIMES .1. BROMIDE AND PENTAFLUOROBENZOIC ACID

Comparison of observed and predicted solute transport under field cond itions is a necessary component for validating various modeling approa ches. The objectives of this study were to compare transport of Br- an d pentafluorobenzoic acid (PFBA) under cropped and fallow conditions f or 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 held 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 Ga llantin Co., MT. The transport of two tracers (Br- and PFBA) was measu red at three depths (0.36, 0.66, and 0.96 m) for three water regimes u nder 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 decreas ing water application. Solute transport in cropped columns was signifi cantly slower than fallow as a result of higher evapotranspiration. Co mparison of Br- and PFBA BTCs under cropped conditions suggested signi ficantly greater plant uptake of Br- compared to PFBA. Predicted volum etric water contents, evaporation (under fallow conditions), and solut e BTCs using LEACHM agreed favorably with observed results, Under crop ped conditions, agreement between predicted and observed soil water co ntents was less favorable, most likely a result of inaccurate assumpti ons concerning depth-dependent root water uptake. LEACHM-predicted mea n travel times (d) agreed within 22% of observed PFBA values for fallo w conditions and within 9% of observed PFBA values for cropped conditi ons. Results indicated that adequate predictions of the transport of P FBA were obtained using LEACHM when coupled with independent estimates of model input parameters.