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Abstract. To investigate relations between solute transport, soil properties, and experimental conditions, we summarize results from leaching experiments that we carried out in a range of soils, at different scales (column (0.3-1.0 rn ID, 1.0 rn length) and field plot scale), and using different leaching rates (0.5-30 cm d-•). The lateral mixing regime and longitudinal dispersion were derived from time series of tracer concentrations at several depths in the soil. Field-and column-scale transport were similar in loam and silt loam soils. The mixing regime was related to soil morphological features, such as vertical tongues, stratification, macropores, and a water-repellent layer. The dispersion increased in all soils more than linearly with increasing leaching rate, implying that the dispersivity is not an intrinsic soil characteristic. The change of dispersivity with leaching rate was linked to the unsaturated hydraulic conductivity using a multidomain conceptualization of the pore space.

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The effect of flow rate and flow regime on solute transport in two soils, a sandy‐loam (Glossudalf) and loam (Udifluvent), was investigated. For each soil type, leaching experiments were carried out in two large undisturbed soil columns (0.3‐m i.d., 1‐m length) for three different steady‐state flow rates and three (sandy loam), or two (loam) transient flow regimes. Solute concentrations were measured in the drain water, cf, and in situ, cr, using time domain reflectometry (TDR). In order to approximate the transient by a steady‐state flow transport process, a solute penetration depth coordinate, ζ, was used. Breakthrough curves (BTCs) of cr and cf were used to optimize parameters of the convection–dispersion equation (CDE). In the sandy loam, the CDE described transport for steady‐state and transient flow conditions well and relevant CDE model parameters could be derived from BTCs of cr. In the loam soil, due to the activation of macropores, lateral solute mixing decreased with increasing flow rate, which resulted in an increase of dispersivity with increasing depth for higher flow rates. Since bypass flow and transport through macropores is barely apparent in time series of concentrations measured in situ, cr, CDE parameters derived from BTCs of cr were inconsistent with parameters derived from BTCs of cf when bypass flow was important. The dispersivity increased with increasing flow rate in both soil types and an effective or flux‐weighted average flow rate rather than a time‐averaged flow rate was used to derive the relation between the dispersivity and the flow rate.

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