SUSCEPTIBILITY OF SOILS TO PREFERENTIAL FLOW OF WATER - A FIELD-STUDY

Flow pathways of water and solutes in soils form distinct patterns, wh ich are not a priori predictable. Macropore structure is a prime cause , but other factors, such as differing initial or boundary conditions, may also predispose a soil to produce bypassing of infiltrating water . This study was conducted to assess the flow pathways of water in dif ferent soils and to investigate the effect of initial water content on the flow pattern. Dye-tracing experiments were carried out at 14 diff erent field sites. The sites represent a good portion of soils used fo r agricultural crop production in Switzerland. Each site consisted of two 1.4 by 1.4 m plots, one of which had been covered with a plastic r oof for two months before the experiment to achieve different initial water contents. Forty millimeters of water containing the dye Brillian t Blue FCF (C.I. Food Blue 2) were applied within 8 hours onto the plo ts with a sprinkling apparatus. One day after irrigation the plots wer e excavated, and the stained pattern was examined on a vertical 1 by 1 m soil profile. The spatial structure of flow patterns showed remarka ble differences. In most soils, water bypassed the soil matrix. In som e soils, dye penetrated beyond 1 m depth, whereas in others it remaine d in the top 50 cm. Structured soils were more prone to produce bypass flow, deep dye penetration, and pulse splitting than nonstructured so ils. The initial water content had a less pronounced effect in some so ils and no effect in others.