Characterizing preferential transport during flood irrigation of a heavy clay soil using the dye vitasyn blau

Common irrigation procedures in many parts of the world involve flood irrigation. Observations during recent years have, however, indicated that preferential flow may be an important phenomenon that could affect the outcome of both irrigation and fertilizer application for this irrigation type. To analyze and visualize solute transport by preferential flow mechanisms, field experiments were carried out at the Cherfech Agricultural Research Station in northern Tunisia. Paths of solute transport were investigated by a 0.05-m ponded pulse of Vitasyn Blau dye allowed to infiltrate into the unsaturated heavy clay soil. Two experimental plots were prepared for the infiltration of dye by leveling the surface. Leveling was done to have similar surface conditions at the two plots. After infiltration, a trench was dug at the border of the dye-injected region to document the dye patterns vertically and horizontally. The patterns indicated strong dependence on the horizontal structure of the soil. In general, the dye was to a major extent transported along distinct cracks and ped faces in the prismatic silty clay. Only the upper 0-0.05 m depth of the tilled soil had almost a complete dye coverage. Below this depth, preferential dye patches start to develop and dye coverage is rapidly decreasing to below 50% at 0.15 m depth. Even though transport through the prismatic silty clay mainly followed cracks between the prisms, it is estimated that only 10-20% of these vertical cracks are active in conducting solutes. The results from the present study have implications for soil management in areas using flood and furrow irrigation with soils susceptible to preferential flow. Tillage depth is a factor that is likely to have a great impact on the amount of water that reaches the soil matrix. Increasing tillage depth and other management practices that break up the structure and make the soil more homogeneous will increase the efficiency of irrigation and reduce the susceptibility for preferential flow