Characterization of preferential flow pathways through Paddy bunds with dye tracer tests

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In terraced paddy landscapes, water losses through the bunds surrounding paddy rice (Oryza sativa L.) fields may significantly reduce water use efficiency. The objective of this research was to identify the dominant pathways active for water flow and solute transport in bunds, and to detect differences between the bunds and adjacent field areas. Two earthen bunds aged 20 and 100 yr were investigated by means of dye tracer experiments in Jiangxi Province, China. Most prominently, the bunds were shown to be distinctly more vulnerable to preferential transport of solutes toward the groundwater than the actual paddy fields. The younger bund showed greater water losses than the old one, caused by a greater macroporosity, lower bulk density, and a less effective hard pan. Three successively operating transport processes could be distinguished: (i) dye solution infiltrated vertically into the field and laterally into the bund; (ii) the plow pan underneath the field and the hard pan in the bund then acted as flow barriers, and horizontal spreading occurred on top of the hard pan toward the outer bund and cross-flow to the adjacent field was observed; and (iii) macropore flow occurred through the hard pan and partly in the subsoil down to the groundwater level, initiated on top of the saturated hard pan and routed through earthworm burrows and root channels as well as shrinkage cracks in the younger bund. In conclusion, compaction of a bund may significantly reduce infiltration rates but does not prevent preferential solute losses.

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