Reducing bypass flow through a dry, cracked and previously puddled rice soil
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Reducing water losses through bypass flow is important in rice-growing areas (rainfed and irrigated) where water losses during land preparation for rice are high owing to soil cracking on the surface and a relatively permeable subsoil. Bypass flow processes were studied in a cracked, previously puddled rice soil. Vertical continuity of soil cracks (10-30 mm in width) was determined in the field using a morphological staining technique. Cracks reached a depth of 0.65 m. An infiltration experiment showed that water was mainly absorbed in the subsoil between 0.2 and 0.5 m depth. In a laboratory experiment, simulated rainfall was applied to large undisturbed soil columns taken from the field. High bypassing ratios (greater than 0.9) were obtained for low (10 mm h-1) and high (30 mm h-1) rain intensities. Shallow surface tillage (0-50 mm) in the field reduced bypass flow, and laboratory experiments showed 45-60% water savings as a result of making the cracks discontinuous. For rainfed rice areas, introduction of this tillage practice may lead to earlier transplanting and therefore reduced risk of late drought. In some areas this may broaden the scope for a second crop. In irrigated rice systems, water savings during land preparation may allow an increase in the command area of an irrigation system.
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