AbstractWhen diuron is used as a soil‐residual herbicide in irrigation canals, water quality is protected in standard practice by ‘flushing’–the diversion of the first discharge of water to the drainage system before irrigation is resumed. In three canals treated with diuron at about 40–45 kg ha−1, flushing for 4 h was relatively inefficient, removing 15 or less of the residual diuron in the sediment. In one other canal approximately 40 was removed during flushing. Cumulative loss of diuron was proportional to the square root of the time of water discharge. The effect of the time interval between treatment and flushing was investigated in trials on soils of typically heavy texture. When flushing closely followed treatment about 6 kg diuron ha−1was removed but in three trials, for a range of times from treatment of 5–15 weeks, losses were similar and about 2.5 kg ha−1, suggesting that a restriction of longer than 5 weeks is unlikely to reduce contamination. In a contrasting soil of sandier texture greater movement of diuron away from the sediment surface probably contributed to a five‐fold reduction of diuron extracted in flushing. These data were used to calculate the effects of flushing time on both the concentrations of diuron in water discharged to drains for subsequent re‐use and the concentrations in the irrigation water, to aid assessment of hazard to crops. Flushing times required for residues in water to reach 0.1 μg ml−1, calculated by extrapolation, were in the order of days so that it may be necessary to restrict the use of diuron to farms which can retain the discharge water within their own boundaries, or to areas where dilution in receiving waters can
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