Deep percolation of excess irrigation water was measured on a 0.62‐ha bare soil field. Deep percolation rates were determined from bromide added to the soil as a tracer. Core samples were taken at a depth to 2.7 m within 14 subplots after each irrigation. The depth of the maximum bromide concentration was uniformly distributed over the field. The average deep percolation rate was determined from the arithmetic mean of the tracer velocities and water content. The deep percolation rate calculated from the tracer velocities was about five times greater than determined from a water balance. The discrepancy between the tracer and water balance rates indicates that much of the water is moving in preferential paths. The water balance may overestimate the travel times of solutes and pollutants to the groundwater.
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Rice et al. (1986) studied this question.