Transport of Conservative Tracers in the Field Under Intermittent Flood Irrigation

The velocity of downward percolating water measured using tracer concentration profiles was compared to the macroscopic velocity calculated from a water balance. Experiments were performed in a 37‐m2 field plot under intermittent flood irrigation. Five different soil water tracers were used to follow the movement of six applied water pulses over a period of 10 months under two irrigation regimes. Mean water velocities measured with the tracers averaged 60–70% greater than the calculated macroscopic velocity. The results indicated that a significant portion of the applied water moved downward through preferential flow paths even under unsaturated conditions. The measured velocities were not sensitive to irrigation frequency as long as total water applied remained constant. The data indicate that simple water balance models may overestimate arrival times of surface‐applied water and chemicals to ground water.