TRANSPORT OF SOLUTES AS AFFECTED BY IRRIGATION METHOD

The leaching behavior of agricultural chemicals depends on the applica tion method of both the chemical and water. This study investigated th e leaching behavior of conservative tracers under intermittent flood a nd drip irrigation on a clay loam soil. Slug applications of conservat ive tracers were applied sequentially to a single 37-m(2) plot and lea ched under the two irrigation regimes with the same daily application rate. Suction lysimeters, installed at seven depths (0.3, 0.6, 1.0, 1. 4, 1.8, 2.4, and 3.0 m) and replicated four times, were used to withdr aw daily soil solution samples during leaching. Pore water velocities, v, and dispersion coefficients, D, for a one-dimensional advection-di spersion model were fit to the resulting concentration vs. time curves by nonlinear least squares. Pore water velocities were slightly great er on average under drip irrigation and were more uniform across the p lot (smaller variance) than under flood irrigation. Preferential flow was observed under both irrigation regimes. Breakthrough curves showed considerably more spreading under intermittent flood irrigation than under drip, indicating greater hydrodynamic dispersion under flood irr igation. Fitted D values were, on average, nearly three times greater under flood irrigation than under drip. Allowing free water at the soi l surface during flood irrigation increased the dispersion of tracer c ompared with the drip irrigation system, where water was applied at a rate less than the saturated conductivity. The increase in D is most l ikely due to small-scale spatial variability of the soil's hydraulic p roperties.