FIELD-SCALE TRANSPORT OF BROMIDE UNDER VARIABLE CONDITIONS OBSERVED IN A FURROW-IRRIGATED FIELD

An experiment was conducted during the summer of 1991 to monitor field scale movement of bromide (Br-) under the variable conditions observe d in a 0.81-ha furrow-irrigated fallow field. Twenty-one evenly spaced sampling stations were established, each consisting of a soil solutio n sampler at 0.3-m depth, and a neutron probe access tube to a depth o f 2.1 m. A narrow pulse of Br- tracer was applied by injection through a solid-set sprinkler irrigation system and was subsequently transpor ted downwards by 26 mm of sprinkler irrigation. Three furrow irrigatio n events were conducted at three-week intervals during the 63-day stud y period. Soil samples were taken at each station in 0.15-m increments to a depth of 2.4 m prior to each irrigation and at the end of the st udy period. The three furrow-irrigation events moved the solute front to 0.23-, 0.38-, and 1.13-m depth, respectively. Three piston flow mod els based on Darcy flow, field capacity, and the Transfer Function mod el (TFM) were used to predict the movement of the solute. The Darcy fl ow model under-estimated solute movement by 18% to 43% after each irri gation event. The TFM predicted the movement of solute with reasonable accuracy after the first irrigation but overestimated solute movement for subsequent irrigations. The fatter was attributed to the need to calibrate the TFM at deeper depths. The field capacity model was more consistent with observed irrigations, and predicted the movement of so lute with reasonable accuracy for the first and second irrigation. How ever, the field capacity model underestimated the final si position. I t was hypothesized that preferential flow occurred during the third ir rigation, since the infiltration rates increased during this irrigatio n and the final measured position of Br- was significantly greater tha n predicted by the piston flow models.