Field Scale Transport of Bromide Under Variable Conditions Observed in a Furrow-irrigated Field

Abstract

An experiment was conducted during the summer of 1991 to monitor field scale movement of bromide (Br-) under the variable conditions observed in a 0.81-ha furrow-irrigated fallow field. Twenty-one evenly spaced sampling stations were established, each consisting of a soil solution sampler at 0.3-m depth, and a neutron probe access tube to a depth of 2.1 m. A narrow pulse of Br- tracer was applied by injection through a solid-set sprinkler irrigation system and was subsequently transported downwards by 26 mm of sprinkler irrigation. Three furrow irrigation events were conducted at three-week intervals during the 63-day study 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 study period. The three furrow-irrigation events moved the solute front to 0.23-, 0.38-, and 1.13-m depth, respectively. Three piston flow models based on Darcy flow, field capacity, and the Transfer Function model (TFM) were used to predict the movement of the solute. The Darcy flow model under-estimated solute movement by 18% to 43% after each irrigation event. The TFM predicted the movement of solute with reasonable accuracy after the first irrigation but overestimated solute movement for subsequent irrigations. The latter 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 solute with reasonable accuracy for the first and second irrigation. However, the field capacity model underestimated the final Br- position. It was hypothesized that preferential flow occurred during the third irrigation, since the infiltration rates increased during this irrigation and the final measured position of Br- was significantly greater than predicted by the piston flow models.