Bromide and atrazine leaching in furrow- and sprinkler-irrigated corn

Irrigation method is an important consideration in the management of nutrients and pesticides. A 2-yr study was undertaken to evaluate Br- and atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)- 1,3,5-triazine-2,4-diamine] leaching following uniform spray application in a corn (Zea mays L.) field under three irrigation treatments: (i) furrow irrigation with water placement in every furrow (EF), (ii) furrow irrigation with water placement (at twice the rate) in alternate furrows (AF), and (iii) sprinkler irrigation (SP). The soil, a Fort Collins clay loam (fine-loamy, mixed, superactive, mesic Aridic Haplustalf), was ridged and furrowed for all irrigation treatments. In both years of the study, Br- movement under all three irrigation treatments was dominated by lateral flow into the ridge and/or dry furrow positions. The deepest Br- leaching was found in the SP treatment, but with irrigation scheduled at 100% of evapotranspiration (ET) in no case in either year did Br- mass below the root zone (1.2 m) exceed 3% of applied mass. Comparing the furrow irrigation treatments, applying water at twice the rate to alternative furrows neither increased nor decreased the plot averaged downward movement of Br-. Atrazine movement was largely unaffected by the type and placement of irrigation, though in each treatment the downward leaching of atrazine was much greater than expected, suggesting nonequilibrium conditions and/or movement with a mobile reactive phase. A modified version of the unsaturated flow and transport code CHAIN-2D, which included two-dimensional root water and solute uptake with partitioning of the soil water into mobile and immobile regions, accurately simulated the Br- concentration profiles. The model will be useful in evaluating management alternatives for the placement of water and chemicals that minimize losses below the root zone.