Bromide and Atrazine Leaching in Furrow‐ and Sprinkler‐Irrigated Corn

Abstract

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.