Identifying Source Areas of Ammonium Fluxes Sampled Above a Louisiana Sugar Cane Field

Average diurnal and nocturnal ammonium (NH4+) concentrations were sampled from two heights (2.89 m and 5.18 m) above a sugarcane field in St. Gabriel, Louisiana between May and July of 2011. Sonic anemometers at sampling both heights collected turbulence data over the sampling periods. These data were input into the analytic crosswind integrated footprint model of Horst & Weil (1994) in an attempt to identify source areas of the sampled NH4+. This model assumes Gaussian dispersion in the crosswind direction and homogeneous surface cover. These assumptions can introduce error because the surface cover in the study areas is only homogenous within the field, with cattle pastures, ponds, ditches, roads, buildings, trees and the Mississippi River in the surrounding region. Though this model inherently makes false assumptions to simplify calculations it can still be used as a guide to help identify general source regions of the NH4+ samples. The footprints will likely extend beyond the immediate sugarcane field under stable and neutral conditions. They will likely be contained within the field during unstable conditions. The NH4+ fluxes vary with many factors including stability, wind speed and direction, humidity, and temperature as the footprints cover various surfaces.