Chaparral Conversion to Increase Streamflow in Arizona: Sequential Treatments Extend Duration of Nitrate Loss to Stream Water

Abstract

Abstract Converting an Arizona chaparral watershed to grass cover in two stages by means of herbicide treatments increased stream flow and the concentration of nitrate ions in the stream water. Nitrate rose to remarkably high concentrations (83 ppm) and was exported from the watershed in extraordinary amounts (up to 143 kg/ha/yr) in comparison with a control watershed (1.0 kg/ha/yr). Abnormal concentrations of nitrate in streamflow persisted for more years from the two-stage treated watershed than from a companion watershed converted in one stage. Monthly mean weighted nitrate concentrations fluctuated in an annual cyclic pattern that closely corresponded with major rainfall events and the annual stream discharge cycle, which peaked during the winter rainy season. A possible consequence of such large releases of nitrate ions into stream water is eutrophication of downstream watercourses and reservoirs. In planning for chaparral conversions to enhance water yield, forage production, and wildlife habitat, consideration should be given to the nitrate-release phenomenon associated with conversion to reduce possible adverse environmental impacts. For. Sci. 33(1):89-103.