Constructed wetlands could be components of low-tech systems to treat and reuse wastewater in arid region. A key function of the wetland would be to provide additional N removal. To improve design criteria, a sequential model of nitrogen transformations (organic N --> ammonium: ammonium --> nitrate: nitrate --> nitrogen gas) was successfully calibrated and verified for a wetland in Kingman, Arizona. A sequential model has the ability to "recognize" species of nitrogen in the influent and predict species of nitrogen in the effluent. Model scenarios show that increasing nitrification rates in the summer and denitrification rates in the winter would improve nitrogen removal efficiencies. Several lines of evidence suggest that wintertime denitrification may be limited by carbon supply. Winter carbon supply could be augmented by routing a portion of the water through channels planted with dryland vegetation.