ASABE Technical Library - Abstract

Abstract. The Agricultural Policy/Environmental eXtender (APEX) model is a distributed, continuous, daily-time step small watershed-scale hydrologic and water quality model. In this study, the newly developed fecal-derived bacteria fate and transport subroutine was applied and validated using APEX model. The experimental study was conducted in the Toenepi watershed located in the Waikato region of the North Island, New Zealand. The prominent land use is intensive pasture-based dairy, and typically 40 kg of animal waste/animal/day is applied directly onto the land as the cows graze. Sensitivity analysis was conducted using the FAST (Fourier amplitude sensitivity testing) method. The return flow ratio, groundwater residence day, groundwater storage threshold and CN index coefficient were sensitive parameters for stream flow simulation. Stream flow was calibrated with the data from watershed outlet. The accuracy of the model was classified as good. The model performance with regard to E. coli was most sensitive to the initial concentrations of this organism in manure. The average New Zealand value of 2.4 x 10⁶ (1.7 x 10³ - 4.9 x 10⁶) MPN/g provided the best results. This study will contribute to evaluation of best management practices related to grazing at the Toenepi watershed. However, further development of the in-stream bacteria fate and transport model appears to be needed to improve simulations of E. coli concentrations.