Bacteria transport simulation using APEX model in the Toenepi watershed, New Zealand

Authors: HONG, EUNMI - ORISE FELLOW; MUIRHEAD, RICHARD - NATIONAL INSTITUTE OF WATER AND ATMOSPHERIC RESEARCH (NIWA) LTD.; Pachepsky, Yakov; PARK, YONGEUN - U.S. FOREST SERVICE (FS); SHELTON, DANIEL

Submitted to: ASABE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: 7/7/2016
Publication Date: 7/17/2016
Citation: Hong, E., Muirhead, R., Pachepsky, Y.A., Park, Y., Shelton, D.R. 2016. Bacteria transport simulation using APEX model in the Toenepi watershed, New Zealand. ASABE Annual International Meeting. Orlando, FL on July 17-20, 2016.

Interpretive Summary: Microbial water quality is of critical importance for surface freshwater utilization in recreation and irrigation. Fecal indicator bacteria such as Escherichia coli, are used to estimate the degree of fecal contamination and potential presence of pathogenic bacteria in water. Watershed-scale water quality models can be complemented with bacteria fate and transport modules and then can be used to assess the effects of animal waste and manure management on the presence of indicator bacteria in surface waters. We evaluated the bacteria fate and transport module developed for the APEX (Agricultural Policy/Environmental eXtender) model with the monitoring dataset for the Toenepi watershed in New Zealand where intensive grazing is the main land use. We found that the APEX model with the new bacteria module performs reasonably well. However, it tends to underestimate indicator bacteria levels during low flow periods. Further work needs to be done, however, to improve the description of in-stream microbial processes. Results of this work can be useful to farm industry and consultants in that they can help to screen conservation measures directed to meeting regulatory requirements regarding microbial quality of surface freshwater sources.

Technical 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 evalated 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 × 106 (1.7 × 103 - 4.9 × 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.