APEX-M - APEX bacterial add-on module
Water quality models can be essential tools for alleviating pathogen load to water sources, evaluating microbial contamination-related risks, guiding the microbial water quality monitoring, and assessing the effect of agricultural management on the microbial water quality. The Agricultural Policy/Environmental eXtender (APEX) is the watershed-scale water quality model that includes highly detailed representation of agricultural management practices. APEX currently does not have bacterial fate and transport simulation capabilities. The objective of this work was to develop the first APEX bacterial fate and transport add-on that could use the APEX conceptualization of watershed processes, current knowledge about the microbial fate dynamics, and recently introduced concepts of the in-stream microbial fate and transport. The add-on simulates fate and transport of manure-borne and animal waste-borne bacteria. Manure depositions from different days are treated separately. APEX simulates manure erosion and the add-on assumes that bacteria survive in and are exported with manure particulates. Manure weathering and rainfall-triggered bacteria regrowth are accounted for. The two-stage bacteria survival kinetics is simulated in manure and in streams. Bacteria enter stream water with eroded manure, with resuspended sediment at high flow events and with hyporheic transport during the low flow periods. The governing equations are presented, and input variables and output files are described. The modified APEX subroutines, new bacteria fate and transport routines, and examples of input and output files are placed at the website along with this report. Opportunities of expanding the add-on are outlined.