A first order decay function is used to calculate changes in bacteria concentrations (Bowie et al., 1985).

$bact_{lpres,i}=bact_{lpre s,i-1}*exp(\mu _{lpres,die})$ 8:5.1.1

$bact_{pres,i}=bact_{pres,i-1}*exp(\mu _{pres,die})$ 8:5.1.2

where $bact_{lpres,i}$ is the amount of less persistent bacteria present in the reservoir on day $i$ (#cfu/100mL), $bact_{lpres,i-1}$ is the amount of less persistent bacteria present in the reservoir on day $i-1$ (#cfu/100mL), $\mu_{lpres,die}$ is the rate constant for die-off of less persistent bacteria in water bodies (1/day), $bact_{pres,i}$ is the amount of persistent bacteria present in the reservoir on day $i$ (#cfu/100mL), $bact_{pres,i-1}$ is the amount of persistent bacteria present in the reservoir on day $i-1$ (#cfu/100mL), and $\mu_{pres,die}$, is the rate constant for die-off of persistent bacteria in water bodies (1/day).

The die-off rate constants are adjusted for temperature using the equations:

$\mu_{lpres,die} =\mu_{lpres,die,20} * \theta _{bact}^{(T_{water}-20)}$ 8:5.1.3

$\mu_{pres,die} =\mu_{pres,die,20} * \theta _{bact}^{(T_{water}-20)}$ 8:5.1.4

where $\mu_{lpres,die}$, is the rate constant for die-off of less persistent bacteria in water bodies (1/day), $\mu_{pres,die}$ is the rate constant for die-off of persistent bacteria in water bodies (1/day), $\mu_{lpres,die,20}$ is the rate constant for die-off of less persistent bacteria in water bodies at 20$\degree$C (1/day), $\mu_{pres,die,20}$ is the rate constant for die-off of persistent bacteria in water bodies at 20$\degree$C (1/day), $\theta _{bact}$ is the temperature adjustment factor for bacteria die-off/re-growth, and $T_{water}$ is the water temperature ($\degree$C).

Table 8:5-1: SWAT+ input variables that pertain to bacteria die-off in the water bodies.

SWAT+ calculates loading of pathogens and indicator bacteria for pathogens from land areas in the watershed. In reservoirs, bacteria die-off is the only process modeled.