Algae

Suspended algal biomass is assumed to be directly proportional to chlorophyll $a$. Therefore, the algal biomass loading to the stream can be estimated as the chlorophyll $a$ loading from the land area. Cluis et al. (1988) developed a relationship between the nutrient enrichment index (total N: total P), chlorophyll $a$, and algal growth potential in the North Yamaska River, Canada.

$(AGP+chla)*v_{surf}=f*(\frac{TN}{TP})^g$ 4:5.1.1

where $AGP$ is the algal growth potential (mg/L), $chla$ is the chlorophyll $a$ concentration in the surface runoff ($\mu g$/L), $v_{surf}$ is the surface runoff flow rate (m$^3$/s), $TN$ is the total Kjeldahl nitrogen load (kmoles), $TP$ is the total phosphorus load (kmoles), $f$ is a coefficient and $g$ is an exponent.

The chlorophyll $a$ concentration in surface runoff is calculated in SWAT+ using a simplified version of Cluis et al.’s exponential function (1988):

$chla=0$ if $(v_{surf}<10^{-5}m^3/s)$or $(TP$ and $TN <10^{-6})$ 4:5.1.2

$chla =\frac{0.5*10^{2.7}}{v_{surf}}$ if $v_{surf}>10^{-5}m^3/s$ or ($TP$ and $TN>10^{-6}$) 4:5.1.3

$chla=\frac{0.5*10^{0.5}}{v_{surf}}$ if $v_{surf}>10^{-5}m^3/s$, $TP<10^{-6}$ and $TN>10^{-6}$ 4:5.1.4