Phosphorus/Chlorophyll a Correlations

A number of empirically derived equations have been developed to calculate chlorophyll a level as a function of total phosphorus concentration. SWAT+ uses an equation developed by Rast and Lee (1978) to calculate the chlorophyll a concentration in the water body.

$Chla=0.551*p^{0.76}$ 8:3.3.1

where $Chla$ is the chlorophyll $a$ concentration ($\mu$g/L) and $p$ is the total phosphorus concentration ($\mu$g/L).

The equation has been modified to include a user-defined coefficient:

$Chla=Chla_{co}*0.551*p^{0.76}$ 8:3.3.2

The user-defined coefficient, $Chla_{co}$, is included to allow the user to adjust the predicted chlorophyll $a$ concentration for limitations of nutrients other than phosphorus. When $Chla_{co}$ is set to 1.00, equation 8:3.3.2 is equivalent to equation 8:3.3.1. For most water bodies, the original equation will be adequate.

While evaluation of water quality by secchi-disk depth measurements is subjective, some general correlations between secchi-disk depth and public perception of water quality have been made. One such correlation made for Annebessacook Lake in Maine (EPA, 1980) is given in Table 8:3-3.

Table 8:3-3: Relationship between secchi-disk depth and public perception of water quality.

Secchi-disk depth (m)	Public perception of water quality
0.0-0.9	gross pollution; water body totally unsuitable for recreation
1.0-1.9	algae blooms still evident; quality unacceptable for most uses
2.0-2.9	some complaints of declining water quality; some impairment of water use
3.0-3.9	satisfactory quality; no impairment of water use
4.0-4.9	excellent water quality; a positive factor encouraging use of lake
5.0+	exceptional quality

Table 8:3-4: SWAT+ input variables that impact eutrophication calculations in ponds, wetlands and reservoirs.

Variable Name	Definition	Input File
CHLA	$Chla_{co}$ :variable for calculation of chlorophyll a concentration in a pond	.pnd
CHLAW	$Chla_{co}$:variable for calculation of chlorophyll a concentration in a wetland	.pnd
CHLAR	$Chla_{co}$ :variable for calculation of chlorophyll a concentration in a reservoir	.lwq
SECCI	$SD_{co}$:variable for calculation of secchi-disk depth in a pond	.pnd
SECCIW	$SD_{co}$ :variable for calculation of secchi-disk depth in a wetland	.pnd
SECCIR	$SD_{co}$:variable for calculation of secchi-disk depth in a reservoir	.lwq