> For the complete documentation index, see [llms.txt](https://swatplus.gitbook.io/io-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://swatplus.gitbook.io/io-docs/theoretical-documentation/section-8-water-bodies/nutrients-in-water-bodies/eutrophication/phosphorus-chlorophyll-a-correlations.md).

# 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       |


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