# mid\_p\_stl

A negative settling rate indicates that the reservoir sediments are a source of P. A positive settling rate indicates that the reservoir sediments are a sink for P.

For natural lakes, measured phosphorus settling velocities most frequently fall in the range of 5 to 20 m/year although values less than 1 m/year to over 200 m/year have been reported (Chapra, 1997). Panuska and Robertson (1999) noted that the range in apparent settling velocity values for man-made reservoirs tends to be significantly greater than for natural lakes. Higgins and Kim (1981) reported phosphorus apparent settling velocity values from –90 to 269 m/year for 18 reservoirs in Tennessee with a median value of 42.2 m/year. For 27 Midwestern reservoirs, Walker and Kiihner (1978) reported phosphorus apparent settling velocities ranging from –1 to 125 m/year with an average value of 12.7 m/year.

The table below lists recommended apparent settling velocity values for phosphorus (Panuska and Robertson, 1999):

| Water body nutrient dynamics                                | Settling velocity \[m/year] |
| ----------------------------------------------------------- | --------------------------- |
| Shallow water bodies with high net internal phosphorus flux | <= 0                        |
| Water bodies with moderate net internal phosphorus flux     | 1 - 5                       |
| Water bodies with minimal net internal phosphorus flux      | 5 - 16                      |
| Water bodies with high net internal phosphorus removal      | > 16                        |