> 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/introduction-1/reservoirs/sediment.res/d50.md).

# d50

SWAT+ calculates the median sediment particle diameter for impoundments located within a landscape unit using the equation

$$
d\_{50}=exp⁡(0.41\*\frac{m\_c}{100}+2.71\*\frac{m\_{silt}}{100}+5.7\*\frac{m\_s}{100})
$$

where $$d\_{50}$$ is the median particle size of the sediment (µm), $$m\_c$$ is percent clay in the surface soil layer, $$m\_{silt}$$ is the percent silt in the surface soil layer, and $$m\_s$$ is the percent sand in the surface soil layer.

Because reservoirs are located on the channel network and receive sediment from the entire area upstream, defaulting the sand, silt, and clay fractions to those of a single LSU or HRU in the upstream area is not appropriate. Instead, the user may set the median particle size diameter to a representative value.

The table below lists the D50 values of different sediment classes:

| Sediment class | Size \[µm] | Approximate size             |
| -------------- | ---------- | ---------------------------- |
| Boulders       | > 256,000  | > Volley ball                |
| Cobbles        | > 64,000   | > Tennis ball                |
| Pebbles        | > 2,000    | > Match Head                 |
| Coarse Sand    | 1,500      |                              |
| Medium Sand    | 375        |                              |
| Fine Sand      | 94         |                              |
| Coarse Silt    | 47         |                              |
| Medium Silt    | 11.7       | Not visible to the human eye |
| Fine Silt      | 4.9        |                              |
| Clay           | 1.95       |                              |


---

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