# 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 | | --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://swatplus.gitbook.io/io-docs/introduction-1/reservoirs/sediment.res/d50.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.