> 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/aquifers/aquifer.aqu/spec_yld.md).

# spec\_yld

The specific yield is defined as the ratio of the volume of water that drains by gravity to the total volume of rock. This variable is required to calculate groundwater height fluctuations.