> For the complete documentation index, see [llms.txt](https://water-strategy.gitbook.io/waterstrategy/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://water-strategy.gitbook.io/waterstrategy/tutorials/creating-a-reservoir-system/adding-reservoir-details/background-on-evaporation-and-precipitation-on-reservoirs.md).
# Background on Evaporation and Precipitation on Reservoirs
## Quantifying Evaporation and Precipitation flow
**Evaporation losses** and added storage via **precipitation** are major components of a reservoir's mass balance. Evaporation and precipitation rates are both generally measured in length/time. In metric units this is often **mm/day**.
To get volumetric daily flow rates which are required for Pywr, these rates are then multiplied by the **Area** of the reservoir. In the metric templates in WaterStrategy this Area is generally expressed in **Km2**.
**For a template that uses flow in ****Mm3/day****, a conversion of 0.001 is required to obtain ****Mm3/day****.**
*Evaporation (mm/day) \* Area (km) \* 0.001 = Mm3/day*
**For a template that uses flow in Ml/day, no conversion is required to obtain Ml/day.**
*Evaporation (mm/day) \* Area (km) \* 1 = Ml/day*
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