# Open water evaporation Open water evaporation is determined in reservoirs and lakes, only when the reservoir and/or lake module is used. A nominal map should be provided with the extent per open water body. Besides, a map should be provided with the fraction per cell occupied by open water. These maps are used to partly switch of the soil-water processes in the corresponding cells, depending on the fraction of the cell occupied by open water. --- # 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://futurewater.gitbook.io/sphy-manual-3.0/manual/sphy-manual/build-your-own-sphy-model/open-water-evaporation.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.