# area\_ps For SWAT+ to calculate the reservoir surface area each day, the surface area at two different water volumes must be defined. Variables referring to the principal spillway can be thought of as variables referring to the normal reservoir storage volume, while variables referring to the emergency spillway can be thought of as variables referring to the maximum reservoir storage volume. If a shapefile is used to implement reservoirs in QSWAT+, the interface will assume that the reservoir surface area at principal spillway equals the area of the polygon. --- # 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/hydrology.res/area_ps.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.