# hru\_ps The fraction of HRU area at principal spillway is multiplied with the HRU area to calculate the principal surface area in ha. However, this value is only used for estimating the initial wetland area. For daily wetland simulations, the model assumes for simplicity that the entire HRU is wet, so the HRU area is used instead of the principal surface area to calculate the principal volume of the wetland. Accordingly, the parameter hru\_ps has very limited impact on wetland calculations. --- # 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/wetlands/hydrology.wet/hru_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.