# Surface Area

The surface area of the reservoir is needed to calculate the amount of precipitation falling on the water body as well as the amount of evaporation and seepage. Surface area varies with change in the volume of water stored in the reservoir. The surface area is updated daily using the equation:

                    $$SA=\beta\_{sa}\*V^{expsa}$$                                                             8:1.1.2

where $$SA$$ is the surface area of the water body (ha), $$\beta\_{sa}$$ is a coefficient, $$V$$ is the volume of water in the impoundment (m$$^3$$ H$$\_2$$O), and $$expsa$$ is an exponent.

            The coefficient, $$\beta\_{sa}$$, and exponent, $$expsa$$, are calculated by solving equation 8:1.1.2 using two known points. The two known points are surface area and volume information provided for the principal and emergency spillways.

           $$expsa=\frac{log\_{10}(SA\_{em})-log\_{10}(SA\_{pr})}{log\_{10}(V\_{em})-log\_{10}(V\_{pr})}$$                                                8:1.1.3

            $$\beta\_{sa}=(\frac{SA\_{em}}{V\_{em}})^{expsa}$$                                                                    8:1.1.4

where $$SA\_{em}$$ is the surface area of the reservoir when filled to the emergency spillway (ha), $$SA\_{pr}$$ is the surface area of the reservoir when filled to the principal spillway (ha), $$V\_{em}$$ is the volume of water held in the reservoir when filled to the emergency spillway   (m$$^3$$ H$$*2$$O), and $$V*{pr}$$ is the volume of water held in the reservoir when filled to the principal spillway (m$$^3$$ H$$\_2$$O).​      


---

# 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/theoretical-documentation/section-8-water-bodies/impoundment-water-routing/reservoirs/surface-area.md?ask=<question>
```

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.