A more thorough explanation:

Surplus water refers to water that is not needed for the reasonable beneficial uses of those who have priority rights. It is also known as excess water.

Examples of surplus water include water running off irrigated ground and water that is left over after a chemical or manufacturing process.

For instance, when farmers irrigate their crops, some of the water runs off and is not used by the plants. This excess water is considered surplus water. Similarly, in a manufacturing process, some water may be left over after the process is complete. This water is also considered surplus water.

Overall, surplus water is any water that is not needed for its intended purpose and can be used by others for their own purposes.