Which component is not typically included in a river basin water balance?

• A. Precipitation
• B. Inflows
• C. Evapotranspiration
• D. Ultraviolet disinfection

Answer: (D)

In a river basin water balance, the focus is on quantities of water moving in, out, or being stored within the basin. The main terms are precipitation that adds water, inflows from upstream or groundwater that bring in water, and losses such as evaporation and transpiration that remove water, along with any changes in storage in lakes, soils, and aquifers. Ultraviolet disinfection is a water treatment step that occurs after water has been collected or conveyed for use; it does not change the amount of water in the basin—it only alters water quality. Because it doesn’t affect the volume or flow balance, it isn’t included in the hydrologic water balance. Therefore, ultraviolet disinfection is the best answer. Precipitation, inflows, and evapotranspiration are all integral parts of the balance since they represent inputs and outputs of water within the basin.