What phenomenon occurs during the first few minutes of storm water runoff?

The phenomenon known as "first flush" occurs during the initial moments of storm water runoff. This refers to the initial surge of storm water that is generated when rain begins to fall and water starts to flow across surfaces. During this period, the runoff often carries a higher concentration of pollutants, including sediments, oils, and other contaminants that have accumulated on impervious surfaces since the last rainfall.

This initial runoff is significant because it can lead to increased pollution loads entering waterways, making it critical for storm water management practices. Understanding the first flush helps operators in planning and implementing effective storm water management strategies, such as creating retention basins or using green infrastructure, to mitigate pollutants and protect aquatic environments.

Other choices refer to different processes that occur in the water cycle and storm water management but do not specifically highlight the critical initial conditions of storm water runoff after a rain event. For instance, ground saturation refers to the condition when soil is fully soaked with water, which typically occurs later in the storm event. Water diversion relates to the management tactics used to redirect water flow but doesn't characterize the initial runoff phase. Similarly, evaporation increase deals with the phase after rain has stopped and does not pertain to the immediate impact of the first moments of storm water runoff.