Runoff
Runoff is precipitation that flows over the land surface into streams, rivers, and lakes instead of soaking into the ground or evaporating.
Definition
Runoff is rain or melted snow that flows across the ground into ditches, creeks, and rivers instead of soaking in, occurring whenever rainfall exceeds the soil's infiltration rate, the soil is already saturated or frozen, or the surface is impervious like pavement or roofs. It follows two main mechanisms: infiltration-excess overland flow, where rainfall intensity exceeds infiltration capacity on compacted or paved surfaces, and saturation-excess flow, where the water table rises to meet the surface in humid forested regions, and a stream's hydrograph blends this surface runoff with shallow subsurface interflow and groundwater-fed baseflow. Runoff is the main pathway carrying sediment, nutrients, and pollutants from land into streams and is what drives flooding, and urbanization measurably raises peak discharge and shortens the time it takes a storm's water to reach a stream.
Example
Water streaming across a parking lot toward a storm drain during a downpour is runoff, and so is muddy water rushing down a hillside trail, while a forest and a parking lot receiving the same storm produce very different results, the forest releasing under $10\%$ of the rain as runoff and the parking lot over $90\%$. Charlotte's Little Sugar Creek rises quickly after storms because roads and roofs shed water fast, watersheds with more than $25\%$ impervious cover are typically severely degraded, and the Mississippi River carries so much runoff-borne nitrogen that it creates a hypoxic dead zone in the Gulf of Mexico each summer.
Key Insight
Pavement and roofs generate more runoff because water cannot soak through them, which is why cities flood more easily than forests, and runoff is the fastest pathway in the whole water cycle but also the hardest to manage, since water that infiltrates takes months or years to reach a stream and arrives filtered, while water that runs off arrives in hours carrying whatever was on the ground.