When rainwater falls over a watershed, its fate is determined by multiple factors, such as the relief of the watershed, the permeability of the surface it falls on, the presence and density of plant life, and the drainage density of the watershed.
Infiltration occurs when rainwater falls over a low-relief, permeable surface such as sand or other loose sediments. The water soaks into the ground and becomes groundwater. Evapotranspiration is a combination of direct evaporation and transpiration. Transpiration occurs when water is drawn up by plants and re-released into the atmosphere as water vapor.
Runoff occurs when rainwater falls on higher-relief or impermeable surface. Instead of soaking into the ground, the water travels over the surface of the watershed and flows into a drainage channel, such as a natural stream or a stormwater pipe. Runoff water moves through a watershed much more quickly than water that has infiltrated.
Drainage Density refers to the total length of channels found in a watershed, including both natural channels and stormwater drainage infrastructure. When stormwater pipes are added to a watershed, the drainage density is increased. High drainage density leads to increased runoff and reduced infiltration and evapotranspiration in a watershed.