What initiates shallow concentrated flow in a watershed?

Shallow concentrated flow in a watershed is primarily initiated when water begins to form rills. Rills are small, shallow channels that develop on the surface of the soil due to the movement of water during rainfall events or melting snow. As water accumulates and flows over the land surface, it can start to erode the soil, creating these small channels. Once rills are formed and the water concentration increases in these pathways, it can lead to more substantial runoff, creating the conditions for shallow concentrated flow. This process is an essential part of the hydrological cycle in a watershed, influencing sediment transport and erosion patterns.

The other options relate to different concepts in hydrology or watershed management. The flow in large rivers typically involves deeper, more established channels rather than the shallow flow dynamics seen in rills. High evaporation rates do not contribute directly to flow mechanisms but indicate a loss of water that may affect surface runoff. Deforestation activities can lead to increased erosion and runoff but do not directly initiate shallow concentrated flow unless they result in conditions conducive to rill formation.