Definition - What does Rill Erosion mean?
Rill erosion is a type of erosion that results in small, yet well defined streams. It happens when water from rainfall does not soak into the soil, but runs across it instead. The rills or small channels (often only 30cm deep) are caused when water running across the surface of the ground gathers in a natural depression in the soil, and erosion is concentrated as the water flows through the depression.
Safeopedia explains Rill Erosion
Any type of soil erosion can pose a problem in agriculture as it means loss of topsoil, which contains most of the nutrients needed to grow crops. Many different organisms live in this soil, which interact with the minerals in rocks and produce fertile soil. Erosion of any kind, therefore, means the washing away of fertile soil in the top layer.