Water is a solvent for vital plant nutrients. The presence and movement of water through the soil affect the availability of these nutrients to the plant. Water will drain quickly across a sloped surface. This is called surface drainage. Additionally, there is internal drainage.
Gravity moves water vertically through the soil.
Even in highly sloped terrain, water drains internally through the soil as well as over the surface. Texture, structure and physical condition of surface and subsoil layers affect vertical drainage and capacity of soil to store water. The ideal moisture level is reached when water occupies one half of the pore space in soil structure. Soil is saturated when all the pore space is filled with water. Saturated soil has no oxygen in its pore spaces.