Runoff specifically refers to the water leaving an area of drainage and flowing across the land surface to points of lower elevation. It is not the water flowing beneath the surface of the ground. This type of water flow is called throughflow . A thin water layer forms that begins to move because of the influence of slope and gravity. Flowing water accumulates in depressions.
Depressions overflow and form small rills .
Rills merge to form larger streams and rivers . Streams and rivers then flow into lakes or oceans . On a global scale, runoff occurs because of the imbalance between evaporation and precipitation over the Earth's land and ocean surfaces. Areas having the most runoff are those with high rates of precipitation and low rates of evaporation. Lvovitch, M.L. 1972.