During, as well as shortly after a storm event, stream flow is dominated by storm flow resulting from overland flow and lateral subsurface flow. Between rainfall events, stream flow is dominated by base flow resulting from ground water discharge. Runoff is that portion of rainfall that leaves a land area as stream flow. Water yield is the sum of stream flow and ground water discharge from a contributing land area. Because the sediment, chemicals, heat, and biota in water is an important component of the hydrologic cycle, the hydrologist must be concerned with water quality.
Water quality refers to the physical, chemical, and biological characteristics of water.
Erosion and nutrient cycling are natural processes which are linked to the hydrologic cycle. Soil generation and landscape development are in part products of weathering and the movement of sediment through the hydrologic cycle. Nutrients are naturally added and removed from land areas as water cycles thought the land area. As water flows through a land area it exchanges heat energy with the surrounding environment. The natural levels of erosion, sediment transport, nutrient transport, etc., are referred to as background levels.