The rainfall-runoff relationship for any rainstorm depends on the dynamic interaction between rain intensity, soil infiltration and surface storage.

Runoff occurs whenever rain intensity exceeds the infiltration capacity of the soil

Runoff from a given rainstorm is a function of:

i.Rainfall intensity distribution and sequence, during a particular rainstorm event

ii.Soil infiltration rates

iii.The soil surface storage capacity

Factors Affecting Runoff:

Climatic factors 

1. Type of precipitation 
2. Rainfall intensity
3.  Duration of rainfall
4.  Rainfall distribution
5.  Direction , velocity of prevailing wind
6. Annual rainfall 
   

Physiographic factors

1. Size of watershed
2. Shape of watershed
3. Slope of watershed
4. Orientation of watershed
5.  Land use
6. Soil moisture & type

Runoff Processes 

1. The paths water can take in moving to a stream are illustrated in Figure 1. Precipitation may be in the form of rain or snow. Vegetation may intercept some fraction of precipitation. Precipitation that penetrates the vegetation is referred to as throughfall and may consist of both precipitation that does not contact the vegetation, or that drops or drains off the vegetation after being intercepted.
2. A large fraction of intercepted water is commonly evaporated back to the atmosphere. There is also flux of water to the atmosphere through transpiration of the vegetation and evaporation from soil and water bodies.
3. The surface water input available for the generation of runoff consists of throughfall and snowmelt. This surface water input may accumulate on the surface in depression storage, or flow overland towards the streams as overland flow, or infiltrate into the soil, where it may flow laterally towards the stream contributing to interflow.
4. Infiltrated water may also percolate through deeper soil and rock layers into the groundwater. The water table is the surface below which the soil and rock is saturated and at pressure greater than atmospheric. This serves as the boundary between the saturated zone containing groundwater and unsaturated zone. Water added to the groundwater is referred to as groundwater recharge.
5. Immediately above the water table is a region of soil that is close to saturation, due to water being held by capillary forces. This is referred to as the capillary fringe.
6. Lateral drainage of the groundwater into streams is referred to as baseflow, because it sustains streamflow during rainless periods.
7. Subsurface water, either from interflow or from groundwater may flow back across the land surface to add to overland flow. This is referred to as return flow. Overland flow and shallower interflow processes that transport water to the stream within the time scale of approximately a day or so are classified as runoff.
8. Water that percolates to the groundwater moves at much lower velocities and reaches the stream over longer periods of time such as weeks, months or even years. The terms quick flow and delayed flow are also used to describe and distinguish between runoff and baseflow.
9. Runoff includes surface runoff (overland flow) and subsurface runoff or subsurface stormflow (interflow).