A runoff models or rainfall-runoff model describes how rainfall is converted into runoff in a drainage basin. More precisely, it produces a surface runoff hydrograph in response to a rainfall event, represented by and input as a hyetograph.
Rainfall-runoff models need to be calibrated before they can be used.
Figure 6. Non-linear reservoir with pre-reservoir for recharge
The Nash Model uses a cascade of linear reservoirs to predict streamflow. Software for this model exists.
Surface runoff is the unconfined flow of water over the ground surface, in contrast to channel runoff. It occurs when excess rainwater, stormwater, meltwater, or other sources, can no longer sufficiently rapidly infiltrate in the soil. This can occur when the soil is saturated by water to its full capacity, and the rain arrives more quickly than the soil can absorb it. Surface runoff often occurs because impervious areas do not allow water to soak into the ground. Furthermore, runoff can occur either through natural or human-made processes.
Runoff flowing into a stormwater drain
Surface runoff from a hillside after soil is saturated
Urban surface water runoff
Urban runoff flowing into a storm drain
