Run-off describes the movement of water from rainfall or snowmelt across the land surface into channels, streams and rivers to finally enter into the ocean. Runoff happens when rainfall or snowmelt occurs on soil which is fully saturated or frozen or when a large amount of rain falls over a short period because the soil cannot absorb the rainfall water. In models runoff is generally estimated as the difference between rainfall and evaporation.

In the future century it is estimated that more intense rainfall and more frequent floods could result in greater surface runoff; this leads to less water recharging ground water stores and subsequently influences the amount of water available to maintain rivers during dry periods. These are important water resources issues and being able to predict what occurs regional is useful to managers in order to ensure a sustainable water supply for human and ecosystem activities.

Runoff is one of the variables generated by the JULES model and the modelled outputs are mapped here.
The JULES model, represent the land surface and total runoff is calculated by adding surface and subsurface flow.