Conceptual model for the element-based approach to rainfall runoff.

• Lateral flow is possible at the surface and in the saturated zone only.
• Field evidence (Dunne) suggests that flood producing runoff will either result from subsurface flow or surface flow resulting from a variable area in which soil storage capacity rather than infiltration capacity is exceeded.
• Storage in the root zone and saturated zone is recorded in terms of a deficit from the capacity for the respective zone.
• Because calculations are done on a per-element basis, the storage and deficit quantities can be thought of as volumes.