Flow through a watershed

Very little of the water in a river actually rains out onto it. Rather, it travels across and under the landscape to reach the stream.

Learning Goals

• Know the four main ways that water moves through a watershed: overland flow (saturation excess or infiltration excess), interflow, and groundwater
• Understand how these mechanisms can relate to the arrival of water in rivers

Videos

Anne Jefferson is a hydrology professor at Kent State (and also an active science blogger!). We’re going to learn a bit of our hydrology from her: she’s the real pro!

In these lessons, Anne will teach us about the ways that water flows through catchments and to rivers.

But first, a good visual

(psst…. this is saturation excess overland flow)

Infiltration excess overland flow

When it’s raining harder than water can infiltrate into the ground, water instead flows across the surface.

Saturation excess overland flow

When the ground is completely filled with as much water it can hold, there’s nowhere else for the remaining runoff to go except across the surface.

Interflow (or subsurface stormflow)

Shallow subsurface flow moves towards the stream: slower than surface water, faster than deep groundwater.

Groundwater

Groundwater slowly moves through the deep subsurface, but can provide persistent “baseflow” to rivers in humid climates.

The arrows show the paths taken by groundwater through the subsurface, including flow paths towards the river (at right). Note that it takes days to (many!) years for this water to reach the river. This lies in contrast to the rapid interflow and overland flow that cause flooding. Image: USGS

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