Overland flow

Runoff

All water transported out of a watershed by streams
 • Overland flow
• Interflow
• Baseflow

Horton Overland flow

• water that has not infiltrated

• Flows as a rough sheet of water

• seen in smooth things

interflow

short path, water stays in ground for short amount of time

Baseflow : Path 2

• sustains rivers when no rainfall 

• slow release of water from groundthe

• Infiltrated water reaches the water table

slow sustaining flow in stream

Depression storage : Hortons overland flow

Occurs when precip rate > infil capacity
• Excess water fills small surface depressions
• Stored water does not contribute to stormflow
• Eventually infiltrates or evaporates
• Once depressions filled, then overland flow occurs

Rainfall and infiltration in Horton Overland

• are not constant

• does not necessarily
  occur in all areas of the basin at once

For there to be Horton Overland Flow

• Precipitation rate > infiltration capacity
  • Depressions fill first, then overland flow
  • Subsurface not necessarily saturated

Subsurface stormflow : Path 3

• increased subsurface runoff
  due to precip

  •  Moves slower than Hortonian flow

  • Smaller proportion of event precip

  • faster than baseflow

• happens anywhere water infiltrates

When some surfaces are not perfectly smooth 

Depression storage

• We get puddles where the water sits

where would you expect to see overland flow

urban areas

What are good hydrologic conditions

• not a lot of impervious surfaces

• well-developed soil

• vegeatation

All water will infiltrate into the ground when

rainfall is less than infiltration rate

The four main paths runoff can take

1) Hortons overland flow( water does not inflitrate)

2) Groundwater flow( water inflitrates down into ground and remerges at stream)

3) shallow subsurface stormflow (water that infiltrates and makes its way out into the stream)

4) Direct precipitation on saturated ground

Saturation overland flow :Path 4

• elevated water table to ground state

• only happens insatturated ground 

  • no place for infiltrated water to go and The subsurface water discharges to the surface( return flow)

• direct precipitation onto saturated areas

  • precipitation does not infiltrate

• much faster than subsurface flow

Horton model applies best to 

• less vegetated areas

• areas impacted by humans

Variable source area applies to

• Forests in excellent condition

•  Vegetated, deep, well-developed soils with minimal
  human impact

•  Areas where infiltration capacity is high

Areas Prone to Saturation

• High water table

• Shallow impermeable layer

•  Bottom of slopes

• Concave areas

Factors Influencing Runoff

• Precipitation

  •  Interception( plants)

  • Type( mist, drizzle, snow,etc)

  • Duration( how long)

  • Intensity( in/hr)

  •  Spatial distribution( where is it falling)

• Watershed properties
  • Size
  • Shape
  • Topography
  • Aspect
  • Geology
  • Soils