Lec 15: Runoff Generation II

Question 1

What does Q = at all times?

Answer 1

At all times: Q = OF + SSSF + GWF

Question 2

Name two specific overland flow pathways

Answer 2

• infiltration-excess or Hortonian overland flow (HOF)
• saturation-excess overland flow (SOF)

Question 3

Name 5 types of subsurface flow

Answer 3

• Matrix flow (Water moves through small soil pores)
• Perched matrix flow (Water moves through small soil pores above a perching layer)
• The subsurface fill and spill mechanism (Water spills from one depression to another at a soil-confining layer interface)
• Macropore flow (Water moves through larger soil pores)
• Pipeflow (Water moves through large soil cavities)
• Groundwater flow (Water moves below the water table, in soil or rock (in aquifers and in some aquicludes): it is often considered as “deep matrix flow”

Question 4

Describe the steps in the development of shallow subsurface stormflow

Answer 4

1) At the start of an event, percolation occurs vertically
vertical hydraulic conductivity > lateral hydraulic conductivity

2) Soil moisture increases and some water reaches the water table

3) The water table rises towards the soil surface

4) Before the water table reaches the soil surface:
* Most soil pores are filled with water and hydraulic conditions change
* Lateral hydraulic conductivity > Vertical hydraulic conductivity
* Water flows laterally through the soil matrix (SSSF)

Question 5

Describe the steps in the development of perched shallow subsurface stormflow

Answer 5

• At the start of an event, percolation occurs vertically
• Soil moisture increases
• percolating water reaches a less permeable layer that will not accept the wetting front –> saturation develops
• The wetting front (or perched water table) rises towards the soil surface
• Before the perched water table reaches the soil surface:
• Most soil pores are filled with water and hydraulic conditions change
• Lateral hydraulic conductivity > Vertical hydraulic conductivity
• Water flows lateral through the soil matrix (perched SSSF)

Question 6

How does the subsurface fill and spill mechanism occur?

Answer 6

• hillslopes have relatively permeable boundaries that restrict water flow (e.g. unweathered bedrock)
• bedrock ‘‘hollows’’ must be completely filled before continuous flow pathways for subsurface stormflow can develop

Question 7

Why do we care about macropore flow?

Answer 7

Enhanced contaminant transport risk

Question 8

Name the two biopore types and their diameters

How can biopores be altered?

Answer 8

• Fauna-created biopores:
  diameters between 1 and 50 mm near the soil surface
• Flora-created biopores: density
  decreases with depth

all biopores can be physically and chemically altered

Question 9

Define:
- macropore flow initiation
- macropore flow interaction

Answer 9

Macropore flow initiation: Water supply to the macropore

Macropore flow interaction: Water transfer between the macropore and the surrounding soil matrix (also called lateral infiltration)

Question 10

Macropore flow depends on:

Answer 10

• Pore size distribution
• Interconnection between pores

Question 11

Types of macropores in soils

Answer 11

• Fauna : 1 to 50 mm at the surface
• Flora: 35 % volume decreases
  with depth
• Chemical or physical alteration

Question 12

How do macropores influence runoff processes?

Answer 12

INSERT

Question 13

What are subsurface preferential pathways?

Answer 13

Uneven and often rapid movement of water and solutes through soil,
characterized by regions of enhanced flux such that a small fraction of media
(such as wormholes, root holes, cracks) participates in most of the flow

Question 14

Where does preferential flow occur?

Answer 14

• In distinctive structures in the soil where water flows only under gravity
  (e.g., macropores)
• In areas with a higher permeability than the surrounding soil matrix (e.g.,
  soil-bedrock interface, interface between the soil matrix and an impeding
  layer)