Lec 12: Soil water & groundwater II

Question 1

Define the saturated and unsaturated hydrologic soil horizons in terms of water pressure

Answer 1

unsaturated zone: negative water pressure
saturated zone: positive water pressure

Question 2

What is infiltration?

Answer 2

The flow of water into the ground through the earth surface.

• Infiltration may be regarded as either a rate or a total ([L T-1] or [L])

rate: infiltration rate (f(t))
total: cumulative infiltration (F(t))

Question 3

what is cumulative infiltration?

Answer 3

depth of infiltration from the beginning of rainfall to a particular time t.

Question 4

What is a wetting front?

Answer 4

A wetting front is present when the change of soil moisture content with depth is
so great that there is a sharp discontinuity in the soil profile, between the wet soil
above and the dry soil below.

Question 5

What is time of ponding?

Answer 5

time at which the surface soil layer becomes saturated
after the rainfall rate exceeds the hydraulic conductivity.

Question 6

How can infiltration be determined

Answer 6

field measures
- Infiltrometer: An artificial application of water to enclosed sample areas

models
- Horton

Question 7

define f(t), f0, fc

Answer 7

f(t) = infiltration rate
f0 = initial infiltration rate of a given soil
fc = infiltration capacity of a given soil

Question 8

control factors on infiltration (5)

Answer 8

• soil surface conditions
• soil subsurface conditions
• hydrophobicity
• flow influences
• influencing factors

Question 9

What soil factors affect subsurface conditions and therefore influence infiltration? (6)

Answer 9

Subsurface conditions
– Soil
* Texture
* Porosity
* Depth
* Shrink and swell
* Layering
* Spatial variability

Question 10

Rank the following soil types by highest to lowest infiltration rate
- clay soils
- high organic matter soils
- sandy soils

Answer 10

• Sandy soils have the highest infiltration rates
• Clay soils have the lowest infiltration rates
• High organic matter improves infiltration rates

Question 11

Besides soil type, what factors affect soil infiltration? (4)

Answer 11

– Root system
– Water table depth
– Subsurface drainage
– Hydraulic conductivity

Question 12

What factors affect soil surface conditions?

Answer 12

– Roughness and slope
– Cracking and crusting
– Surface sealing
– Land use and vegetation cover

Question 13

Which factors affect both soil subsurface and surface conditions, and therefore affect soil infiltration?

Answer 13

• Compaction through mechanical processes, e.g., plowing, overgrazing
• Frost-freeze-thaw cycles
• Litter layer, organic matter
• Antecedent soil water conditions
• Chemical, biological and microbial activity

Question 14

how does hydrophobicity impact soil infiltration?

What is it driven by?

Answer 14

• Hydrophobic soil causes water to collect on the soil surface rather than
  infiltrate into the ground (water repellency)
• Driven by extreme dryness and/or fire

Question 15

What flow influences impact infiltration?

Answer 15

– Ponding
– Soil and water temperature