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Geog370Hdrlgy > Soil Water > Flashcards

Soil Water Flashcards

1
Q

SMAP

A

Soil Moisture Active Passive

  • New satellite
  • Will produce global maps of soil moisture to help improve knowledge of water and carbon cycles and water management
  • Can access 1st few cm’s of surface, even through thick vegetation
  • Radar is already broken but radiometer still functions
  • ~3 days to cover the planet with a large swath
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2
Q

Vadose Zone

A
  • Unsaturated Zone
  • Zone of aeration
  • Realm of soil moisture
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3
Q

Phreatic Zone

A
  • Zone of saturation

- Realm of Ground water

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4
Q

How is soil the 1st line of defense?

A

It filters contaminants such as pathogens from water before it enters groundwater

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5
Q

What are the possible pathways for rain?

A
  • Evaporate
  • Runoff
  • Infiltrate
  • Drawn by plants and transpired by weeds, crops, and trees
  • Soil moisture
  • Soil Water
  • Ground water
  • Groundwater flow
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6
Q

Why study soil water?

A
  • Flood prediction
  • Contaminant migration (mitigation?)
  • Erosion
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7
Q

What does soil water have to become in order to reach the ground water zone?

A

Gravitational water

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8
Q

Why is soil water study important for agriculture?

A

Knowing the soil type and how it interacts with water can help understand how much the plants require and if the soil water is enough or if irrigation is necessary

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9
Q

Why is soil water study important for Geotechnical engineering?

A

Water content and how it relates to hazards (sink holes, etc)

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10
Q

What is soil made of?

A
  • Mineral ~45%
  • Air ~20-30% (air can be displaced by water, saturated soil has 0% air)
  • Water ~20-30%
  • Organics ~5%
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11
Q

What are the 3 main particle sizes in soil? What are other less common sizes?

A 
Common:
- Clay
- Silt
- Sand
Less Common:
- Pebbles
- Rocks
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12
Q

What is the organic matter in soil composed of?

A
  • Organisms 10%
  • Roots 10%
  • Humus 80%
  • -> Content can vary significantly
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13
Q

What is the clay fraction?

A
  • Highly weathered particles
  • Mostly clay minerals, plagioclase
  • Adsorbs soil water very effectively
  • determines a soils hydrological properties
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14
Q

Adsorption

A

Water in a surface layer, stuck through electromagnetic forces (more or less hydrogen bonds)

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15
Q

What, above all, determines a soil’s hydrological properties?

A

The clay fraction

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16
Q

What is the sand and silt fractions?

A
  • Less weathered particles
  • Less reactive/charged particles
  • Retains less soil water
  • Larger pores, higher hydraulic conductivity
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17
Q

Clay soil vs. Sandy soil

A

Clay:
Greater total pore volume = Greater porosity, less permeable
Sand:
Less total pore volume = Less porosity, more permeable

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18
Q

Components of Soil Texture

A

Soil triangle can be used anywhere on the planet

  • lithology
  • angularity vs. roundness
  • complexity
  • Particle size
  • Mineral layers
  • Sorting
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19
Q

Soil properties

A
  • Grain size, type
  • Sorting, variability in size
  • Bulk density, packing
  • Water content, degree of saturation
  • Hydraulic conductivity
  • Thermal state
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20
Q

What is the Gravimetric water content?

A

The mass fraction of water in the soil

θg = Mass water / Mass soil

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21
Q

Gravitational water

A

The water in a soil that is freely available for gravitational drainage

i. e. free water in the pore space
- Saturated

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22
Q

Hygroscopic water

A

Held tightly to a particle

- Unavailable to plants, but possible a little available to the atmosphere

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23
Q

Field Capacity

A
  • The maximum water content that a soil can hold against gravity
  • Gravitational power exceeds capillary power and the water moves down/drains through
  • Water in a soil that can be retained against gravity through adsorbed water and capillary pressure
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24
Q

Matric Potential

A

= capillary pressure = suction (negative pressure)

  • Equivalent term fro pressure head from Bernoulli’s equation and is often used to describe negative pressure in the unsaturated zone
  • Commonly associated with capillary pressure due to surface tension from the soil minerals/grains. This capillary pressure is a form of suction or “negative pressure” which draws water up into the soil
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25
Q

Permanent wilting point

A

Water content at which plants cannot draw water out of the soil

26
Q

Available water

A

Water available to plants
θa = θfc - θpwp
Available water = Field capacity - permanent wilting point

27
Q

Capillary water

A

Water held in micropores

  • available water
  • plant roots can absorb this
28
Q

Adsorbed water

A

Hygroscopic

- Dry soil

29
Q

Soil texture vs. percent soil water

A

Sand has more potential for gravitational water while finer particles have more potential for hydroscopic water and less for gravity water
- Loams have a higher percentage of soil water overall

30
Q

Phreatic zone

A
  • Ground water zone
  • Saturated zone
  • P water > P atmosphere
31
Q

Vadose Zone

A
  • Unsaturated zone
  • Root zone, intermediate zone, Capillary fringe
  • Pwater
32
Q

Water Table

A

P water = P atmosphere

33
Q

Hydraulic Head, h

A

Total potential

  • Gravitational potential (elevation)
  • Pressure/matric potential
  • Function of elevation and Pressure
34
Q

Bernoulli’s Equation for hydraulic head, h

A

Total energy = KE + PE + ‘pressure’ energy
- Reduces to h = z + P/density*g (ie head = elevation + pressure)


35
Q

Bernoulli’s Equation applied to soils

A

Head = elevation + matric potential
pF = log10 (-ψ)
- Log scale for convenience

36
Q

pF = 2

A

~ the wilting point

37
Q

Surface tension

A

is a function of the interaction between the two materials at the interface

38
Q

Matric potential (suction) vs. Capillary pore size

A

Suction is greater with smaller pore spaces

39
Q

Matric potential (suction) vs. Volumetric moisture content

A

Suction is a function of moisture content
- Suction is also a function of where it is wetting or drying
- An also antecedent conditions & soil type
- Suction is stronger with drier soils
ie inverse relationship with moisture content
- evaporation can draw energy up when elevation hasn’t changed

40
Q

Why does the Matric potential (suction) vs. Volumetric moisture content change and how

A
  • When wetting the small pores fill first (highest suction) and water content increases slowly
  • When drying the large pores empty first so there is a rapid decline in water
41
Q

What are three things that relate to matric potential?

A
  • Pore size
  • Volumetric moisture content
  • Pore structure
42
Q

Negative Energy (cm)

A

Negative energy = upwards flow or suction

  • Evaporation increases negative energy & suction
  • Evaporation also changes the elevation of hydraulic head and brings it into negative energy
  • Evap causes moisture decrease + Suction increase
43
Q

Positive Energy (cm)

A

Positive energy = Downwards flow or gravitational

44
Q

Types of flow

A
  • Ponded & non-ponded infiltration
  • Macropore flow
  • Fingering flow
  • Funnel flow
45
Q

Types of flow:

Ponded vs. non-ponded infiltration

A

Ponded: Infiltrating from a pond, puddle, lake, etc and may or may not be flowing
Non-Ponded: From rain, high infiltration or low precipitation rate where it would be less likely to pool

46
Q

Types of flow:

Macropore Flow

A

> 30 micrometers

  • often created by animals or inverts (earthworms)
  • Water easily makes it into this zone but then depends on the end of this and the micropores next
47
Q

Types of flow:

Fingering flow

A
  • Fingers of water flow down

- Soil profile can change the pattern or stop the flow

48
Q

Types of flow:

Funnel Flow

A
  • Large roots, rocks
  • Preferential flow along these less penetrable surfaces
  • Changes volumetric content in an area
49
Q

Why does hydraulic conductivity decrease when matric potential increases?

A
  • Small clay pores increase suction but they are not well connected
  • Clay holds lots of water but it isn’t available to plants or atmosphere
50
Q

Hydraulic conductivity, K

A

A measure of how effectively water moves through a substrate under a given hydraulic (head or potential) gradient.
- Units m/s or cm/day

51
Q

Permeability κ (m2 or cm2) vs. Hydraulic Diffusivity, Kd (m2/s)

A
  • Almost synonymous, with different definitions

- All measure how well water moves through a porous medium

52
Q

Hydraulic conductivity as a function of matric potential

A
  • High water content increases water flow in coarse soils
  • Water stores increase with clay and high matric potential, therefore low water content and water easily flows through fine soils
  • Hydraulic conductivity decreases rapidly in course soils as matric potential increases
  • Hydraulic conductivity decreases slowly as matric potential increases in clays
53
Q

K (vol. water content) Vs. Vol. Water Content

A
  • At high water contents coarse soils have a higher hydraulic conductivity than fine soils
  • At low water contents fine soils have higher hydraulic conductivity than coarse soils because the small pores per unit volume are filled with water and because most of the large pores are empty in course soils
  • At low water content the tension head (matric potential of vol. water content) is higher (less negative) in fine soils
54
Q

Soil Water Characteristic Curve

A
  • At low water content the tension head (matric potential of vol. water content) is higher (less negative) in fine soils
  • Matric potential (tension head) is high with low water content
  • Hydraulic conductivity, K, is higher with higher water content
55
Q

Soil Measurements

A
  • Double Ring Infiltrometer
  • Tensiometer
  • Rainfall simulator
56
Q

Soil water content

A
  • Oven drying, ceramic block, GPR, TDR

- Pressure plate

57
Q

What causes water contents below hygroscopic?

A
  • Human interference

- not a normal range of water contents than an undisturbed soil can have

58
Q

Why can’t plants easily access hygroscopic water?

A

Because the water is held to tightly to the soil particles

- That is why it is known as the permanent wilting point

59
Q

Why can’t plants easily access gravitational water?

A

It flows through the soil profile and isn’t easy to be uptaken by the plants
- Known as field capacity

60
Q

What is the potential when adding calcium to soil (in fertilizer)

A
  • Calcium may reduce surface tension and water may preferentially flow into micropores where it is held longer and can be accessed by plants longer
  • Flow is preferential through macropores usually and the water flows through these faster
  • Micropores may hold water longer

Geog370Hdrlgy (9 decks)

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