6-1

GEOG415 Lecture 6: Soil Water

Infiltration

Movement of water into soil. Importance?

- flood prediction

- erosion

- agriculture

- water resources

Infiltration capacity

Expressed in the same unit as rainfall intensity (mm hr-1).

Significant overland flow occurs when infiltration capacity isexceeded by heavy rainfall.

Dunne and Leopold (1978, Fig. 6-1)

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Infiltration process

Driving force

- gravity

- surface tension (capillary force) … “wicking”

Finer soils have stronger capillary force. Why?

h ∝

Dunne and Leopold (1978, Fig. 6-2)

‘Permeability’ of soil … depends on pore sizes. Why?

areaflow

perimeterdriving force ∝

resistance ∝

Infiltration capacity decreases with time during a storm event.

- reduction of capillary force

- pore blockage by fines

- soil swelling

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Controls of infiltration

Three processes are involved in infiltration:

- entry

-storage

-transmission

What control these processes?

- pore sizes

- macro features (cracks, root holes, etc.)

- depth of the permeable soil

- rainfall characteristics

- antecedent soil moisture condition.

- frozen soil

0

50

100

0 20 40 60time (min)

infil

tratio

n ra

te (m

m/h

r) grasscultivation

Infiltration test, Saskatoon

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Effects of vegetative cover

- protects soil surface from direct impacts of rain drops.

- enhances soil structures and macro features

- ??

Effects of cultivation?

“Zero till” … an alternative to conventional cultivation.

Infiltration measurement

Single-ring infiltrometer- constant water level in the ring- overestimates infiltration. Why?

- good for comparing the relative magnitudes of infiltration.

Plot/watershed study

The amount of overland flow during a storm is measured andcompared against the total precipitation. This method is usedto estimate spatially averaged infiltration capacity.

Does this really measure infiltration capacity?

See next page for an example →

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Dunne and Leopold (1978, Fig. 6-6)

Estimation from soil/vegetation type

A rough, semi-quantitative estimate of infiltration capacity can be made from soil type and the characteristics of the surface(e.g. vegetation).

See DL, p.171.

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Soil moisture storage

Soil moisture is a minor component in terms of relativequantity (0.064 % of world’s fresh water reserve), but it is avery important component of the hydrologic cycle. Why?

Soil water is held by “tension”, which refers to the force thatsoil particles exerts to “pull” water molecules against gravity.

Dunne and Leopold (1978, Fig. 6-8)

Tension consists of capillaryforce and osmotic force.

Capillary force is generallymuch higher. Exception?

Unit of tension

1 atm = 101.3 × 103 N m-2 = 10.3 m of water column

The soil absorb water through the porous cup.

What happens to pressure in the air pocket?When will the flow stop?

We can estimate soil tension from the pressure in the air pocket.

air pocket

water

ceramic cup

vacuum gauge

Soil tension is measured by a tensiometer.

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Soil moisture measurement

Gravimetric method- Weigh a known volume of sample.- Dry it in an oven, and weigh again.- Accurate, but non-repeatable.

Neutron scattering method- Neutrons are emitted from a probe placed in a tube.- They are scattered and slowed down by hydrogen atoms.- The number of returning “slow” neutrons is related to

moisture content by a calibration procedure.- Sampling volume : 30 cm radius.

Time domain reflectometry (TDR)- Speed of electromagnetic (EM) wave

3.0 × 108 m/s in air 0.33 × 108 m/s in waterHigher water content results in slower velocity.

- Send EM waves into the soil and measure the velocity.- High spatial resolution (< 10 cm).- Usually limited to a small depth (1-2 m).- EM waves are easily lost in saline soils.

neutron source

counter

TDR box

EM waves

Reflected at the endof wave guides

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0.01

0.1

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10

100

0 0.1 0.2 0.3 0.4 0.5water content

soil

tens

ion

(m)

field

laboratory

Clay-loam soil, Saskatoon

Soil-moisture characteristics

The relationship between water content and soil tension. This is measurable in the field and laboratory.

Field capacity refers to the soil-moisture condition after free drainage of soil. It is commonly represented by soil tension of 1 m.

Why?

Wilting point refers to the condition when all plant-accessible water is depleted. It is commonly represented by soil tension of 150 m.

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Soil-moisture monitoring in a clay-loam soil under a wheat field near Saskatoon.

0.3

0.32

0.34

0.36

0.38

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6/25 7/10 7/25 8/9 8/24 9/8 9/23 10/8

wat

er c

onte

nt

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4

soil

tens

ion

(m)

water cont.tension

Heavy rain on Aug. 6

Harvest in early Sep.

1994

Average water content in top 60 cmSoil tension at 40 cm.

Soil water content in a sandy loam at a depth of 0.3 m.

Dunne and Leopold (1978, Fig. 6-10)

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Porosity of most natural soils =

Field capacity depends on the soil ‘texture’. Why?

Dunne and Leopold (1978, Fig. 6-9)

Note that the ‘heavy textured’ soil may have a fair amount of water, but this water is not accessible to plants.

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Movement of soil water

Driving force: gravity and the gradient of soil tension.

Water has a tendency to move to drier regions.

Soil’s ability to transmit water is called hydraulic conductivity.

Conductivity increases with increasing moisture. Why?

saturated soil drier soil

0rain water content ?

Gravity-driven flow. Why?

High gradient of tension“wetting front”

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Dunne and Leopold (1978, Fig. 6-14)

Water is slowly redistributed in the soil after the rainfall event.

Gravity and tension gradient are still the driving force, but flow resistance is much higher in unsaturated soil. Why?

What is the implication on evaporation and transpiration?

Summer fallow in the prairies

Dry land agriculture requires leaving the land cultivated but unseeded every 3-4 years. This practice is called summer fallow. Very little evaporation occurs on the summer-fallow field and soil moisture is conserved during the summer, which reduces the moisture deficit in the following year.