Surface Hydrology: Soil water storage and percolation

Question 1

Define the Unsaturated Zone.

Answer 1

The portion of the subsurface above the groundwater table where the soil and rock contain both air and water in their pores.

Question 2

Name the three outgoing fluxes that precipitation is distributed into by the unsaturated zone.

Answer 2

Overland flow
Recharge
Evapotranspiration

Question 3

The unsaturated zone does not provide any water storage capacity.

Answer 3

False

Question 4

What is soil?

Answer 4

Soil is a mixture of solids (rock and organic material), liquids, and gases.

Question 5

What are the components of soil by volume and mass?

Answer 5

Solids (VS, MS), Liquids (VL, ML), Gases (VG, MG - negligible mass)

Question 6

Give the formula for dry bulk density (ρB).

Answer 6

ρB = MS / VT

Question 7

Give the formula for porosity (ε).

Answer 7

ε = (VL + VG) / VT or ε = e / (1+e) = (ρS - ρB) / ρS

Question 8

The void ratio (e) is given by ________.

Answer 8

e = (VL + VG) / VS

Question 9

What is the typical dry bulk density (ρB) for clay soils?

Answer 9

A) 1.1 g/cm³

Question 10

Porosity is lower in sands and gravels compared to clay. (True/False)

Answer 10

True

Question 11

What is volumetric moisture content (θ)?

Answer 11

θ = VL / VT

Question 12

What is gravimetric moisture content (θG)?

Answer 12

θG = ML / MS

Question 13

State the formula linking θ and θG.

Answer 13

θ = θG × (ρB / ρW)

Question 14

What are the two categories of soil hydraulic properties?

Answer 14

Water storage properties: θ(ψ) relationship; Water transmission properties: K(ψ) relationship

Question 15

The matric potential (ψ) represents ________.

Answer 15

The potential energy of water related to capillary retention.

Question 16

Which force is least important in determining soil water potential?

A) Capillarity
B) Adsorption
C) Osmosis
D) Gravity

Answer 16

C) Osmosis

Question 17

Define Capillarity.

Answer 17

The tendency for a liquid in a capillary tube to rise or fall as a result of differences in surface tension between the interface of two mediums.

Question 18

What is the soil water potential (φ)?

Answer 18

The potential energy of soil water relative to water in a standard reference state.

Question 19

Kinetic and elastic energy are important in the unsaturated zone. (True/False)

Answer 19

False

Question 20

State the formula for capillary rise.

Answer 20

h = (2γ cos θ) / (ρgr)

Question 21

How does pore size affect water retention in soils?

Answer 21

Smaller pores retain water more strongly against gravity.

Question 22

Define matric suction.

Answer 22

The pressure head associated with capillarity; it is negative compared to external pressure.

Question 23

State the formula linking matric potential (φ) and matric suction (ψ).

Answer 23

ψ = φ / (ρg)

Question 24

What does the soil moisture characteristic curve represent?

Answer 24

The relation between matric suction (ψ) and volumetric water content (θ).

Question 25

At what point are all soil pores filled with water?

Answer 25

Saturation point.

Question 26

At field capacity, soil drains easily under gravity. (True/False)

Answer 26

False

Question 27

At what matric suction value does the wilting point occur?

Answer 27

15 bar or 15000 cm

Question 28

What drives the movement of soil water?

Answer 28

Differences in hydraulic head (h).

Question 29

State Darcy’s Law for water flow in soil.

Answer 29

v = -K(θ) ∇h

Question 30

What happens to hydraulic conductivity as suction increases?

Answer 30

Conductivity decreases as larger pores empty first.

Question 31

Which soil type tends to have higher conductivity?

Answer 31

Sandy soil

Question 32

What experiment is proposed for capillary rise and retention?

Answer 32

Measure weight loss of a banana plant over a week.

Question 33

Capillary rise is stronger in larger pores. (True/False)

Answer 33

False

Question 34

In which direction does unsaturated zone flow predominantly occur?

Answer 34

Vertically downward under gravity.

Question 35

Name two hydrological contributions from the unsaturated zone to a catchment's hydrograph.

Answer 35

Stormflow and Baseflow.

Question 36

What is meant by expressing water stores in units of depth?

Answer 36

Volume of water per unit area, e.g., 1 mm = 1 liter/m².

Question 37

Describe the gravimetric method for measuring soil water content.

Answer 37

Weigh a soil sample, dry it, weigh it again, then use θG = (MT - MS) / MS.

Question 38

What is Time Domain Reflectometry (TDR)?

Answer 38

A method that measures soil moisture by detecting changes in dielectric constant.

Question 39

TDR integrates water content over the entire probe length. (True/False)

Answer 39

True

Question 40

What is measured in field experiments related to overland flow?

Answer 40

Overland flow as a proportion of precipitation.

Question 41

Formula for hydraulic head?

Answer 41

h = ψ + z

Question 42

What is the field capacity?

Answer 42

Occurs when excess water has drained away. Beyond this point soil will not drain further because retention forces exceed the g force

Question 43

Formula for soil moisture deficit?

Answer 43

∫(between d and 0) VG/VT dy

Question 44

Formula for actual water content?

Answer 44

∫(between d and 0) VL/VT dy

Question 45

When does saturation excess overland flow occur?

Answer 45

When SMD = 0

Question 46

When does infiltraiton excess overland flow occur?

Answer 46

When P>i