Soil Water: Movement and Availability

Question 1

Three Forces Affecting Potential Energy of Soil Water

Answer 1

Gravitational Force. Always pulls water from higher to lower elevation

Matric Forces. Result from the attraction between water molecules and solid surfaces. Greatly reduce the energy state of water near soil particle surfaces.

Osmotic Forces. Result from the attraction between water molecules and solutes. Reduces the energy state of water in the soil solution.

The difference in energy state of water from one location to another in soil determines the direction and rate of water movement.

Question 2

What does capillarity have to do with soil water? Equation?

Answer 2

Capillary tubes are small diameter tubes.
If the walls of the tube has an affinity for water (hydrophilic) water will rise in the tube.
The height of rise is inversely proportional to the radius of the tube. In the case of water and a glass tube the height of rise is given by

h=0.15/r
height=0.15/radius of tube

Soil pores function like capillary tubes

Question 3

Measurement of water status

Answer 3

Volumetric water content, θv
Volume of water present per unit volume of dry soil
Usually expressed as: m3 H2O/m3 soil
Important for determining plant water supply and managing irrigation.
Gravimetric water content, θg
Mass of water present per unit mass of dry soil
Usually expressed as: g H2O/kg soil
Can be converted to θv if bulk density (Db) is known.
θv = Db x θg

Question 4

Three types of water movement in soil

Answer 4

Saturated Flow (gravitational)
Unsaturated Flow (matrix)
Vapor Movement

Question 5

Saturated flow- and equation?

Answer 5

Quantity of water per unit time (Q/t) that can flow through a column of saturated soil is expressed by Darcy’s law:

Q/t=AKsat(Change in potential/L)

Q = quantity of water
t = time
A = cross sectional area of the column
ΔΨ = change in water potential between the ends of the column
L = length of the column
Ksat = saturated hydraulic conductivity

Question 6

Tortuosity.

Answer 6

Soil pores are not smooth and straight like glass capillaries. They are rough, twist and turn, and change size and shape. The more tortuous the pathway, the slower will be water flow.

Question 7

Unsaturated Flow

Answer 7

Water movement in soils when larger pores (macropores) are filled with air and micropores are filled with water.
Most water movement in upland soils is unsaturated flow.
Unsaturated flow is critical for water supply to plant roots
Matric potential, not gravity, is the driving force.
Water moves from higher to lower matric potential
From less negative potential to more negative potential
From wetter to drier. From a region of thicker moisture films to thinner moisture films.
Flow rate (hydraulic conductivity) decreases as moisture content decreases (as water potential decreases)

Question 8

At high water potential levels (high moisture content)…
vs.
At low potential levels (low moisture content)…

Answer 8

(high)Hydraulic conductivity is greater in coarse textured than in fine textured soil.
More larger pores are still water filled and there are more larger pores in coarse textured soil (sandy soil)

(low)Hydraulic conductivity is greater in fine textured soil than in coarse textured soil.
Coarse textured soil has few connected fine pores so water cannot move.

Question 9

Infiltration.

Answer 9

The entry of free water into the soil at the soil-atmosphere interface.
Occurs under saturated conditions
The rate of infiltration usually slows with time if the soil is quite dry when infiltration begins

Question 10

Percolation

Answer 10

The downward movement of water through the soil profile
Involves both saturated and unsaturated flow
The boundary between wet and dry soil during percolation is the wetting front

Question 11

Maximum retentive capacity.

Answer 11

Amount of water a soil can hold when saturated (all pores filled with water).

Question 12

Field Capacity

Answer 12

Volume of water in the soil when gravitational water has drained out. Only water held against gravity remains. Usually around -10 to -30 kPa

Question 13

Wilting coefficient or permanent wilting point

Answer 13

Volume of water in soil when remaining water can no longer be removed by plant roots. Matric potential is lower than root potential, around -1500 kPa for most plants.

Question 14

Plant available water

Answer 14

Volume of water in soil between field capacity and permanent wilting point.

Question 15

Soil organic matter?

Answer 15

Has very large water holding capacity, acts like a sponge

As organic matter content of soils increases, the soil water holding capacity will increase

Question 16

As organic matter content of soils _______, the soil water holding capacity will ______

Answer 16

increase; increase