Soils 205-90
Lecture 6- Description and Flow of Water in Soil
Videos Pages in text.
| 8,9 |
195-213 |
A. Expression of Soil Water Content- either by weight (gravimetric) or volume (volumetric)
1. Volumetric water content = qv
qv = Volume of water (ml)
Total volume of soil (cm3)
%qv = qv * 100%
2. Gravimetric water content (qg or Pw) = % by weight
Pw = weight of water (g)
weight of oven dried soil (g)
%Pw = Pw * 100%
Conversion from gravimetric to volumetric weight:
qv = Pw * Db
Pw = qv/Db
B. Water retention
1. Saturation
Saturation is a state of high free energy where qv = % pore space. At saturation the potential = 0.
Gravity will remove the water until the attraction of the soil solids is greater than the gravitational force.
When gravitational forces = matrix forces, you have reached...
2. Field capacity
When downward drainage of water caused by gravity ceases, the soil is said to be at field capacity.
Typically occurs 2-3 days after saturation
Soil water potential is approximately -10 to -30 kPa
-33 kPa (-1/3 bar) is used to recreate this condition in the laboratory
Water content at Field capacity
Water held against gravity will then by used by plants, evaporate, or move to areas of lower potential. Plants will consume water until the attraction for the water by the soil forces is greater that the attraction exerted by the plants. This water content is called the...
3. Permanent Wilting point
At the perm. wilting point most plants can no longer pull the water out of a drying soil.
Potential is about -1500 kPa or -15 bars
4. Hygroscopic coefficient
refers to an air dry soils where the pores are empty and the only water left is adsorbed to the soil.
Soil water potential is about -3100 kPa or -31 bars
C. Soil water content and potential- soils with different textures will not have the same water content at a given potential
D. Plant available water
- the % of water that is between field capacity and the perm. wilting point
- depends on soil texture
E. Water flow in soils
1. Saturated flow
- occurs at water contents greater than field capacity (> -33kPa or -1/3 bar)
- gravity is the driving force
- volume of flow is a function of the driving force (F), and the hydraulic conductivity (K) of the porous media.
V = K * F
Hydraulic conductivity (K)
-due to inherent properties of the porous media, mainly the size and continuity of pores
-maximum value occurs at saturation
2. Unsaturated flow
-occurs when there is air in the soil pores, potential is < -33kPa or -1/3 bar
-volume of flow is dependent on the driving force and hydraulic conductivity, just as in saturated flow, but the
driving forces are matric and osmotic potential.
-unsaturated flow, is therefore, a function of water content
Driving forces- unsaturated flow
Matric forces are usually the driving forces of this type of flow. Differences in matric potentials create a GRADIENT and flow takes place in response to this gradient.
3. Vapor flow
-Occurs when the soil is dry
-Soil potential is <-1500 kPa or -15 bars
-Water can no longer move unless it evaporates
4. Flow in stratified soils
sand over clay
Perched water
table may form
clay over sand
