1. Soil Properties and Soil Water Storage
ATM 301 Lecture #6
Soil Properties and Soil Water Storage

2. Most (about 2/3 -3/4) of global land precipitation enters the soil
Most (about 2/3 -3/4) of global land precipitation enters the soil

3. Overland flow
infiltration (~76% of land precip.)
unsaturated soil
redistribution
saturated groundwater

4. Why care about soil water?
About 75% of precipitation over land “infiltrates” to become soil water
Most vegetation gets its water from the soil (forests, crops, etc.)
Precipitation that does not infiltrate can quickly become runoff and lead to flooding and erosion
Groundwater is largely supplied from soil water
Important questions
How much water is in the soil? How do we know?
How quickly can water go into the soil? What are the limits?
How hard plants need to work to pull water out of the soil?
How does water move in and through the soil?
Applications
Irrigation strategies
Flood forecasting
Soil chemistry and composition (nutrients, contaminants, etc.)
Ground water management

5. Properties of soil:
What properties are hydrologically relevant / useful ?
What is the soil made of?
How big are the soil grains?
How much space is there between the soil grains?
How much water is in the soil?
How much water can be in the soil?
How easily does water enter and move through the soil?

6. Properties of soil: Some basic definitions:
Soil volume Vs = Vm + Va + Vw = Vm + Vv
where Vm = vol. for soil mineral, Va = vol. for air, Vw= vol. for water, and Vv = vol. for voids or pores in the soil.
Soil bulk density: the dry density of the soil:
b=Mm/Vs = Mm/(Va+Vm) = Mm/(Vv+Vm)
constant in in time, but increases with depth.
Particle density: the dry density of the particles: m=Mm/Vm 2650 kg/m3
Porosity (): is the proportion of pore spaces in a volume of soil:  = Vv/Vs = (Va+Vw)/Vs = b (kg/m3)/2650 Vm Vv

7. Ranges of Porosities of soil:
Porosity decreases with grain size

8. Properties of soil: 1. Texture (particle size)
We often want to classify fraction of various soil particle sizes (i.e., texture), as they affect the storage and movement of water
We can quantify this using soil sieves
Pass soil through a series of progressively finer meshes by shaking
Each mesh stops all grains > some specified diameter
Weighing contents of each sieve to get “%-finer”
Can be done wet or dry
Size classes (USDA):
Gravel: >2mm
Sand: mm
Silt: mm
Clay: <0.002mm

10. Properties of soil: 1.Texture (particle size)
Can plot results as a “grain size distribution curve”
Clay
Silt
gravel
sand
%- finer
0.002
0.05 2
Diameter (mm)

11. Properties of soil: 1. Texture (particle size)
Can plot results as a “soil-texture triangle”
ex.
53% sand
44% silt
3% clay
95% sand
45% clay
Dingman Fig. 7-5

12. Properties of soil: 2. Composition
Classify the soil by chemical and biological composition
Biological vs. mineral content (we’ll ignore biological component)
Type of minerals present
Clay minerals are a particularly important component
Aluminosilicate minerals
contain alumina Al(OH)6 & Silica SiO3
Kaolinite, illlite, smectite, …
Mostly form as a byproduct of weathering of other minerals…often present as clay grain sizes
Has distinct hydrological properties
Effective at absorbing and retaining water

14. Water Storage in Soil:
1. Volumetric water content  (or water content or soil-moisture content): the ratio of water volume to soil volume:
 = Vw / Vs
where Vw= vol. for water, and Vs = soil vol.
Soil water storage =  x Layer depth
Measurement of : Gravimetric method:
 = (Mswet – Msdry)/(w Vs)
A soil sample with Vs vol. and Mswet mass is oven-dried at 105oC for 24 hrs.
Other methods for measuring , see pp , Box 7.2

15. Distribution of Soil Depth in cm (Webb et al. 1993)
1 inch = 2.54 cm

16. Water Storage in Soil:
2. Saturation or wetness  : is the proportion of pores that contain water:
 = Vw / Vv = Vw/(Va+Vw) =  / 
where Vw= vol. for water, Va = vol. for air and Vv = vol. for pores
Effective saturation * : for practically available water
* = ( - r) / ( - r)
where r = the permanent residual water content (0.05)

17. Forms of Soil Water Storage
Water is held in soil in various forms and not all is available to plants.
Chemical water is part of the molecular structure of soil minerals and unavailable to plants.
Gravitational water is held in large soil pores and rapidly drains out within a day or two after rain.
Capillary water is held in small soil pores that does not drain out under gravity and is the main source of water for plants.
At saturation, all pores are full of water, but after a day or so, all gravitational water drains out, leaving the soil at field capacity (or water holding capacity).
Plants then draw water out of the capillary pores, readily at first and then with greater difficulty, until no more can be withdrawn and the water left is in the micro-pores.
The soil is then at wilting point, and without water addition, plants die.

18. Changes in Soil Water Storage

19. Field Capacity (or Water Holding Capacity):
Drainage due to gravity because negligible within a few days after saturation
Field Capacity (fc)is the soil water content at which the gravity-drainage rate becomes “negligible” (e.g., 0.1mm/day).
This happens when the vertical water pressure gradient balances the downward gravitational gradient:
dp = -g dz
(similar to atmospheric hydrostatic Eq)

20. Field capacity y= log10Tfc where n, r , rG are van Genuchten
parameters, and Kh is the saturated
hydraulic conductivity (see p. 347,
Dingman)
y= log10Tfc
Tfc = days drain to field capacity
Tfc = 10y

21. Soil Water Holding Capacity
Water holding capacity (mm/cm depth of soil) of main texture groups.  Figures are averages and vary with structure and organic matter differences.
Texture
Field Capacity
Wilting point
Available water
Coarse sand
0.6
0.2
0.4
Fine sand
1.0
Loamy sand
1.4
0.8
Sandy loam
2.0
1.2
Light sandy clay loam
2.3
1.3
Loam
2.7
1.5
Sandy clay loam
2.8
Clay loam
3.2
1.8
Clay
4.0
2.5
Self-mulching clay
4.5
Source: Department of Agriculture Bulletin 462, 1960
10%
40%

22. Permanent Wilting Point:
Surface evaporation or plant uptake for transpiration can remove water from a soil that has reached field capacity.
But there is a limit to the suction (negative pressure) by plants
In hydrology, this pressure limit is usually -15 bar (=-15,000cm of water =-1470kPa=-14.5 atm)
The water content at this pressure limit is defined as the permanent wilting point: pwp  (-15bar)
Using the van Genuchten relations (Table 7.2 on p.336, Dingman), one gets:
Wilting point:
high for clay,
low for silt and sand

23. Soil-water Status: Available Water Content (a):
The difference between the field capacity and the permanent wilting point is the available water content for plant use:
a = fc  pwp
Soil-water Status:
θ ≈ 0
θ = θfc
θ = θpwp
θ = ϕ

24. Hydrologic Soil Horizons (or layers):
Vadose Zone
(unsaturated zone)
p=w g (z’-z’o)
Phreatic Zone
(saturated zone)
p=w g (z’-z’o)

25. Root Zone Depth: varies with vegetation types and locations

26. SMOS Satellite-derived Root Zone Soil Moisture Content (in fraction of soil volume)
SMOS=Soil Moisture and Ocean Salinity Satellite

27. Soil Moisture Content Patterns

28. Summary of Soil Properties:
1. Porosity (): the fraction of pore spaces in a soil sample. Ranges from ~0.35 to 0.55, decreases as grain size increases.
2. Texture: fractions of clay, silt and sand particles. Determines water storage and movement.
3. Soil Types:
Size classes (USDA):
Gravel: >2mm
Sand: mm
Silt: mm
Clay: <0.002mm

29. Plot the Diameter.vs. % data, estimate
the % at Diameter=0.05mm.
2. Determine the % of clay, silt
and sand in the sample 1.3:
Clay (D<0.002mm): %
Silt (0.002<D<0.05mm): =29%
Sand (0.05<D<2mm): =34%
Gravel (D>2mm): =31%
3. Re-normalize % to be for particles<2mm:
Clay: 6% * 100/69 = 8.7%
Silt: 29%*100/69 = 42.0%
sand:34%*100/69= 49.3%
4. Find the soil texture class from the triangle:

30. x Properties of soil: 1. Texture (particle size) ex. 1.3: 8.7% clay
42.0% silt
49.3% sand
Class: Loam
95% sand
45% clay
Dingman Fig. 7-5 x

31. Summary of Water Storage in Soil:
1. Volumetric water content : the ratio of water volume to soil volume:
 = Vw / Vs = (Mwet – Mdry)/(w Vs)
2. Saturation or wetness  : the fraction of the pores that contain water:
 =  / 
3. Effective saturation * : for practically available water
* = ( - r) / ( - r)
where r = the permanent residual water content (0.05) unavailable to plants
4. Changes in water storage:
5. Water holding capacity (or field capacity):
the maximum water in the soil after gravitational draining.
Ranges from ~10% in sandy soil to ~40% in clay.

32. Equilibrium Soil-Water Profiles:
Finer soil texture has higher water content
Water content increase with depth
Equilibrium Soil-Water Profiles:
the vertical water-content profile above the water table under the local mean recharge rate.
A higher water table maintains a wetter soil

33. Soil Moisture Measurements
Direct methods: volumetric water content () can be derived using the known volume of the soil sample and a drying oven. Most common method.
Other measurements: various instruments (see pp )
- Tensiometer
- time-domain reflectometry (TDR), neutron probe, frequency domain sensor, capacitance probe, amplitude domain reflectometry, electrical resistivity tomography, ground penetrating radar (GPR)
Satellite remote sensing method:
Satellite microwave remote sensing is used to estimate soil moisture
based on the large contrast between the dielectric properties (mainly
emissivity) of wet and dry soils. Canopy water can contaminate the signal.

34. Calculate the volume of the samples: Vb = Depth x Area = d. ( r2) = 0
Calculate the volume of the samples: Vb = Depth x Area = d * ( r2) = 0.1m x x 0.025m x 0.025m = x 10-4 m3
Water content:  = (Mwet – Mdry)/(w Vb) = (302.5 – 264.8) x 10-3 kg / (1000 kg/m3 x x 10-4 m3) = 0.192
Bulk density b = Mdry / Vb = x 10-3 kg/ (1.9635x10-4 m3) = kg/m3
Porosity  = 1 - b / m = 1 – /2650 = 0.49
Degree of saturation  =  /  = / 0.49 = 0.39