Soil Water Flashcards
SMAP
Soil Moisture Active Passive
- New satellite
- Will produce global maps of soil moisture to help improve knowledge of water and carbon cycles and water management
- Can access 1st few cm’s of surface, even through thick vegetation
- Radar is already broken but radiometer still functions
- ~3 days to cover the planet with a large swath
Vadose Zone
- Unsaturated Zone
- Zone of aeration
- Realm of soil moisture
Phreatic Zone
- Zone of saturation
- Realm of Ground water
How is soil the 1st line of defense?
It filters contaminants such as pathogens from water before it enters groundwater
What are the possible pathways for rain?
- Evaporate
- Runoff
- Infiltrate
- Drawn by plants and transpired by weeds, crops, and trees
- Soil moisture
- Soil Water
- Ground water
- Groundwater flow
Why study soil water?
- Flood prediction
- Contaminant migration (mitigation?)
- Erosion
What does soil water have to become in order to reach the ground water zone?
Gravitational water
Why is soil water study important for agriculture?
Knowing the soil type and how it interacts with water can help understand how much the plants require and if the soil water is enough or if irrigation is necessary
Why is soil water study important for Geotechnical engineering?
Water content and how it relates to hazards (sink holes, etc)
What is soil made of?
- Mineral ~45%
- Air ~20-30% (air can be displaced by water, saturated soil has 0% air)
- Water ~20-30%
- Organics ~5%
What are the 3 main particle sizes in soil? What are other less common sizes?
Common: - Clay - Silt - Sand Less Common: - Pebbles - Rocks
What is the organic matter in soil composed of?
- Organisms 10%
- Roots 10%
- Humus 80%
- -> Content can vary significantly
What is the clay fraction?
- Highly weathered particles
- Mostly clay minerals, plagioclase
- Adsorbs soil water very effectively
- determines a soils hydrological properties
Adsorption
Water in a surface layer, stuck through electromagnetic forces (more or less hydrogen bonds)
What, above all, determines a soil’s hydrological properties?
The clay fraction
What is the sand and silt fractions?
- Less weathered particles
- Less reactive/charged particles
- Retains less soil water
- Larger pores, higher hydraulic conductivity
Clay soil vs. Sandy soil
Clay:
Greater total pore volume = Greater porosity, less permeable
Sand:
Less total pore volume = Less porosity, more permeable
Components of Soil Texture
Soil triangle can be used anywhere on the planet
- lithology
- angularity vs. roundness
- complexity
- Particle size
- Mineral layers
- Sorting
Soil properties
- Grain size, type
- Sorting, variability in size
- Bulk density, packing
- Water content, degree of saturation
- Hydraulic conductivity
- Thermal state
What is the Gravimetric water content?
The mass fraction of water in the soil
θg = Mass water / Mass soil
Gravitational water
The water in a soil that is freely available for gravitational drainage
i. e. free water in the pore space
- Saturated
Hygroscopic water
Held tightly to a particle
- Unavailable to plants, but possible a little available to the atmosphere
Field Capacity
- The maximum water content that a soil can hold against gravity
- Gravitational power exceeds capillary power and the water moves down/drains through
- Water in a soil that can be retained against gravity through adsorbed water and capillary pressure
Matric Potential
= capillary pressure = suction (negative pressure)
- Equivalent term fro pressure head from Bernoulli’s equation and is often used to describe negative pressure in the unsaturated zone
- Commonly associated with capillary pressure due to surface tension from the soil minerals/grains. This capillary pressure is a form of suction or “negative pressure” which draws water up into the soil
Permanent wilting point
Water content at which plants cannot draw water out of the soil
Available water
Water available to plants
θa = θfc - θpwp
Available water = Field capacity - permanent wilting point
Capillary water
Water held in micropores
- available water
- plant roots can absorb this
Adsorbed water
Hygroscopic
- Dry soil
Soil texture vs. percent soil water
Sand has more potential for gravitational water while finer particles have more potential for hydroscopic water and less for gravity water
- Loams have a higher percentage of soil water overall
Phreatic zone
- Ground water zone
- Saturated zone
- P water > P atmosphere
Vadose Zone
- Unsaturated zone
- Root zone, intermediate zone, Capillary fringe
- Pwater
Water Table
P water = P atmosphere
Hydraulic Head, h
Total potential
- Gravitational potential (elevation)
- Pressure/matric potential
- Function of elevation and Pressure
Bernoulli’s Equation for hydraulic head, h
Total energy = KE + PE + ‘pressure’ energy
- Reduces to h = z + P/density*g (ie head = elevation + pressure)

Bernoulli’s Equation applied to soils
Head = elevation + matric potential
pF = log10 (-ψ)
- Log scale for convenience
pF = 2
~ the wilting point
Surface tension
is a function of the interaction between the two materials at the interface
Matric potential (suction) vs. Capillary pore size
Suction is greater with smaller pore spaces
Matric potential (suction) vs. Volumetric moisture content
Suction is a function of moisture content
- Suction is also a function of where it is wetting or drying
- An also antecedent conditions & soil type
- Suction is stronger with drier soils
ie inverse relationship with moisture content
- evaporation can draw energy up when elevation hasn’t changed
Why does the Matric potential (suction) vs. Volumetric moisture content change and how
- When wetting the small pores fill first (highest suction) and water content increases slowly
- When drying the large pores empty first so there is a rapid decline in water
What are three things that relate to matric potential?
- Pore size
- Volumetric moisture content
- Pore structure
Negative Energy (cm)
Negative energy = upwards flow or suction
- Evaporation increases negative energy & suction
- Evaporation also changes the elevation of hydraulic head and brings it into negative energy
- Evap causes moisture decrease + Suction increase
Positive Energy (cm)
Positive energy = Downwards flow or gravitational
Types of flow
- Ponded & non-ponded infiltration
- Macropore flow
- Fingering flow
- Funnel flow
Types of flow:
Ponded vs. non-ponded infiltration
Ponded: Infiltrating from a pond, puddle, lake, etc and may or may not be flowing
Non-Ponded: From rain, high infiltration or low precipitation rate where it would be less likely to pool
Types of flow:
Macropore Flow
> 30 micrometers
- often created by animals or inverts (earthworms)
- Water easily makes it into this zone but then depends on the end of this and the micropores next
Types of flow:
Fingering flow
- Fingers of water flow down
- Soil profile can change the pattern or stop the flow
Types of flow:
Funnel Flow
- Large roots, rocks
- Preferential flow along these less penetrable surfaces
- Changes volumetric content in an area
Why does hydraulic conductivity decrease when matric potential increases?
- Small clay pores increase suction but they are not well connected
- Clay holds lots of water but it isn’t available to plants or atmosphere
Hydraulic conductivity, K
A measure of how effectively water moves through a substrate under a given hydraulic (head or potential) gradient.
- Units m/s or cm/day
Permeability κ (m2 or cm2) vs. Hydraulic Diffusivity, Kd (m2/s)
- Almost synonymous, with different definitions
- All measure how well water moves through a porous medium
Hydraulic conductivity as a function of matric potential
- High water content increases water flow in coarse soils
- Water stores increase with clay and high matric potential, therefore low water content and water easily flows through fine soils
- Hydraulic conductivity decreases rapidly in course soils as matric potential increases
- Hydraulic conductivity decreases slowly as matric potential increases in clays
K (vol. water content) Vs. Vol. Water Content
- At high water contents coarse soils have a higher hydraulic conductivity than fine soils
- At low water contents fine soils have higher hydraulic conductivity than coarse soils because the small pores per unit volume are filled with water and because most of the large pores are empty in course soils
- At low water content the tension head (matric potential of vol. water content) is higher (less negative) in fine soils
Soil Water Characteristic Curve
- At low water content the tension head (matric potential of vol. water content) is higher (less negative) in fine soils
- Matric potential (tension head) is high with low water content
- Hydraulic conductivity, K, is higher with higher water content
Soil Measurements
- Double Ring Infiltrometer
- Tensiometer
- Rainfall simulator
Soil water content
- Oven drying, ceramic block, GPR, TDR
- Pressure plate
What causes water contents below hygroscopic?
- Human interference
- not a normal range of water contents than an undisturbed soil can have
Why can’t plants easily access hygroscopic water?
Because the water is held to tightly to the soil particles
- That is why it is known as the permanent wilting point
Why can’t plants easily access gravitational water?
It flows through the soil profile and isn’t easy to be uptaken by the plants
- Known as field capacity
What is the potential when adding calcium to soil (in fertilizer)
- Calcium may reduce surface tension and water may preferentially flow into micropores where it is held longer and can be accessed by plants longer
- Flow is preferential through macropores usually and the water flows through these faster
- Micropores may hold water longer
