Extra Reading Flashcards
Hydrophobicity:
Commonly associated with fungal mycelia
Savage et al, 1969
Hydrophobicity:
Bug fires vaporise more organic compounds creating a stronger, more continuous hydrophobic layer.
Tiedemann et al, 1979
Hydrophobicty:
Stronger in coarser textured soils.
Huffman et al, 2001
Infiltration:
Infiltration capacity is a control of surface runoff.
Horton, 1933
Infiltration:
Wind action and differential temperatures aid in re-opening soil pores after rain.
Horton, 1933
Infiltration:
Increase of infiltration capacity immediately following cultivation of soils.
Horton, 1940
Infiltration:
Rough surfaces may have lower infiltration capacities.
Horton, 1940
Infiltration:
4 tension disc experiment on loamy soils.
TDR measured water at depth of 2cm below tension disc, with varying hydraulic head.
Measuring transport of contaminants.
Simunek et al, 1999
Unsaturated zone:
Part of the soil where water contents are less than the soil porosity.
Nielsen et al, 1986
Unsaturated zone:
Surface and ground waters are linked by the unsaturated zone
Nielsen et al, 1986
Hysteresis:
Enhanced by presence of rapped air, soil shrinking and swelling and rates of wetting and drying
Davidson et al, 1966
Hysteresis:
Keeps water contents higher in the initially wetted part of the soil, while reducing wetting front in the drier soil
Dracos, 1984
Unsaturated zone:
Temperature effects viscosity and surface tension.
Nielsen et al, 1986
Preferential flow:
In well structured soils, water moves along cracks
Bouma, 1991
Preferential flow:
Sandy soils create ‘fingers’
Hillel, 1987
Preferential flow:
Dyes used to trace flows but are destructive and you cannot do repeats.
Ehlers, 1975
Flury et al, 1994
Preferential flow:
Penetration depth is deeper in wet soils
Flury et al, 1994
Preferential flow:
Structure must be considered when pesticides are used.
Poor structure, sandy soils are more susceptible.
Flury et al, 1994
Influences on water movement in soils
Soil bulk density
Soil strength
Soil OM
Soil chemical properties
Processes that slow infiltration capacity…
Deterioration of soil structure Crust formation Detachment of particles(blocked pores) Clay swelling Air entrapment below wetting front
Factors influencing infiltration…
Topography
Macropores
Hysteresis
Effects of soil properties on infiltration…
Water content - infiltration is faster on initially dry soil
Soil texture - reduced infiltration on fine textured soils - associated with porosity.
Infiltrability depends on…
Time of event Initial soil water content Hydraulic conductivity Soil surface Presence of impeding layers.
What is field capacity and what is the UK matric potential value used for it?
The water content of the soil after it has been saturated and allowed to drain against gravity for 48 hours… Half wet, half dry.
Matric potential -5kPa
Factors effecting field capacity…
Texture Type of clay-montmorillonite has high fc. Organic matter Depth of initial wetting Impeding layers Evaportranspiration
What is bulk density… Including the equation.
The mass of dry soil per unit of volume (including pore space)
Pb = Ms/Vt
Where ms is mass of solids and vt is total vol.
What happens to bulk density when the ground is compacted?
There is less pore space, therefore the bulk density is higher.
Why does bulk density increase with depth?
Pressure.
What is the bulk density of clay like?
Clays have low bulk density because of their many micropores
In agricultural soils: 1.00-1.60gcm3
What is the bulk density of sands like?
Sands have a higher bulk density because all the partials lie close.
In agriculture soils: 1.20-1.80gcm3
Pore sizes…go.
Luxmoore 1991
Micropores: 100 micrometers
What is soil water potential?
Potential energy possessed by water held in any position within the soil.
The difference in energy levels determines direction and rate of flow.
High energy state when pore sizes are small.
What is water flow in soils caused by?
OM Chemistry Topography Structure Stoniness Horizons Compaction
What is runoff influenced by?
Soil type Cracks Type of rock OM content Vegetation Topography Type or rainfall/intensity
What is overland flow?
If the rate of water supply at the soil surface exceeds the infiltration capacity.
Laminar flow=shallow/gentle
Turbulent flow=erosive
What is through flow?
Lateral downslope movement within the soil profile.
Diffuse flow (slow) Macropore flow (fast) Pipe flow (much larger scale)
What is groundwater flow?
Need knowledge of parent material including pressure, density, viscosity and homogeneity.
What is Channel precipitation?
Less than 5% contribution to runoff.
Environmental implications of runoff…
Pesticide pollution soil loss flooding decrease in soil quality Landslides Toxic waste
What does hydrograph separation enable you to do?
Enables separation of runoff compartment from baseflow compartment.
From where runoff begins to the corresponding point on the regression limb.
What are the two main hydrographic separations in runoff?
Theoretical separation
Experimental separation - using tracers to find water source (isotopic and geo chemical separation)
Factors effecting the shape the hydrograph
Climate - rainfall intensity. Regression limb would be more gradual and slow for snow.
Topography - size/shape of basin. Slope of land.
Ponding/storage
Geology.
What is tortuosity?
The average ratio of the actual roundabout path to the straight flow path.
Wiggly line : straight line
Pressure cell for measuring matric potential
Good to -98kPa, takes a long time to reach equilibrium, works by applying positive air pressure
Pressure plate for measuring matric potential
Good to -1500kPa, often used in labs, unsuited for large cores and determination at the wet end of the range
Psychrometer to measure matric potential
Psychrometer: Range -98 to -3000kPa, measures contribution of matric and osmotic potential
Tensiometer to measure matric potential
Good to -85kPa, inexpensive and quick can measure whole profile but slow to respond/equilibrate, requires excellent soil contact
How does a tensiometer work?
Water is drawn into soil through porous cup until the pressure in the cup & soil are in equilibrium (Bouma, 1981)
Advantages of using a tensiometer to measure unsaturated hydraulic conductivity
- Inexpensive
- Quick
- Variable length so can measure whole profile
- Can be used to control irrigation schedules for crops
Disadvantages of using a tensiometer to measure unsaturated hydraulic conductivity
Doesn’t measure matric potential as low as PWP, effective up to -85kPa
• Slow to respond/equilibrate
• Requires excellent soil contact
Unsaturated hydraulic conductivity decreases as:
Volumetric water content decreases
Tortuosity increases
Cross-sectional area of water flow decreases
Drag forces increase
Advantages of TDR - measuring unsaturated hydraulic conductivity
Fast, precise, non-destructive measurement (Roth et al., 1990)
•Minimal calibration
• Continuous measurement/automation – rapid results
Disadvantages of TDR - measuring unsaturated hydraulic conductivity and porosity
Measurement area only a few cm’s beyond probe
Problems with clay soils and organic soils
Neutron probe to measure soil water content. Porosity.
Measures how fast neurons are converted to slow neutrons is dependant on soil water content
Advantages of neutron probe
Non destructive
High precision
Disadvantages of neutron probe
Expensive
Health hazard - need training
What can be used to measure the water content of a soil
Oven drying: 105 degrees for 8 hours
Neutron probe
Time domain reflectometry
What would you use to hydrostatic (positive pressure potential) of saturated soils
Piezometer
Open tube in soil
Bottom is perforated to allow soil water to enter
Digital reading
What would you use to matric potential of unsaturated soils
Tensiometer
Water drawn into soil through porous cup until pressure in the cup and soil in equilibrium
Name the two flow types in saturated flow
Steady state flow - intensity does not change
Transient flow - does change with time, realistic
USE DARCYS LAW TO MEASURE
Darcy’s law
The flow of water in soils is proportional to the hydraulic conductivity (ease of flow)
The flow of water is dependant on the differences in hydraulic potential between two locations in the soil
The rate of flow is equal to hydraulic conductivity multiplied by hydraulic head.
Limitations of Darcy’s law
Steady flow - flux must remain constant
Uniform medium - assumption
Preferential flow - no account for horizons and a change in flow due to structure etc.
How can hydraulic conductivity be measured
Calculated using a permeameter, hydraulic head needs to stay constant
Remember lab work.
Factors control Ksat
Porosity
Tortuosity
Pore size distribution - poiseuilles law - total flow rate is proportional to R4. A sand with large pores may have a higher Ksat than clay with small pore.
Measuring infiltration
Double ring infiltration-2 rings with water ponded inside, measurement of rate from inside ring, outside used to reduce lateral flow.
Sprinkle infiltrometers-excess water is sprayed on the surface at a measured rate and runoff independently collected. Pop for catchments studies
Tension disc infiltrometer-water held at tension. Determines unsaturated hydraulic conductivity
Pressure ring infiltration.
Hydrophobicity:
caused by leaching of hydrophobic compounds, such as aliphatic hydrocarbons from the litter and humus layers.
Huffman et al, 2001
