Soil Physics Flashcards
Soil Particles Sizes
Sand (20+ Micron)
Silt (2-20 micron)
Clay (2 Micron)
Measuring Particle Size
Mechanical Sieving
Sedimentation –> larger particles settle first (stokes law)
Bulk Density
Cultivated topsoil 0.9-1.3g/cm3
Dry soil = fan forced oven at 105 degrees for 24 hours
Particle Density
Just the solid material in the soil (doesn’t account pore space)
Average soil density = 2.65g/cm3
Total Porosity
The ration of the volume of pores divided by the volume of the whole soil
Gravimetric Soil Moisture
Proportion of moisture as a ratio of the weight of water to the mass of soil
Ignores bulk Density
Measured on disturbed or intact soil
Volumetric Soil Moisture
Proportion of moisture as a ratio of the volume of water to the volume of soil.
Considers bulk density
Measured on intact soil
Poiseuille’s Law
The rate of volume flow through a soil pore is proportional to the fourth power of the radius of the pore (assumes laminar flow)
Affected by friction along side walls
Sees soil as a bundle of different tubes –> gross oversimplification
Saturated Flow influenced by:
Size and slope of soil, its permeability/hydraulic conductivity.
Assumptions of Darcy’s Law
Laminar flow not turbulent flow
Heterogenous and isotropic soil
Constant temperature
Static
Only works with water
But works in most cases except really fast or really slow
Importance of saturated flow
Soil Structure
Likelihood of Dam failure
Ground water flow
Catchment hydrology
Understanding infiltration
Infiltration versus Ksat
Infiltration results from both gravity flow and capillary flow, Ksat only considers gravity flow
Soil Infiltrometer
Automated measurement of water level
On board calculation of infiltration rate and hydraulic conductivity
Measure surface and subsurface infiltration and hydraulic conductivity
Laser based measurement of water depth
Accurate to within 1 mm
Real time display
Automated data logging
WiFi data to mobile phone
Sturdy quad leg design
3D printed components
Double Ring Infiltrometer
Ignores capillary flow to some extent
Prone to overestimation
Works best with large rings and very shallow ponding
Subsoil Saturated Hydraulic Conductivity
Glover Equation
Guelph Semi-empirical equation
Altering bulk density
Tillage –> increase
OM –> decrease
Wet soil –> increase
What is field capacity
Field capacity is the soil water content after the soil has been saturated and allowed to drain freely for about 24 to 48 hours.
Matric Potential Important for understanding:
Water flow
Irrigation scheduling (using matric potential is more accurate)
Plant water stress
Soil water availability
Water Potential
Amount of work (force x distance) required to transport water from one pressure and elevation to another pressure or elevation.
Types of water potential
o Gravitational .
o Osmotic
o Matric
Matric Potential
Matric potential results from the capillary and adsorptive forces of the soil matrix. These forces attract and hold water in the soil and reduce its potential energy below that of free water, i.e., work would have to be done
to remove water from the soil. Thus, the matric potential is always negative.
Matric Potential and Capillary
As soil dries matric potential increases (more negative) due to increased capillary and adhesive forces binding water to soil.
* Capillarity is due to two forces.
o the attractive force of water for the solids on the walls of pores
o the surface tension of water, due to the attraction of water molecules for each other.
Pore diameter (um) =
30/matric potential (m)
FC, PWP and PAW Matric Potentials
Sat = 0kPa
FC = -10kPa
PWP = -1500kPa
PAW = -10 to -1500kPa
Classification of Soil pores
o Macroporosity = Moisture at sat - moisture at FC : Transmission pores
o Mesoporosity = Moisture at FC - Moisture at WP : Storage pores (capillary)
o Microporosity = Moisture < WP : Residual pores (not available)
Hysteresis
- Drying Curve occurs at higher potential than the wetting curve.
- Hysteresis due to ink bottle effect.
- There is a difference between wetting and drying curves.
Van Genuchten
Soil Moisture Curve (moisture versus matric potential)
S shaped curve
Measuring Matric Potential
Tensiometer
* Pulls water out the same way plant pull water out the soil
* If they get too dry, they stop working
Where does saturated and unsaturated flow occur?
- Saturated flow occurs between soil particles
- Unsaturated flow occurs in thin layers surrounding soil
Hydraulic conductivity in dry soil
- The dryer the soil the slower the hydraulic conductivity
o Because there is more friction
Richards Equation
Darcy’s law is combined with the continuity equation, we obtain Richards’ equation
* The equation is never solved, it gets to a point where the changes in soil water become so small as to no longer matter.
Flow rates change non-linearly
Infiltration
results from the combination of saturated, unsaturated and vapour flow, as water enters the soil profile.
Results from capillary and gravity forces
Infiltration influenced by:
Soil Moisture
Soil Structure
Depth of Ponding
Entrapped Air
Surface Crusts
Infiltration excess flow
AKA Hortian Flow
Irrigation rate exceeding infiltration rate
Infiltration Rate
Decreases over time
Early flow is dominated by unsaturated sorptive (capillary) flow.
Later flow is dominated by saturated flow (Flatline steady state)
Piston flow
New water pushing old water out of the soil profile
Infiltration calculation
Green Ampt and Phillips Equation
Measuring Infiltration in lab and field
- Laboratory methods
o Constant head
o Falling head
o Disk Permeameter - Field methods.
o Single ponded ring
o Dual ponded rings
o Twin ponded rings
o Tension Infiltrometer (disk permeameter).
o Drip Infiltrometer
4 ways of scheduling irrigation
- Evapotranspiration (replace what is lost)
o Weather station & leaf area
o Class A pan evaporation & leaf area - Soil Moisture / matric potential sensor
o Canopy temperature
o Guestimation
Issue with field capacity definition
Assumes uniform soil structure and texture and no evaporation
Refill Point
- Point at which the effort to extract water substantially reduces crop growth
- Refill Point: most crops -30 to -50 kPa, Pasture -80-100 kPa
Soil Moisture Sensors and Probes
Single Point
Single Sensor - mobile
Multi-depth –> fixed
Soil Matric Potential Sensors
Gypsum Block
Cheap, breakdown
Tensiometer (most fail below -80kPa)
ICT - auto filling
Decagons, MPS
Soil Moisture Probes
o Capacitance – EnviroSCAN, ECHO probes, Theta probe, Gopher, Diviner
o Neutron Probe
Old but most accurate. Measures water content by neutron bombardment of water molecules in soil
Good for vertosols
* Expensive, radioactive, can’t be logged, requires licencing.
o Time Domain Reflectometry (TDR) – TRIME
Presence of water slows it down
o Electrical resistance
o Frequency Domain – Campbell scientific
Not suited to vertic soils
FAO56
- Procedure for estimating crop and pasture evapotranspiration
o Penman- Montheith equation & Crop factor
FAO56 Approach
- Rainfall
- Wind
- Humidity
- Solar radiation
- Temperature
- Available soil water
- Root depth
- Crop stage / leaf area
- Evaporation from the soil
- Transpiration from the plants (affected by leaf area, solar radiation, humidity, wind speed, temp).
Crop Factor accounts and doesn’t account for:
- Kc factor accounts for
o Crop type
o Crop stage
Initial
Mid
End - Kc Doesn’t account for
o Soil moisture availability
o Waterlogging
o Pests and disease
o Mulch and compost
o Unexpected stomata control (wind)
Irrigation Efficiency definition
- Irrigation efficiency (%): water lost via transpiration as a % of water added through irrigation.
Irrigation losses and gains
o Before paddock
Seepage
Leaks
Distribution Losses
Operation Losses
Evaporation
o Within paddock
Application losses off-target (irrigate wrong thing)
Evaporation
Non-recycled runoff
to soil
from soil
* Gains (stored water)
o Water retained in soil (how much irrigation applied is stored in crop root zone and not below it?)
o Change in soil water
o Change in root zone soil moisture
o Crop water use (transpiration)
o Crop Growth
Application efficiency vs effectiveness
- Application efficiency
o Proportion of applied water retained within the crop root zone - Application effectiveness
o Proportion of rootzone that is filled (brought to field capacity*). - High application efficiency – Low effectiveness
o None of the water ran-off or leaked below the root zone but the entire root zone was not wet up. - High effectiveness – Low application efficiency
o All the root zone has wet up, but irrigation leached below the root zone and runoff the end.
Instantaneous Application Rate
- IAR: The rate (mm/hr) at which irrigation is applied to the soil
o If the instantaneous application rate is greater than the soil infiltration rate, irrigation ponds on the surface or runs of.
Benefits of VRI
- Easy to use – saves time
- Efficient watering
- Customized to specific field needs
- Different application rates for different soils or crops saves water, energy and fertilizer/chemicals
- Reduces over watering on laterals and part circle pivots
- Saves water as individual sprinklers or zones can be turned off over tracks, drains, creeks, bridges etc.
- Decreases and eliminates watering in low or flooded areas.
- Reduces leaching and runoffs on tighter soil areas
- Less track maintenance
- Best used in areas with highly variable soils, or infrastructure under the pivot
Problems with VRI
- Very low adoption about 5% of all pivots sold
- Requires skills and time to generate the prescription maps
- Poor irrigation prescription support
- Soil infiltration rates cannot be remotely mapped.
- System can only reduce irrigation application to over wet areas, not increase irrigation.
- Best for reducing waterlogging in low lying areas rather than ensuring soil in elevated areas are irrigated to field capacity
- A lot of work and hassle to get it working properly
- Makes a simple piece of equipment highly complex.
Bill Cotching Rules of Drainage
- Investigate in the wet, install drains in dry
- Plan, whole farm perspective – were will the water go
- Check for outfall
- Staged drainage development
a. Drain the landscape first – identify and intercept water flow. Then drain the soil and flat areas.
b. Build drainage slowly rather than overinvesting in one hit.
How to identify a waterlogged soil -
- Surface water ponding (gumboots)
- Reeds and Rushes growing
- Seepage areas
- Soggy to walk on
- Pugged soil
- Sulphurous smelling soil
- Machinery ruts
- Winter grass, broadleaf weeds
- Mottling and gleyed colours
- Soil doesn’t need to be saturated or inundated to be “too wet”
Causes of wet soils
- Landscape
o High regional watertable
o Break in slope/foot slope – seepage
o Flooding - Soil slow permeability
o Perched water table
o Degraded structure
o Nin-wetting sands
o Surface mat
Surface Drains
- Open/trench drain
- Reverse-bank interceptor drain
o Duplex soils
o Manages sheet erosion and drainage
o Only do it in a chromosol or kurosol don’t want tunnel erosion - Cut-off drain
- Contour/ spoon drain
- Hump and Hollow
o Use this when water has nowhere to go
o Wreck structure
o Essentially ‘last resort’ when no outfall
Subsurface Drain
- Mole drain
o Last 15-20 years when right
o 2 weeks when wrong
o If it works, its cheap
o Not suitable to all soils
o Stable clay
o Non-dispersive clay
o Moisture content critical
o 40-60cm depth
o About 10-30m spacing
o Cost at 20m, spacing $2500/ha
o Needs to be moist to get smearing and dry to get fracturing - Slotted Ag Pipe
o Single pass
o 15-20km/hr
Draining Acid Sulphate Soils?!
- Soils containing iron pyrite or iron sulphides (jarosite)
- Coastal areas (tropics) and inland swaps (temperate)
- Oxidise in air to form sulphuric acid
- Drainage causes anerobic jarosite to oxidise, uncontrolled sulphuric acid discharge
- pH as low as 2.0 impacts waterways, infrastructure, fish kills
- DO NOT DRAIN ACID SILPHATE SOILS
Soil Suitability for slotted ag pipe drains
- Suitable for structure medium clays, clay loams etc.
- Ideal for soils with moderate hydraulic conductivity >60mm/hr
- Soils with low hydraulic conductivity aren’t suitable for subsoil drainage. Require very close drain spacing to be effective
- Soils should be at least 50cm deep
- Suitable for soils that slake but not highly dispersive soils (ESP>15) due to likelihood of drains silting up
- Avoid acid sulphate soils
Catchment Hydrology Key Processes
- Rainfall
- Evaporation
- Infiltration
- Soil water movement (mostly unsaturated)
- Groundwater flow (saturated)
- Subsurface lateral flow (saturated)
- Runoff (overland flow)
- Stream flow
Curve Number
- Empirical method to partition the amount of rainfall into either runoff or infiltration
- Essentially, it’s a fix for when you don’t have rainfall intensity or soil data, but rainfall is expected to exceed the soil infiltration rate.
- Consists of 4 soil types, 3 soil conditions (structure), and multiple vegetation / crop types.
o More than four soil types, assumes three soil structure conditions (Good, okay, bad)
Tipping bucket model
Soil water is held below field capacity
* Moisture more than field capacity (ie between saturation and field capacity) drains to the next soil layer.
* Described as a tipping bucket
* Each soil layer is a bucket.
* Drainage (tipping) below the rootzone (last bucket) becomes deep drainage to groundwater
Stream flow
- Runoff due to
o Saturation excess – filled the soil profile
o Infiltration excess – rainfall rate greater than soil infiltration rate.
Sources of stream flow
- Direct rainfall
- Groundwater
- Subsurface lateral flows
- Surface runoff
o Saturation excess runoff
o Infiltration excess runoff - Variable Source Area contribution
Rainfall
- Green-Ampt
- Phillips Equation
- Infiltration = steady state + sorptivity (steady state flow over square root of time) = gravity flow + capillary flow
- Saturated hydraulic conductivity always under infiltration and doesn’t change with time.
- Unsaturated hydraulic conductivity changes with matric potential.
o Matric potential (pressure)
o Sat to unsat flow flow rate drops dramatically due to getting affected by tortuosity and tension on walls - This relates to Pouselli law flow rate = diameter to the fourth power
- Infiltration has 2 parts saturated and unsaturated
Centre Pivot
- Greatest innovation since tractor
- Undulating land brought into productivity
- Issue when instantaneous application rate is more than infiltration rate
- When we get ponding due to irrigation being too long shorter irrigation (redesign the farm) break into two pivots
o Change the vegetation type/crop rotation
o More Carbon and green manure fix soil structure
o Fix infiltration with better soil.
o Go to a 24-hour irrigation schedule.
o Add wetters to irrigation water (but expensive)
Irrigation Scheduling FAO56
- FAO56 evapotranspiration
o Either need pan evaporation or weather station
o And a crop factor
These get you reference ET or ETo
o Pan evaporation ETo x crop factor = ETactual
o Weather station ETo reference
Need rain, temperature, humidity, and wind speed
Use Penman Monteith equation
o Crop factor from FAO or NVDI (leaf area) from satellite
o Replace what is lost (water) once you’re at FC if you did if at the refill point it would get stressed.
o Use this to manage irrigation by staying between FC and refill.
o Start monitoring once soil profile is full
