soil physics: Infiltration & evaporation Flashcards
what are the 4 types of pores in the soil, what are they a function of, and how do they affect conductivity
macro and micropores describe size, related to texture. macropores release water rapidly = rapid conductivity and the inverse for micro
continuous and noncontinuous (noncontinuous pores caused by compaction) which reduces conductivity
what is the saturated hydraulic conductivity, what is high conductivity, and what is the measurement
this measures how fast the water moves in soil, this relates to the type of pores. high conductivity is when pores are large enough for water and air and are continuous
Ksat or mm/day
how will management be affected by a soil with high Available water capacity and high hydraulic conductivity
this means soil can store a lot of water however it will drain quickly therefore irrigators need to apply constant water however in small amounts
what is darcys law
-(Q/A) = -ks * (H + L/L)
the flow rate of water in soil is a function of potential difference and conductivity
when does runoff occur from a conductivity perspective
when the application eg rainwater irrigation, exceeds the infiltration rate
how does erosion occur through runoff (3 ways)
- splash impact (little)
- sheet (soil moves as a sheet)
- rill (water moves over weak point)
depends on slope and cover
how do you measure the hydraulic conductivity in the field
constant head permeameter, measure infiltration per minute, estimate using the slope of the flat line (not rapid flow)
what happens when there is no potential difference
no movement will occur
what happens when there are different texture layers in the soil profile (Ksat)
subsurface lateral flow due to difference in hydraulic conductivity cannot penetrate layer beneath then there will be evaportation, slow penetration or if on hill more lateral flow
what is the water pressure in darcys law
height of water above the soil
what is the wetting front
the interspace between the unchanged soil and the saturated zone after infiltration.
what is the short term infiltration rate
water moves rapidly into the soil controlled by sorptivity, texture and moisture content
what is the long term infiltration rate
water moves steadily due to hydraulic conductivity structure and texture
what is slaking
in soils with low OM that cannot hold aggregates together, moisture will rapidly infiltrate aggregate and it will break due to pressure
what is the effect of slaking after the water is dried
if on topsoil a crust can form which reduces infiltration and seedling emergence, some particles will settle and create a compact subsurface
what is soil dispersion
soils high in sodium. clay particles will separate on wetting due to the repulsion of sodium.
what do slaking and dispersion tests measure
the strength of aggregates
what is the effect of dispersive soils
swell when wet which restricts gas and water flow, can form a crust upon drying, particles can block pores in the subsoil
what is preferential flow
bigger pores will contribute most to water flow, flow rate is pore radius squared.
what is the effect of cracked soils
continuous cracking or large macropores can enhance the transport of chemicals to groundwater as it does not filter through the soil profile
what is the best type of irrigation
aim to minimize runoff, drip irrigation is best
what controls evaporation ( environmental factors)
sun, moisture content of the soil, soil cover
heavy rainfall on wet soil = higher evaporation
what are seasonal differences in rainfall (ET)
higher in summer ~ 1mm per day
compared to winter ~0.6mm per day
the best method to control evaporation in dams
vapor concentrates with adjacent pores enhancing diffusive fluxes meaning the best method is something with no small gaps eg squares or hexagonal shapes, not circles
what is potential evapotranspiration (PET)
expected et based on climate, heat radiation and temperature
what is actual ET
bounded by amount of precipitation
what happens when PET is higher than ET
the larger than gap between ET and PET the more stress occurs to the plant, this is a water deficit
what are 3 methods of reducing Evaporation
- plastic mulch (expensive used for high-value crops)
- stubble terrain
- cover crops
what are the 4 benefits of cover crops
- increase biodiversity
- reduced evaporation
- increases OM and structure
- reduced erosion
what are 2 methods of measuring ET
- flux tower to measure the change in moisture content of air
- lysimeter measure water loss in a box
what is reference ETo
measured using penman monteith formula is the ET for a well-watered grass using amount of radiation and temp per day
what is crop ETc
estimated per crop using crop factor Kc * reference ETo
what is crop water use index, what does it measure and what is a factor of
yield kg/ ETc mm the amount of crop production per ET unit. depends on soil AWC, crops’ ability to convert water to biomass, and access water.
kg/ha/mm
what are some limitations of CWUI
not a real measure of water use efficiency as it doesn’t show where the water came from or how it is used eg rice (irrigation with high CWUI).
2. it can only be compared with the same crop as a benchmark, not between different species.
what are the benefits of CWUI
can help identify environmental and management constraints to the crop by comparing against bench mark
how do you stabilize soil temperature and what is the additional benefit
mulch has high heat capacity and can reduce soil temperature and prevent ET
7 positives of good soil structure
- increase crop yields
- provides habitat for organisms
- increases infiltration of water and gas
- increases root access
- decreases erosion
- flood prevention
- increase carbon
