soil physics: Infiltration & evaporation

Question 1

what are the 4 types of pores in the soil, what are they a function of, and how do they affect conductivity

Answer 1

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

Question 2

what is the saturated hydraulic conductivity, what is high conductivity, and what is the measurement

Answer 2

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

Question 3

how will management be affected by a soil with high Available water capacity and high hydraulic conductivity

Answer 3

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

Question 4

what is darcys law

Answer 4

-(Q/A) = -ks * (H + L/L)

the flow rate of water in soil is a function of potential difference and conductivity

Question 5

when does runoff occur from a conductivity perspective

Answer 5

when the application eg rainwater irrigation, exceeds the infiltration rate

Question 6

how does erosion occur through runoff (3 ways)

Answer 6

1. splash impact (little)
2. sheet (soil moves as a sheet)
3. rill (water moves over weak point)

depends on slope and cover

Question 7

how do you measure the hydraulic conductivity in the field

Answer 7

constant head permeameter, measure infiltration per minute, estimate using the slope of the flat line (not rapid flow)

Question 8

what happens when there is no potential difference

Answer 8

no movement will occur

Question 9

what happens when there are different texture layers in the soil profile (Ksat)

Answer 9

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

Question 10

what is the water pressure in darcys law

Answer 10

height of water above the soil

Question 11

what is the wetting front

Answer 11

the interspace between the unchanged soil and the saturated zone after infiltration.

Question 12

what is the short term infiltration rate

Answer 12

water moves rapidly into the soil controlled by sorptivity, texture and moisture content

Question 13

what is the long term infiltration rate

Answer 13

water moves steadily due to hydraulic conductivity structure and texture

Question 14

what is slaking

Answer 14

in soils with low OM that cannot hold aggregates together, moisture will rapidly infiltrate aggregate and it will break due to pressure

Question 15

what is the effect of slaking after the water is dried

Answer 15

if on topsoil a crust can form which reduces infiltration and seedling emergence, some particles will settle and create a compact subsurface

Question 16

what is soil dispersion

Answer 16

soils high in sodium. clay particles will separate on wetting due to the repulsion of sodium.

Question 17

what do slaking and dispersion tests measure

Answer 17

the strength of aggregates

Question 18

what is the effect of dispersive soils

Answer 18

swell when wet which restricts gas and water flow, can form a crust upon drying, particles can block pores in the subsoil

Question 19

what is preferential flow

Answer 19

bigger pores will contribute most to water flow, flow rate is pore radius squared.

Question 20

what is the effect of cracked soils

Answer 20

continuous cracking or large macropores can enhance the transport of chemicals to groundwater as it does not filter through the soil profile

Question 21

what is the best type of irrigation

Answer 21

aim to minimize runoff, drip irrigation is best

Question 22

what controls evaporation ( environmental factors)

Answer 22

sun, moisture content of the soil, soil cover

heavy rainfall on wet soil = higher evaporation

Question 23

what are seasonal differences in rainfall (ET)

Answer 23

higher in summer ~ 1mm per day

compared to winter ~0.6mm per day

Question 24

the best method to control evaporation in dams

Answer 24

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

Question 25

what is potential evapotranspiration (PET)

Answer 25

expected et based on climate, heat radiation and temperature

Question 26

what is actual ET

Answer 26

bounded by amount of precipitation

Question 27

what happens when PET is higher than ET

Answer 27

the larger than gap between ET and PET the more stress occurs to the plant, this is a water deficit

Question 28

what are 3 methods of reducing Evaporation

Answer 28

1. plastic mulch (expensive used for high-value crops)
2. stubble terrain
3. cover crops

Question 29

what are the 4 benefits of cover crops

Answer 29

1. increase biodiversity
2. reduced evaporation
3. increases OM and structure
4. reduced erosion

Question 30

what are 2 methods of measuring ET

Answer 30

1. flux tower to measure the change in moisture content of air
2. lysimeter measure water loss in a box

Question 31

what is reference ETo

Answer 31

measured using penman monteith formula is the ET for a well-watered grass using amount of radiation and temp per day

Question 32

what is crop ETc

Answer 32

estimated per crop using crop factor Kc * reference ETo

Question 33

what is crop water use index, what does it measure and what is a factor of

Answer 33

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

Question 34

what are some limitations of CWUI

Answer 34

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.

Question 35

what are the benefits of CWUI

Answer 35

can help identify environmental and management constraints to the crop by comparing against bench mark

Question 36

how do you stabilize soil temperature and what is the additional benefit

Answer 36

mulch has high heat capacity and can reduce soil temperature and prevent ET

Question 37

7 positives of good soil structure

Answer 37

1. increase crop yields
2. provides habitat for organisms
3. increases infiltration of water and gas
4. increases root access
5. decreases erosion
6. flood prevention
7. increase carbon