soil hyraulic conductivity Flashcards
how much water is in the earths soils? (total water percent)
soils = 0.001 (0.05 % of fresh water) groundwater = 1.7
how can you manage soil water in 4 ways in preparation for drought and climate change?
- breed drought-tolerant crops
- conservation agriculture
- hydrological seasonal forecasting
- understanding soil water characteristics
what are 3 aspects of conservation agriculture
- no till to preserve structure
- constant ground cover
- diverse crops adn pasture to increase OM
how does soil structure effect water conservation
better sturcture will lead to increase infiltration and soil moisture.
degraded structure will lead to more runoff and soil will be less effective at storing water
what are the two ways soils water is expressed and what are their units?
- gravimetrically (wetness w g/g)
2. volumetrically/porosity (θ m3/m3 and dimensionless)
calculation for gravimeteric water content
w = Mass liquid/mass soil
* 100 for percent
Calculation for volumetric water content
θ = voume liquid (=mass liquid)/volume soil
*100 for percent
calculation for θ using wetness
w * bulk density (p)
converting mm to volume (ML)
100 mm/ha = 1 ML/ha
4 methods to measure water content (field or lab)
- bulk density cores
- sensors (probes)
- permanent feild equipment
- satellite soil mositure (active or passive measure temp ) only surface of the soil
3 statuses of the water in soil
- saturation
- field capacity
- wilting point
what is saturation
all pores are filled with water not all water is available
what is field capacity
~2 days after saturation, gravoty drains excess water from pores and some water is held
what is wilting point
water is held very tighly by soil and it is not available to plants (too much energy to absorb)
what 2 factors describes the state of water in soils
the water content (quantity volume %)
in relation to the water potential (quality) (wettest possible condition of the soil related to texture and bulk density
eg 20%water is high quality for sand and low quality for clay
which soil sandy, loam, clay has the most available water
aggregated loam. clay can hold more water however most in unavailable because it is held tightly to particles
where does water flow in relation to potential
high potential to low potential eg a wet sand to a dry sand.
what are the 2 reducing forces of soil water (output)
- evaporation
2. gravity (drainage)
what are the 2 holding forces of water
- adsorption (surrounding particles)
2. capillarity (in pores)
what are the 2 forces that drive capillarity
- adhesive
2. cohesive
what are adhesive forces
attraction of water molecules to solid surfaces (attraction of dissimilar substances). at low moisture content forms a thin layer
what are cohesive forces and when does this occur
water molecules pulling each other together, this can move water upwards. mostly at high water content
what is the water potential at saturation (J/kg)
0 J/kg (positve)
most soils are less than this (negative)
what are the 3 pathways water flows (high to low potential)
- precipitation (downwards + gravity)
- evaporation (upwards)
- Plant uptake (lateral/upwards)
how do plants uptake water
this is not due to an exertion of energy by the plants rather a different in potential. the lowest potential is in the top leaves (eg -1000 kPa) compared to high potential at roots (eg -10 kPa)
what units describe water potential (3 ways)
- energy per mass (j/kg)
- energy per volume (kPa)
- pressure head (m)
1 j/kg = 1 kPa = 0.1m
what is the simple capillary model
height in cm (the smaller the capillary the higher the water)
the height of the water is inversly proportional to the radius of the capilary
what is the capillary rise formula
height cm = 0.15/radius capillary cm
what is the capillary fringe
the space between the saturated zone (ground water) to the unsaturated zone intermediate zone/soil zone
what causes capillary rise to become an issue (eg AUS)
groundwater is high in salinity. when native vegetation is removed and either left bare or replaced with shallow crops the groudwater level becomes higher and capillary forces increase salinity in the soil.
what is a non wetting sand and wetting sand
hydrophobic soil, water that sits ontop of soil with >90 degree contact angle. in comparison to wetting sand angle = 0
what causes non wetting sands
coating of sand particles with fungi (microbes) gums and waxes. caused by vegetation high in lipids (issue in SA and WA)
how does clay fix non wetting sands
high surface area of clay means substance cannot coat all particles
what is another way you can fix non wetting sands
using wetting agents such as detergents to increase surface area
what is the main issue with non wetting soils
soils will not equally moisten therefore cannot crop
what is a duplex soil and what can be done to fix hydropobic topsoil
soil profile marked by distinct textures (texture contrast) sandy topsoil with clay subsoil. these can be mixed by devling and spading
what is the water retention curve/soil moisture characteristic curve
representation between volume of water and potential of water. the morre negative the value (potential) the drier the soil eg 0 = saturated
how to interpret the water retention curve
the lower the potential (eg -10000) the smaller the pore size.
the higher the potential (eg -1) the larger the pore size
the most pores iswhere the graph is the flattest, if the graph is steep there is even distribution of pores
what is field capacity
(-10 kpa, 1m) water in soil after saturated flow drained (pores smaller than 30mm diameter
what is permanent wilting point
-1500 pka, 150m
planst cannot exert enough energy to absorb water
what is available water capacity
field capacity - wilting point = AWC throughout whole depth of soil (mmwater/mmsoil)
how do you get the height of water using AWC
AWC X thickness of soil
height of water (using AWC mm) values rating
<100 small
>200 large
what is Plant available water capacity
AWC mm of all layers up until root depth eg add AWC of all layers until that thickness
what is the equivilant of feild capacity
drained upper limit (DUL)
what is the equivilant of permanent wiling point
Crop lower limit (CLL)
when do you irrigate
halfway between PWP and FC
AWC ratings (m3/m3)
<0.1 small
>0.2 large
