atm 301 lecture #6 soil properties and soil water storage
TRANSCRIPT
http://www3.geosc.psu.edu/~dmb53/DaveSTELLA/Water/soil%20water/soil_water.htm
Most (about 2/3 -3/4) of global land precipitation enters the soil
infiltration (~76% of land precip.)
redistributionunsaturated soil
saturated groundwater
Overland flow
Why care about soil water?• About 75% of precipitation over land “infiltrates” to become soil water • Most vegetation gets its water from the soil (forests, crops, etc.)• Precipitation that does not infiltrate can quickly become runoff and lead
to flooding and erosion• Groundwater is largely supplied from soil water
Applications• Irrigation strategies• Flood forecasting• Soil chemistry and composition (nutrients,
contaminants, etc.)• Ground water management
http://www.facstaff.bucknell.edu/mvigeant/univ270_05/jake_aq/irrigation.htm
http://blog.timesunion.com/healthcare/rivers-hit-record-levels-in-capital-region/2689/irene-flooding/
Important questions• How much water is in the soil? How do we know?• How quickly can water go into the soil? What are the
limits?• How hard plants need to work to pull water out of
the soil?• How does water move in and through the soil?
Properties of soil:
What properties are hydrologically relevant / useful ?
• What is the soil made of?
• How big are the soil grains?
• How much space is there between the soil grains?
• How much water is in the soil?
• How much water can be in the soil?
• How easily does water enter and move through the soil?
Properties of soil:
Some basic definitions:
• Soil volume Vs = Vm + Va + Vw = Vm + Vv
where Vm = vol. for soil mineral, Va = vol. for air, Vw= vol. for
water, and Vv = vol. for voids or pores in the soil.
• Soil bulk density: the dry density of the soil:
b=Mm/Vs = Mm/(Va+Vm) = Mm/(Vv+Vm)
constant in in time, but increases with depth.
• Porosity (): is the proportion of pore spaces in a volume of
soil: = Vv/Vs = (Va+Vw)/Vs = 1 - b (kg/m3)/2650
Vm
Vv
We often want to classify fraction of various soil particle sizes (i.e., texture), as they affect the storage and movement of water
We can quantify this using soil sieves• Pass soil through a series of progressively finer
meshes by shaking• Each mesh stops all grains > some specified
diameter• Weighing contents of each sieve to get “%-finer”• Can be done wet or dry
http://www.turfdiag.com/SoilTexture101.htm
Properties of soil: 1. Texture (particle size)
Size classes (USDA):• Gravel: >2mm• Sand: 0.05-2mm• Silt: 0.002-0.05mm• Clay: <0.002mm
Properties of soil: 1.Texture (particle size)
Can plot results as a “grain size distribution curve”
Silt sand gravelClay
%- fi
ner
Diameter (mm)0.002 0.05 2
Can plot results as a “soil-texture triangle”
Dingman Fig. 7-5
Properties of soil: 1. Texture (particle size)
ex. 53% sand44% silt3% clay
Properties of soil: 2. Composition
Clay minerals are a particularly important component• Aluminosilicate minerals
• contain alumina Al(OH)6 & Silica SiO3
• Kaolinite, illlite, smectite, …• Mostly form as a byproduct of weathering of other
minerals…often present as clay grain sizes• Has distinct hydrological properties
• Effective at absorbing and retaining water
Classify the soil by chemical and biological composition• Biological vs. mineral content (we’ll ignore
biological component)• Type of minerals present
http://www.reading.ac.uk/CFAM/Imageofthemonth/Imageofthemonth2008/cfam_february08.aspx
http://razorfamilyfarms.com/gardening/soil-composition/
Water Storage in Soil:
1. Volumetric water content (or water content or soil-moisture content): the ratio of water volume to soil volume:
= Vw / Vs
where Vw= vol. for water, and Vs = soil vol.
• Soil water storage = x Layer depth
• Measurement of : Gravimetric method:
= (Mswet – Msdry)/(w Vs)
A soil sample with Vs vol. and Mswet mass is oven-dried at 105oC for 24 hrs.
• Other methods for measuring , see pp.320-322, Box 7.2
Water Storage in Soil:
2. Saturation or wetness : is the proportion of pores that contain water:
= Vw / Vv = Vw/(Va+Vw) = /
where Vw= vol. for water, Va = vol. for air and Vv = vol. for pores
• Effective saturation * : for practically available water
* = ( - r) / ( - r)
where r = the permanent residual water content (0.05)
Forms of Soil Water Storage
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• Water is held in soil in various forms and not all is available to plants.
• Chemical water is part of the molecular structure of soil minerals and unavailable to plants.
• Gravitational water is held in large soil pores and rapidly drains out within a day or two after rain.
• Capillary water is held in small soil pores that does not drain out under gravity and is the main source of water for plants.
• At saturation, all pores are full of water, but after a day or so, all gravitational water drains out, leaving the soil at field capacity (or water holding capacity).
• Plants then draw water out of the capillary pores, readily at first and then with greater difficulty, until no more can be withdrawn and the water left is in the micro-pores.
• The soil is then at wilting point, and without water addition, plants die.
Field Capacity (or Water Holding Capacity):
• Drainage due to gravity because negligible within a few days after saturation
• Field Capacity (fc)is the soil water content at which the gravity-drainage rate becomes “negligible” (e.g., 0.1mm/day).
• This happens when the vertical water pressure gradient balances the downward gravitational gradient:
dp = -g dz
(similar to atmospheric hydrostatic Eq)
Field capacity
y= log10Tfc Tfc = days drain to field capacityTfc = 10y
)](log2[60.0 10 hK
rrG
rfc n
where n, r , rG are van Genuchten parameters, and Kh is the saturatedhydraulic conductivity (see p. 347,Dingman)
Soil Water Holding CapacityWater holding capacity (mm/cm depth of soil) of main texture groups. Figures are averages and vary with structure and organic matter differences.
Texture Field Capacity Wilting point Available water
Coarse sand 0.6 0.2 0.4Fine sand 1.0 0.4 0.6
Loamy sand 1.4 0.6 0.8
Sandy loam 2.0 0.8 1.2Light sandy clay
loam2.3 1.0 1.3
Loam 2.7 1.2 1.5Sandy clay loam 2.8 1.3 1.5
Clay loam 3.2 1.4 1.8Clay 4.0 2.5 1.5
Self-mulching clay 4.5 2.5 2.0
Source: Department of Agriculture Bulletin 462, 1960
40%
10%
Permanent Wilting Point:
• Surface evaporation or plant uptake for transpiration can remove water from a soil that has reached field capacity.
• But there is a limit to the suction (negative pressure) by plants
• In hydrology, this pressure limit is usually -15 bar (=-15,000cm of water =-1470kPa=-14.5 atm)
• The water content at this pressure limit is defined as the permanent wilting point: pwp (-15bar)
• Using the van Genuchten relations (Table 7.2 on p.336, Dingman), one gets:
nnn
rrGrpwp /)1(
)15000(1
Wilting point:
high for clay, low for silt and sand
θ ≈
0Available Water Content (a):
• The difference between the field capacity and the permanent wilting point is the available water content for plant use:
a = fc pwp
• Soil-water Status:
θ =
ϕ
θ =
θfc
θ =
θpw
p
Hydrologic Soil Horizons (or layers):
Phreatic Zone (saturated zone)
Vadose Zone(unsaturated zone)
p=w g (z’-z’o)
p=w g (z’-z’o)
http://www.cesbio.ups-tlse.fr/SMOS_blog/wp-content/uploads/2012/04/SW2F_20120402_6h00pm1.png
SMOS=Soil Moisture and Ocean Salinity Satellite
SMOS Satellite-derived Root Zone Soil Moisture Content (in fraction of soil volume)
Equilibium Soil-Water Profiles: the vertical water-content profile above the water table under the local mean recharge rate.
• Finer soil texture has higher water content• Water content increase with depth
• A higher water table maintains a wetter soil
Soil Moisture Measurements
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• Direct methods: volumetric water content () can be derived using the known volume of the soil sample and a drying oven. Most common method.
• Other measurements: various instruments (see pp. 320-322)
- Tensiometer
- time-domain reflectometry (TDR), neutron probe, frequency domain sensor, capacitance probe, amplitude domain reflectometry, electrical resistivity tomography, ground penetrating radar (GPR)
• Satellite remote sensing method:
Satellite microwave remote sensing is used to estimate soil moisture based on the large contrast between the dielectric properties (mainly emissivity) of wet and dry soils. Canopy water can contaminate the signal.