Soil Water Content. Soil Moisture Content. Water that may be evaporated from soil by heating at 105 0 C to a constant weight. mass of water evaporated (g). Gravimetric moisture content (w) =. mass of dry soil (g). volume of water evaporated (cm 3 ). Volumetric moisture content ( q ) =.
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Soil Water Content
Soil Moisture Content
Water that may be evaporated from soil by heating at 1050C to a constant weight
mass of water evaporated (g)
Gravimetric moisture content (w) =
mass of dry soil (g)
volume of water evaporated (cm3)
Volumetric moisture content (q) =
volume of soil (cm3)
bulk density of soil
q = w *
density of water
mass of dry soil (g)
Bulk density of soil (r) =
volume of soil (cm3)
Example: A soil is sampled by a cylinder measuring 7.6 cm in diameter and 7.6 cm length. Calculate gravimetric and volumetric water contents, and wet and dry bulk densities using the following data:
Volume of cylinder = p*r2*h = 3.14*(7.6/2)2*7.6 = 345 cm3
Weight of wet soil = 1000 – 300 = 700 g
Weight of dry soil = 860 – 300 = 560 g
Dry bulk density = 560/345 = 1.62 g cm-3
Gravimetric moisture content = (700-560)/560 = 0.25 or 25%
Volumetric moisture content = r *w = 1.62*0.25 = 0.41 or 41%
Know how to do these calculations for
quiz on Friday
Weigh drying pan, moist soil subsample + pan,
Oven dry the subsample at 105C for 24 hr,
Weigh the dried soil + pan.
Calculate the moisture content (w):
w = (g moist soil – g dry soil)/(g dry soil – pan)
Rearrange the eqn to solve for dry soil wt.
Dry soil wt = g moist soil / (1 + w)
Methods for measuring soil water content
Direct method
(Gravimetric)
Indirect methods
(need to calibrate)
Electrical properties
Acoustic method
Thermal properties
Chemical methods
Radiation technique
-Neutron scattering
-g- ray attenuation
Electrical Conductance
Dielectric constant
TDR
- Gypsum blocks
- Nylon blocks
- Change in conductance
Principles underlying different methods of
assessment of soil water content
Water Content
Direct
Gravimetric: evaporating water at 1050C (be able to do the calc’ns)
Indirect
Neutron scattering:
Thermalization
Time domain reflectrometry:
Dielectric constant
Soil Water (matric) Potential
In-direct:
Watermark (granular matrix sensor), gypsum block
Direct: Tensiometer
http://www.bae.ncsu.edu/programs/extension/evans/ag452-3.html
W = g water/g dry soil = (wet – dry) / dry soil
Db = g dry soil / cm3 volume soil
Θ = (W x Db) / Dw
Water retention curves: Water content vs pressure or tension
Note: clay holds more water at a specific water potential than sand or loam;
Water is held tighter at a given water content in clay than in sand.
Structure is predominant at low potentials; as soil dries out, texture is more important
www.soils.umn.edu/.../soil2125/doc/s7chp3.htm
The flow of water in soil
Saturated and unsaturated
flow
Saturated flow
Ksat = Q/A x L/(Ψ1 - Ψ2)
where Q is volume of water in time (t)
A is area of cross section
Ksat is saturated hydraulic conductivity of soil (how fast water moves)
L is length of column
Ψ is the water potential at points 1 and 2
Flux can be thought of as water flowing from a hose. The flux is the rate of water discharged by the hose, divided by the cross-sectional area of the hose.
http://soils.usda.gov/technical/technotes/note6fig1.jpg
because at least some of the water is a long distance from solid surfaces
http://www.maf.govt.nz/mafnet/schools/activities/swi/swi-04.htm
http://www.montcalm.org/montcalmold/media/planningeduc/tn_gwa5.jpg
Saturation
unsaturated
wet
dry
Unsaturated flow
Soil moisture content changing with depth
Gravity is not sufficiently strong to exert a significant influence on unsaturated flow because much of the soil water adheres to solid surfaces.
http://www.maf.govt.nz/mafnet/schools/activities/swi/swi-04.htm
http://wwwlb.aub.edu.lb/~webeco/SIM215soilwater_files/image004.gif