Soil Water Content Soil Moisture Content

Water that may be evaporated from soil by heating at 1050C to a constant weight

Gravimetric moisture content (w) =mass of water evaporated (g)

mass of dry soil (g)

Volumetric moisture content () =volume of water evaporated (cm3)

volume of soil (cm3)

= w *bulk density of soil

density of water g

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Bulk density of soil () =mass of dry soil (g)

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:

1. Weight of empty cylinder = 300 g

2. Weight of cylinder + wet soil = 1000 g

3. Weight of cylinder + oven dry (1050C) soil = 860 g

Volume of cylinder = *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 = *w = 1.62*0.25 = 0.41 or 41%

Know how to do these calculations for quiz on Friday

Calculating dry soil weight basis of samples for analysis

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

Radiation technique

Acoustic method

Thermal properties

Chemical methods

Electrical Conductance

Dielectric constant

-Neutron scattering- ray attenuation

- Gypsum blocks- Nylon blocks- Change in conductance

TDR

Principles underlying different methods of assessment of soil water content

Direct

Gravimetric: evaporating water at 1050C (be able to do the calc’ns)

Indirect

Neutron scattering:Thermalization Time domain reflectrometry:

Dielectric constant

Water Content

In-direct:

Watermark (granular matrix sensor), gypsum block

Direct: Tensiometer

Soil Water (matric) Potential

Calibrating field instruments

http://www.bae.ncsu.edu/programs/extension/evans/ag452-3.html

Gently tap a tube into the soil to take an undisturbed sample from the center of

the effective root zone.

Trim the soil at each end of the tube to the tube length so that the soil occupies the

exact tube volume.

Calibration for moisture content• Measure and weigh the tube• Weigh the field moist soil + tube• Oven dry the soil from the tube• Calculate:

W = g water/g dry soil = (wet – dry) / dry soil

Db = g dry soil / cm3 volume soil

Θ = (W x Db) / Dw

• Compare lab moisture content to field measurements

• For water potential, compare water retention curves derived in lab using pressure plates.

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

Effect of structure on water flow

www.soils.umn.edu/.../soil2125/doc/s7chp3.htm

The flow of water in soil

Saturated and unsaturated

flow

Saturated flow

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

Ksat = Q/A x L/(Ψ1 - Ψ2)

http://soils.usda.gov/technical/technotes/note6fig1.jpg

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.

Saturated flow in soils

• The pores are full of water and matric potential is considered to be negligible

because at least some of the water is a long distance from solid surfaces

• Under these conditions, flow is:            Rapid - moving through large pores            Driven by gravity and sometimes

Hydrostatic pressure if water is ponded

http://www.maf.govt.nz/mafnet/schools/activities/swi/swi-04.htm

http://www.montcalm.org/montcalmold/media/planningeduc/tn_gwa5.jpg

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unsaturated

wet dry

Soil moisture content changing with depth

Unsaturated flow

Unsaturated flow – most common in soils• Occurs along soil surfaces, not through large pores.• Driven by matric forces that are much stronger than

gravity.Gravity is not sufficiently strong to exert a significant influence on unsaturated flow because much of the soil water adheres to solid surfaces.

• Unsaturated flow is slow. • Even though the driving force is usually greater than for

saturated flow, the resistance to flow is enormous. • Water will flow toward a lower (more negative) potential

regardless of direction (up, down, laterally). In other words it will flow towards:            drier medium            salty solution            finer texture (small pores)

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http://wwwlb.aub.edu.lb/~webeco/SIM215soilwater_files/image004.gif