Agriculture Reference
In-Depth Information
Box 4.2 Ways of Measuring Soil Water Content
Soilwatercontentismosteasilymeasuredinthelaboratorybyweighingasample
of moist soil, drying it at 105ºC for 48 hours in an oven and reweighing the sample
tomeasuretheweightofoven-dry(o.d.)soil.helossinweightrepresentsthe
weightofwater,expressedasapercentageoftheo.d.soil,accordingtotheequation
Loss in weight of soil sample
We
(B4.2.1)
=
×
100
Gravimetric water content
ight of o.d. soil
Another measure of soil water content is the volumetric water content,
θ
(theta),expressedasm
3
water per m
3
soil,accordingtotheequation
θ
=
Volume of water in the pores
Volume of soil
(B4.2.2)
Volumetricwatercontentcanbemeasureddirectlyintheield(see
“MonitoringSoilWater,”thischapter).Also,because1Mgwateroccupies1m
3
at
normal temperatures, a gravimetric water content can be converted to a
θ
value by
multiplying by the soil's bulk density.
Values of
θ
are useful in the vineyard because they give a direct measure of the
“equivalentdepth”ofwaterperunitareaofthesoil.Forexample,considerasoil
volume of 1 m
3
that has a
θ
value of 0.25 m
3
/m
3
,whichisequivalenttoawaterdepth
of 0.25 m per m
2
surfacearea.AsshowninigureB4.2.1,wemayvisualizethisasa
depth of 250 mm of water in 1-m depth of soil. Expressed this way, soil water content
is directly comparable with volumes of rainfall, irrigation water, and evaporation, all
ofwhicharemeasuredinmillimeters(perm
2
ofsoilsurface).Agoodruleofthumbis
that1mmofrainorirrigationwaterisequivalentto1liter(L)perm
2
.
1 m
1 m
1 m
Figure B4.2.1
Equivalentdepth
of water in 1 m
3
of soil of water
content
θ
equalto0.25m
3
/m
3
.
(White,2003)
250
mm
Equivalent depth
of water for
θ
= 0.25 m
3
/m
3
(continued)
