Agriculture Reference
In-Depth Information
Figure 4.11
Asoilproileshowingaclearlydeinedwettingfront.(White,2006;reprintedwith
permissionofWiley-BlackwellPublishingLtd.)
Box 4.6 Estimating the Depth of Soil Wetting
Rainfall or irrigation of 20 mm means that water is deposited on 1 m
2
of soil
surface to a depth of 20 mm. As it infiltrates, this water can only occupy the soil's
porespace,whichisabout50%byvolume(0.5m
3
/m
3
).husifalltheporespace
is initially air-filled, the wetting front will penetrate to a depth
x
1
, where
=
=
x
1
20
/.
05
40
mm
(B4.6.1)
Ifthereiswaterinthesoilinitially(evenair-drysoilcontainssomewater),the
infiltrating water will penetrate farther. For example, if the initial water content
θ
is 0.1 m
3
/m
3
, the effective pore space to be occupied is 0.4 m
3
/m
3
and the depth of
penetration
x
2
is
=
=
x
2
20
/.
04
50
mm
(B4.6.2)
Somefurtherpenetrationofwateroccursasthewetsoilzonedrainstoits
FC
.Notealsothatwaterdoesnotalwayspenetrateasauniformwettingfront
becauseofpreferentiallowdownwormholesandbetweenaggregates(see“Soil
IniltrationandWetting,”thischapter).
