Delta-T HH2, User Manual

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Water Deficit Explained

103
Water Deficit Explained
Saturation
is the moisture content at which all the air within the pores has been
replaced by water. It’s not a stable situation because the water will immediately start
to drain through. It’s a property of soil type only.
Field Water Capacity
(or
Field Capacity
) is the moisture content obtained
when a saturated soil has been allowed to drain (sometimes taken as 2 days later,
sometimes when drainage has become “negligible”).
It’s a property of soil type only, and typically varies between 0.1 for sandy soils up to
about 0.45m
3
.m
-3
for clay soils.
Wilting Point
(WP) is the moisture content at which a particular crop is unable to
extract any more water. Conventionally this is taken to correspond to a matric
potential of –1500kPa, but it’s really much more variable than that suggests. It’s a
property of soil type and crop type, and can vary between about 0.04 for sandy soils
to 0.22 for clay soils. (Data for organic soils is not available).
Dry
is zero moisture content.
plant water use
plant stressed
waterlogged
oven dry
all water, no soil!
Field Capacity
Wilting point
Through-
drainage
irrigation
Saturation
100%
0%
Time
%
W
at
er
Proportional to
Deficit