Delta-T HH2, User Manual

Page: 120 / 124

Glossary

113
Engineering Units
These are the units of measurement of the reading shown
on the display of the HH2. The units available depend on the
sensing device.
Options:
mV, percentage of soil water by volume (%Vol), cubic metres of
water per cubic meter of soil (m3.m-3), and mm deficit.
You may also save raw mV readings in addition to readings
converted into other units.
Field Capacity
Also called
Field Water Capacity (FWC).
The maximum water
content a soil can hold against gravity, once excess water has
ceased to drain through it, surface run-off has ceased, and the
water content has become relatively stable. This usually takes
several days.
Measurements in the laboratory are made by subjecting the soil to
a standard suction pressure, normally specified as 0.05 bar in the
UK and Europe and 0.15 bar in the USA. Tables of Field Water
Capacity for different soil types are given above on page 106.
Field Capacity Units are correctly stated as mm of water over a
certain depth (e.g. 3mm over 0-70 cm).
Another 'definition' or usage of 'field capacity' is "The wettest soil
profile that can be regularly obtained after through-drainage has
ceased". This is related to the idea that the field has reached its
capacity or full point.
ML1
ThetaProbe type ML1. Soil moisture sensor with one differential
analogue output.
ML2
ThetaProbe type ML2. Soil moisture sensor with one differential
analogue output. All references to ML2 include ML2x.
Permittivity
An physical property describing the electromagnetic behavior of a
material - used to determine both soil water content and
conductivity.
PR1
Profile Probe type PR1. Soil moisture sensing device with 4 or 6
differential analogue outputs, one for each sensor in the probe.
PR2
Profile Probe type PR2. A much improved version of the Profile
Probe!
Plot ID
A unique letter between ‘A’ and ‘Z’. This will default to ‘A’ when
the meter is reset, but the user is able to modify this at any time to
reflect a geographical area in which the meter is being used. The
current value is stored with each reading which is recorded in the
meter.
Refill Point
T
he water content of the soil below which the plant exhibits some
form of stress, and a drop in yield. Stress may be indicated by a
drop in the daily use water. It depends both on soil and crop. One
indicator of the refill point might be that roots start extracting water
at a greater depth than normal. Some crops may be deliberately
taken below the refill point to improve desired attributes.
Rooting Depth
The depth of soil to which the roots extend. The soil depth that
you are interested in for use in the water deficit calculation.