Delta-T HH2, User Manual

Page: 118 / 124

Field Water Capacity Values

111
Finding the Refill Point
The following is reproduced by courtesy of Research Services New England.
When the moisture content is maintained in the "optimum zone" the drying cycle line
on a graph of depth vs. water content appear to change in a "Y" shape.. That is to
say, the majority of the water will be extracted in the top 400 mm of soil, with some
extraction taking place below. This will always represent a plant extracting water
under an "optimum working environment".
When a soil gets near to the refill point, there are three distinctive changes:
1. The extraction lines on the depth graph become parallel.
2. There is usually a sudden deeper extraction of water in the soil profile.
3. Combined with the above two symptoms, there is usually a decline in the daily
water use of the crop.
The refill point may change through the season. A potato grower interested in scab
control may only want to dry down to a soil moisture deficit of 10-20 mm (depending
on soil type), during the high scab risk period. This is commonly less than half of the
technical refill point. An apple grower may wish to stress the crop beyond the refill
point to stop or slow down shoot growth, at certain times of the year.
The "Full" and "Refill" points can also be determined from the time graph from the
change in slope - see the diagram on page 103.
Refill Point
0
200
400
600
800
1000
1200
0 5 10 15 20 25 30 35 40
Water (%Vol)
De
pth
(m
m
)
REFILL
FULL
Ful
l
R
ef
ill
Parallel Lines
Extraction