ASWC ¼ TAM CRðFCSWI SWIÞ ð1Þ
where TAM1 is the plant available soil water content of the root zone
at field capacity after drainage of free water (in mm). Eq. (1) reflects
the assumed direct proportionality between soil water deficit (the
difference in water content at field capacity and current water content)
measured by sensors (FCSWI – SWI), and quantified in units of
available water (TAM – ASWC). Values for FCSWI were determined
by investigating recorded drainage and extraction patterns after a
wetting event. Significant wetting of an already wet root zone will
increase SWI above the FCSWI, causing rapid drainage and decline of
SWI over time. As soon as the SWI reaches FCSWI, drainage rate and
decline in SWI would slow markedly, indicating the transition from
rapid drainage of free water to extraction of water by plants, providing
an indication of the value of FCSWI. Values for CR were determined
by comparing recorded extraction rates for dry days with MyCanesim
simulated extraction rates and adjusting CR values until these
extraction patterns (average rates of decline in simulated and
observed ASWC) matched. CR values can also be determined by
comparing recorded responses to night-time wetting events of
known amounts of water but this was not used here because reliable
irrigation and local rainfall records were not available.
Half-hourly SWI data were transferred from a central Aquacheck
server to the MyCanesim database, and then converted to
ASWC. The ASWC value at 8:00 am is taken as the daily value that
is displayed on soil water graphs (Fig. 1). The user can also manually
upload ASWC data into the database through the MyCanesim
web interface. Users need to specify whether they want simulated
ASWC to be corrected with measured values or not. If the correction
option is chosen, the simulated ASWC at the start of the day
will be reset to the measured value, except on days when rainfall
or irrigation exceeded 15 mm. This exception was required to
avoid potential errors that could be caused by the uncertainty of
whether the wetting event occurred before or after the time of
the measured ASWC.
4. System output
Daily values of ASWC, rainfall and irrigation, crop water status,
canopy cover, cane and sucrose yield are provided via the web in
tables and graphs. Online graphing was done using the freeware
(available for non-commercial use) at http://www.highcharts.com.
This package was chosen out of eleven freeware candidates
because it could display multiple series on multiple axis, display
values on mouse over events, allowed scrolling and zooming, could
export graphs as images, and was compatible with HTML and Javascript.
Some examples of the graphs are shown in Figs. 1–3.
Table 1 is an example of an irrigation advice report from
MyCanesim. Information to support irrigation scheduling decisions
include:
the advised current irrigation status (irrigating or not),
the predicted date of the next action (stop or start irrigating),
the predicted date of the commencement of the drying off period,
and
the current ASWC expressed as a percentage of TAM.
The different programming and data components of the integrated
MyCanesim system are illustrated in Fig. 4.