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November 2015
42
JAMES MOORE,
IrriCheck
FOCUS
Natural resources and energy
Special
Irrigation scheduling to navigate
load shedding and optimise water consumption
W
ith the persistent and rapid rise in energy costs and
increased pressure on our water resources, the South
African irrigation producer is, now more than ever,
faced with the problem of how to produce more, with
less, and to make every drop count.
To add to this, producers have to face an uneven electricity supply,
dubbed “load shedding”, which adds to the pressure by limiting the
hours available to irrigate.
Mr Gerrit (Appel) van Zyl, a partner at IrriCheck (Pty) Ltd, is an engi-
neer and irrigation design and scheduling specialist with more than
20 years’ experience. He reveals that, “On average (depending on
the distance and height at which the water is pumped) it can cost
R1 526 per hectare in electricity alone to grow a maize crop. To
put this into perspective, in 2005/2006 the comparative cost was
R515,41 and in 2010/2011 it stood at R1 047,60. This is an increase
of, on average, 20% per year. Next year, with a 12,69% increase, this
cost will rise to R1 719,65 per hectare and the subsequent year to
R1 937,87 per hectare.”
In order to be sustainable, producers are rapidly introducing new
technologies to improve their effectiveness and yields. In the irriga-
tion farming environment, irrigation scheduling appears to be the
most popular and effective way to achieve that.
What is irrigation scheduling?
According to Van Zyl, irrigation scheduling is best described as the
process where technologies are employed to manage the soil-crop-
climate-economic environment by measuring and calculating the
optimum quantity and timing of irrigation.
This, in combination with the limitations of the irrigation system, is
used to apply water at the correct rate and in accordance with crop
phenological stages in order to maximise water and nutrient use
efficiency, while maximising production and quality.
Why schedule?
Crop water use is influenced by many factors, including, but not
limited to; the climate, the type and size of the crop, crop phenology,
the root system, soil characteristics and irrigation systems. Most of
these factors are variable and interdependent on one another.
“When irrigation producers are unsure when or how much to irri-
gate, they tend to over-irrigate,” says Van Zyl. “Every time we over-
irrigate, precious water and nutrients are washed away (leached)
past the root zone and out of the plant’s reach. In addition to this,
soil oxygen, vital for healthy root growth and beneficial microbial
activity is constantly impaired, potentially leading to anaerobic con-
ditions which inhibits the uptake of water and nutrients and creates
favourable conditions for soil borne diseases, ultimately decreasing
yield potential.”
Irrigation scheduling systems
Currently, the most popular irrigation scheduling hardware is the
continuous logging soil moisture capacitance probe, which is cou-
pled with a GPRS unit. It measures the soil moisture content and
temperature at six depths receiving live data on a two hour basis.
As for software, Van Zyl is of the opinion that an efficient scheduling
system should at least have the ability to measure and analyse the
following:
Soils
Each soil has unique boundaries in terms of water holding ca-
pacity, including field water capacity (FWC), permanent wilting
point (PWP) and readily available water (RAW). Knowing these
boundaries is important when managing different moisture uptake
regimes at key phenological stages of the crop as well as when ap-
plying (safe) controlled stress conditions for crop manipulation.
Most importantly, the system should indicate soil water data in
millimetres of water, and probes should therefore be calibrated to
millimetres per meter water holding capacity in each field.
Crop specific water needs
The software should accurately simulate crop evapotranspira-
tion under non-specific conditions, using the dual crop coefficient
approach. Daily water loss in the form of transpiration from the crop
and evaporation from the soil under specific in situ climatic condi-
tions should be taken into consideration. Because of changing man-
agement practices, crop phenology and climatic conditions, the
software should also be able to automatically determine the crop
evapotranspiration and changing crop coefficients according to
these changing conditions for scheduling purposes.
Climatic parameters
Because atmospheric evaporative demand is the driving force in
crop water use, it is important that an accurate weather forecast is
used for predicting irrigation needs for the coming week and that
actual weather data is measured accurately, ideally making use of an
on-farm weather station.
Irrigation systems
The parameters of the irrigation system, via input data, should be
taken into consideration in order for the software to adjust to the fol-
lowing: The limitations of your irrigation system, different soil types,
unique environmental conditions and plant specific requirements.
Recommendations
Recommendations should be live and in real-time (data should auto-
matically be sent to the software) and in practical units, like millime-
tres, cubic meters or hours and minutes, and should be provided at
least a week in advance. This allows the producer to make quick de-
cisions and plan ahead, for instance, ordering water at least a week