| Furrow irrigating cotton
growers are starting to install more centre pivots and lateral moves
(CP&LMs) every year.
The main reasons are:
• A reduction of irrigation labour requirements of 80 per
cent over that used for traditional furrow irrigation;
• Greater control of soil moisture;
• One bale per hectare average potential yield increase due
to reduced crop waterlogging;
• Greater beneficial capture of in-crop rainfall;
• Overall simplicity of use; and,
• A 30 to 50 per cent reduction in applied water possible
over traditional furrow irrigation.
System capacity
System capacity is the most important design parameter for CP&LM
machines in the Australian cotton industry. Many machines installed
in Australia in the past did not have a system capacity large enough
to successfully grow a cotton crop.
The ‘failures’ associated with low system capacity have
been the single greatest reason for the continuing low uptake of CP&LMs
in Australia. To be successful, these systems must be able to supply
water to irrigated cotton fields at a rate to cater for peak crop evapotranspiration.
In Australia’s highly variable climate, we can’t rely on
timely rainfall to help irrigation systems during peak crop water requirement.
So future systems must be designed without any allowance for supplemental
rainfall.
This discussion assumes no in-crop rainfall and that growers have an
adequate volume of water allocated for the irrigated area under their
CP&LM.
How to manage your system capacity
The system capacity is the maximum possible flow rate that the CP&LM
can apply to the area of an irrigated field (see box story Page 25).
The system capacity of a CP&LM is reduced considerably in the real
world by the number of hours the pump is turned off during the irrigation
cycle.
Also take into account the non-irrigating time necessary for any pesticide
spraying with over-crop sprinklers and the dry travel time of the CP&LM
that you think that you may need.
System capacity is further reduced by losses that occur between the
nozzle on the machine and the crop root zone. The ratio of the water
that actually makes it into the crop root zone, divided by the total
amount of pumped water is called the application efficiency.
For low energy precision application (LEPA) systems, the application
efficiency could be 0.98 and for modern over-crop sprinkler systems,
between 0.9 and 0.95. So a grower running his CP&LM pump for 204
hours (out of 240) throughout a 10 day period during peak crop water
use, using a well-tuned over-crop sprinkler system would be able to
irrigate at a rate of 204/240 x 0.95 = 0.81 of the system capacity.
You might have a system capacity of 14 mm/day, but if the pump only
runs for three quarters of the time, even with a LEPA system, then on
average 10.5 mm/day would be applied into the crop root zone.
Remember that these system capacity values have nothing whatsoever to
do with the amount of water applied by the CP&LMs during each irrigation
pass. The amount of water that is applied per pass is governed by the
pump flow rate and the amount of time that the machine takes to complete
one irrigation pass of the complete irrigated area. Just as a constant
flow rate boomspray operator would reduce speed to apply a greater amount
of water to the field, so too is the average speed of a CP&LM reduced
to apply more water per pass.
Choosing your system capacity
A process for choosing a suggested CP&LM system capacity has been
developed utilising the evapotranspiration maps of Australia recently
developed by the CRC for Catchment Hydrology and the Bureau of Meteorology.
A calibration factor (21.5) has been derived from the system capacities
of CP&LMs across the cotton industry and the January map of average
point potential evapotranspiration (ETo), to allow growers to choose
their location and calculate their own system capacity target.
January was chosen as it represents the period of greatest crop water
use for cotton. The calibration factor assumes a pumping utilisation
rate of 0.85 and the use of a LEPA system with an application efficiency
ratio of 0.98.
Locate the proposed site of your CP&LM on the evapotranspiration
map. Then find the closest lines of evapotranspiration for your particular
location and divide the value by the cotton industry system capacity
calibration factor (21.5). The resulting number will be in millimetres
per day, and is a starting point for decisions regarding the appropriate
system capacity for CP&LM design.
If growers are concerned about the particular value they calculate,
consult appropriately skilled irrigation professionals.
Peak crop evapotranspiration rate
In any growing season, there will always be periods in which crop evapotranspiration
is much higher than average. Systems must be designed to handle these
extremes and minimise the days when crop evapotranspiration exceeds
the system capacity.
When growers choose their irrigation system capacity, they are choosing
the level of risk that the machine will not be able to keep up with
particularly high evaporative days. Growers who are not prepared to
risk the possibility that their CP&LM will ‘not keep up’,
should choose larger CP&LM system capacities.
The real consequences of choosing lower system capacities will be the
reduction in the average amount of water held in the crop root zone
as each passing day extracts, on average, more than the CP&LM system
capacity can supply. This does not necessarily mean crop failure, but
rather the gradual decline in the readily available water supply for
the crop and the potential for crop yield reduction.
Increasing system capacity
For large lateral moves (whose upper size limit is currently controlled
by the maximum flow rate of the largest pumps that manufacturers are
prepared to place upon drive carts) the system capacity can be increased
by cutting the overall irrigated run length. This is a cost effective
and simple matter as no substantial change to the lateral move design
is necessary. But costs could be incurred if changes are necessary to
the field drainage network.
Increasing centre pivot system capacity could involve changes in the
nozzle set at a very minor cost. But alterations in the pump and pipe
diameters, both in the span and supply line, can have significant associated
costs. And if pumps and pipes are incorrectly designed, the lifetime
running costs of the system can be greatly increased.
Remember that choosing larger system capacities for CP&LMs does
not mean that larger water volumes are applied to the crop. It simply
means that there is adequate capacity to cater for the peak crop water
requirements of well-grown cotton when the crop requires it most.
Recent purchases of large lateral moves in the cotton industry have
all been with the largest pump flow rate possible for these machines.
Based on this fact, a range of different field lengths has been calculated
and this is presented in Table 1.
Bigger capacity, lower running cost
One of the largest costs of ownership of CP&LMs is the on-going
pumping energy cost associated with supplying irrigation water through
the machine. Many growers in the past have not completely understood
the implications of purchasing equipment with small diameter pipe spans.
Their overall cost of ownership drastically increased because they purchased
a slightly cheaper (smaller) span pipe configuration.
A slight increase in the up-front capital cost can drastically reduce
long term ownership costs.
A “present worth” analysis of the long-term pumping energy
costs of a large lateral move with four different configurations was
conducted, as shown in Figure 1.
The lowest capital cost option of the four different lateral move designs
consists of 18 small diameter spans. The most expensive design consists
of 14 spans of the larger diameter pipe spans.
Increasing the number of spans with larger pipes, costs an additional
7.9 per cent, but reduces the 15 year pumping energy costs to one third
of that using small diameter pipe spans.
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