Water Quality
Crops that are susceptible to
foliar damage (which affect crop
quality) when salts are absorbed
directly through their leaves, often
display greater yield reductions
than when only exposed to root
zone effects. However, almost no
quantitative data are available
which quantify this effect on yield
and only limited qualitative data
are available to assess the rela-
tive susceptibility of crops to foliar
injury. The DSS therefore evaluates
the degree of leaf scorching expe-
rienced by crops for which data
are available, only in qualitative
terms, and only when the selected
irrigation application method
allows for foliage to be wetted.
The presence of plant nutri-
ents in irrigation water is mostly
considered as an advantage,
since it represents a potential
saving in fertiliser costs. However,
the presence of nutrients may also
complicate fertiliser management,
as high concentrations may stimu-
late excessive vegetative growth
and cause lodging, delayed crop
maturity and poor quality. The
approach adopted for evaluating
the presence of plant nutrients
in irrigation water is to estimate
both the quantity of NPK that will
be added through irrigation and
the contribution their addition
will make to the NPK removed
by a specific crop. The assess-
ment of fitness for use is based
on the contribution that irriga-
tion applications makes towards
the estimated NPK removal by
crops (indirectly the crop nutrient
requirement). When inadvertent
nutrient additions through irriga-
tion are relatively low compared
to crop requirement, it is relatively
easy to accommodate the addi-
tional nutrients as part of normal
nutrient management practices,
and the additional nutrients may
be viewed as beneficial. However,
as inadvertent nutrient additions
increase, it becomes increasingly
difficult to manage the negative
effects associated with unintended
higher fertiliser applications.
The main concern about the pres-
ence of human pathogens in
irrigation water is the risk posed
to food safety (crop quality)
when crops destined for human
consumption are contaminated
during irrigation. The deposition
of pathogens during irrigation is
of particular concern for fruit and
vegetables which are consumed
raw, or which undergo minimal
processing. The levels of E.coli are
used as an indicator of microbial
pathogens, and more specifically
Norovirus, in irrigation water.
Norovirus is recognized as the
most common agent of viral
diarrhea transmitted by irriga-
tion water and by irrigated fresh
produce consumed raw. The risk
of norovirus infection per person
per year is estimated using a dose-
response look-up table, which
gives the risk of norovirus infec-
tion per person per year when
consuming lettuce irrigated with
water of different E.coli concen-
trations. The calculated risk of
exposure is thus based on the total
annual E.coli intake. For crops
other than lettuce, the annual
intake is calculated from the
volume of irrigation water retained
by the crop and how much of the
crop is consumed on an annual
basis, relative to that of lettuce.
Whether a crop is wetted by irri-
gation and the volume of water
retained, is determined by the
irrigation system and anatomy of
the crop. The DSS output reports
the risk of infection as the number
of excess infections per thousand
persons per annum (p.a.).
From a review of available infor-
mation, it was clear that the pesti-
cide concentrations measured in
water resources are low and that
the likelihood is small for irrigation
water to be the source of unac-
ceptable pesticide residues on
produce. Herbicides are the most
widely applied class of pesticides
in South Africa. In view of the
phytotoxic risk they pose to non-
target/sensitive crops, herbicides
are more likely to be of concern
to irrigation farmers. Although
this risk also appears to be low
based on available evidence, it
was decided to include herbicides
as one of the suitability indicators
in the DSS. After glyphosate, atra-
zine is the most widely used herbi-
cide in South Africa. Since atra-
zine is highly mobile compared
to glyphosates, with a significantly
longer half-life, it was selected for
inclusion in the DSS.
Atrazine is used to control annual
broadleaf weeds and certain
grasses in maize, grain sorghum
and sugarcane, as a pre- and/or
post-emergence application. A
problem associated with atrazine
applications (and thus with atra-
zine applications through irriga-
tion) is that the residual atrazine
remaining in the soil may damage
atrazine sensitive follow-on crops.
The DSS calculates the atrazine
dosage applied through irriga-
tion and uses international criteria
to assess a qualitative risk this
dosage poses to sensitive crops.
The dosages recommended by
manufacturers are used to assess
the qualitative risk posed to maize,
grain sorghum and sugarcane.
Suitability indicators
of the effect on irri-
gation equipment
Irrigation water is normally
supplied untreated. It is thus not
chemically stabilised to control the
potential for corrosion or scaling
of irrigation equipment or filtered
so that it can be used directly for
drip irrigation. Corrosion and
scaling of irrigation equipment
and structures are arguably the
primary water quality problems
associated with on-farm irrigation
infrastructure. Either can neces-
sitate the early replacement of
expensive irrigation equipment.
Both corrosion and scaling are the
result of waters having chemical
imbalances. A secondary problem
associated with water constituents
is the clogging of drippers, which
can be of a chemical, biological
or physical nature.
The prediction of corrosion and
scaling is a complex phenomenon
with several factors determining
its outcome, some of which are
very site-specific. Although minor
scaling which forms a protective
layer against corrosion inside
pipes is normally considered
beneficial,
excessive
scaling
reduces flow rates and damages
water
systems,
necessitating
repair or replacement. The most
common cause of scaling is the
precipitation of calcium carbonate
when its saturation is exceeded.
Although less frequent, gypsum
precipitation also occurs in irriga-
tion equipment when water high in
calcium and sulphate is used.
The DSS uses the Langelier
Saturation Index (LI), which is
an approximate measure of the
degree to which water is saturated
with calcium carbonate, as indi-
cator of the likelihood of corrosion
or scaling. A positive LI indicates
that water is over-saturated, and
scaling is likely, while a negative LI
indicates water is under-saturated
with respect to calcium carbonate
and is potentially corrosive. The
DSS uses the LI to determine the
corrosion or scaling potential of
a water sample.
Design, Build, Commission, Maintain
All Water Purification Systems
Purification of water from:
Rivers
Dams
Boreholes
Municipal Outlets
Application specific solutions to various problems such as:
Iron & Manganese (Fe & Mn)
Bacteria & Viruses
Low Alkalinity (Aggressive water)
Brackish water (NaCl—Salt)
Odour & Taste
Water treated for the following uses:
Agricultural
Potable water
Irrigation
Industrial
[email protected]
021 851 2451
De-mineralized water
Process feed water
Waste water recycling
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