Water Quality
Suitability indicators to assess specific
site, risk-based irrigation water quality
by HM du Plessis 1 , JG Annandale 1 and N Benade 2
University of Pretoria 1 and NB Systems 2
I
n the first of this two-paper series, we introduced
readers to a newly developed electronic Decision
Support System (DSS) that helps the user to assess
the fitness for use of irrigation water (du Plessis et.
al., 2019). In this paper, we provide an overview of the
suitability indicators used by the DSS to assess the fitness
for use of water for irrigation.
The DSS provides the user with
an assessment of how water of a
given composition will affect soil
quality, crop yield and quality,
as well as irrigation equipment.
Specific indicators were identi-
fied with which to measure the
suitability of irrigation water for
its intended purpose and criteria
have been established with which
to categorise the effect on each
suitability indicator. The criteria
are used to quantify the effect that
water constituents have on suit-
ability indicators and categorise
the effect as being either ideal,
acceptable, tolerable or unac-
ceptable. Throughout the DSS use
is made of these (colour coded)
fitness for use categories to clas-
sify and display the suitability of
irrigation water (see Table1).
Table 1 A generic description of the fitness for use classification of irrigation water
used by the DSS
Fitness for
use category
26
Description
Ideal A water quality that would not normally impair the fitness of
the water for its intended use
Acceptable A water quality that would exhibit some impairment to the
fitness of the water for its intended use
Tolerable A water quality that would exhibit increasingly
unacceptable impairment to the fitness of the water for its
intended use
Unacceptable A water quality that would exhibit unacceptable impairment
to the fitness of the water for its intended use
SABI | OCTOBER/NOVEMBER 2019
The DSS allows the user to
conduct either a rapid conserva-
tive (Tier 1) or an extensive site-
specific (Tier 2) evaluation of the
suitability of a specific water for
irrigation. Should the Tier 1 FFU
assessment (that makes use of
conservative assumptions) not
indicate potential problems with
any of the suitability indicators,
the water is deemed fit for use on
all crops, under all but the most
exceptional circumstances. On
the other hand, should the Tier
1 assessment identify potential
problems with one or more of
the suitability indicators, a more
detailed, site specific assessment
as provided by a Tier 2 assess-
ment, is indicated. The Tier 2
assessment allows the user to
select more appropriate site
specific variables (such as crop,
irrigation system and manage-
ment, soil texture and climatic
data) to simulate soil-plant-
atmosphere interactions with a
soil water and solute balance
model, and produce a much
more rigorous assessment of soil-
crop-water interactions. Running
the Soil Water Balance (SWB)
model over several years, allows
the calculation of the likelihood of
yield and other parameters falling
in different suitability categories.
Suitability indicators
of soil quality
Four suitability indicators are
used to evaluate the effect water
constituents have on soil quality,
namely root zone salinity, soil
permeability, dissolved organic
carbon loading and trace
element accumulation.
Salinity (salt content) within
the root zone reduces crop
growth by reducing the ability of
plant roots to absorb water from
soil. While the yield of some
crops is already affected at low
soil salinity levels, others display
salt tolerance, with the result that
the range of crops that can be
successfully cultivated decreases
as soil salinity increases. Root
zone salinity is thus an impor-
tant indicator of soil quality, and
the crops that can be cultivated
successfully under irrigation.
The suitability of a soil for crop-
ping is to a large degree deter-
mined by its ability to conduct
water and air (permeability) and
on physical properties that control
the friability of the seedbed
(tilth). In general, increases in
soil sodium concentrations cause
a deterioration of soil physical
properties.
The
undesirable