Flow diagram for evaluating salinity and sodicity impacts of irrigation water
Salinity is the presence of soluble salts in or on soils, or in waters. High salinity
levels in soils may result in reduced plant productivity or, in extreme cases, the
elimination of crops and native vegetation. Salinity related issues are of concern in
many parts of Australia but salinisation is currently considered to be only of minor
importance in New Zealand.
Sodicity is the presence of a high proportion of sodium (Na+) ions relative to
calcium (Ca2+) and magnesium (Mg2+) ions in soil or water. Sodicity degrades soil
structure by breaking down clay aggregates, which makes the soil more erodible
and less permeable to water, and reduces plant growth.
The effects of salinity and sodicity in irrigation waters are very situation-specific,
making it inappropriate to set water quality trigger values for general application.
Factors which need to be considered include: the type of crop being cultivated and
its salt tolerance, the characteristics of the soil under irrigation, soil management
and water management practices, climate and rainfall. A flow diagram for evaluating
salinity and sodicity impacts of irrigation water is shown below (see WQG page 4.2-4
and Figure 4.2.1).
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STEP 1
Identify the soil properties, water quality, climate (rainfall) and management (irrigation application rates) practices for the site in question.
Water Quality
salinity (EC)
sodicity (SAR)
Soil properties
clay % (average root zone)
cation exchange capacity (average root zone)
exchangeable sodium % (at bottom of root zone)
Rainfall
mm / year
Irrigation
mm / year
STEP 2
Estimate the leaching fraction under the proposed irrigation regime using
approaches outlined in section 4.2.4 (see page WQG 4.2-4).
Leaching Fraction
calculated using all input information
STEP 3
Estimate the new average root zone salinity as outlined in this section.
Average root zone
salinity is considered the key limitation to plant growth in
response to salinity and
sodicity levels in irrigation water. However, poor soil
structure can also reduce plant
yields by limiting aeration, water infiltration
and root growth. The likelihood of soil
structural problems induced by
irrigation can be predicted from trigger values derived in
this section.
Average root zone salinity
calculated using
STEP 4
Estimate relative plant yield (although note that the impact of salinity and
sodicity can be
modified by management practices as discussed later in section 4.2.4 (WQG page 4.2-4).
Crop salt tolerance
impact threshold &
yield decline
Management practices
application methods
amelioration techniques
managing variable quality water supplies
Plant response
relative yield
STEP 5
Consider salinity and sodicity problems within the framework of broader
catchment
issues such as regional watertables, groundwater pollution and
surface water quality.
Watertable salinity develops in response to excess
water and salts accumulating in
sensitive parts of the landscape. Excess
water can percolate to groundwaters as a result
of changing climatic
patterns (e.g. frequency and duration of rainfall events), land use or
land
management (including irrigation). Before an irrigation scheme is
developed, the
planning process should include investigation of the
regional hydrogeology to avoid
development of watertable salinity. The
guidelines given here concentrate on localised
effects of irrigation, but
broader salinity issues should not be ignored.
Broader landscape issues
e.g. land use and watertable management
NOTE
Software SALF PREDICT is now available. It estimates the parameters necessary
for a
detailed assessment of irrigation water quality in relation to soil properties,
rainfall, water
quality and plant salt tolerance. The software is based on summer
rainfall areas and
should be used with some caution in winter rainfall areas. It
incorporates many of the
detailed algorithms presented in Volume 3, Section 9.2.3.
The software is provided on the
CD ROM provided with these Guidelines and is
also available from the Queensland
Department of Natural Resources.
A simple initial assessment can be made by measuring
the electrical conductivity
(ECi) and concentrations of sodium (Na+), calcium (Ca2+) and
magnesium (Mg2+)
in irrigation water. Note that EC is expressed in units of dS/m
throughout Section
4.2.4 (1 dS/m = 1000 µS/cm).