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Oktober 2017
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Improve irrigation efficiency by
applying a water balance
T
hrough intensive research, the ARC-
Agricultural Engineering, together
with other disciplines, developed
a South African framework for im-
proved water efficiency. The framework
was applied to re-assess the system effi-
ciency indicators typically used by irrigation
designers when making provision for los-
ses in a system and converting net to gross
irrigation requirements.
The project was funded by the Water Re-
search Commission and the report ‘Stand-
ards and guidelines for improved efficiency
of irrigation water use from dam wall release
to root zone application’ was published re-
cently and is now the approach in water use
efficiency.
Success with irrigated farming can be ob-
tained through efficient irrigation by apply-
ing and understanding the water balance
approach. The water balance approach
can be applied at any level, within defined
boundaries, or across all levels to assess
performance within the whole water man-
agement area.
Studies and research over 40 years in
South Africa on the techniques of flood-,
mobile- and micro-irrigation contributed to
the knowledge base of applying irrigation
methods correctly. The fraction of the wa-
ter abstracted from the source that can be
utilised by the plant, can be called the ben-
eficial water use component and optimised
irrigation water supply is therefore aimed at
maximising this component.
In South Africa an area of 16 000 000 ha has
been cultivated and 1 600 000 ha are being
irrigated. With effective water management
and good subsurface drainage, improved
soil health conditions are being created for
successful irrigation farming to assist with
food security. The resulting approach of
‘measure, assess, evaluate, improve’, pro-
motes an investigative water balance ap-
proach to improve water efficiency.
The basis of the water balance approach
is that any water withdrawn from a catch-
ment for irrigation use contributes either to
storage change, to the consumed fraction,
or to the non-consumed fraction at a point
downstream of the point of abstraction. The
water that is consumed will either be to the
benefit of the intended purpose (beneficial
consumption) or not (non-beneficial con-
sumption). Water that is not consumed but
remains in the system will either be recov-
erable (for re-use) or non-recoverable (lost
to further use). The boundaries are as ex-
plained in
Figure 1
.
In order to apply this framework to irrigation
areas, typical water infrastructure system
components are defined wherein different
scenarios may occur. In South Africa, most
irrigation areas consist of a dam or weir in
a river from which water is released for the
users to abstract, either directly from the
river or in some cases via a canal. Water us-
ers can also abstract water directly from a
shared source, such as a river or dam/reser-
voir, or the scheme-level water source could
be a groundwater aquifer.
Once the water enters the farm, it can either
contribute to storage change (in farm dams),
enter an on-farm water distribution system
FOCUS
Irrigation
Special
FELIX REINDERS,
ARC-Agricultural Engineering
WATER MANAGEMENT
LEVEL
INFRASTRUCTURE SYSTEM COMPONENT
Water source
Dam/reservoir
Aquifer
Bulk conveyance system River
Canal
Irrigation scheme
On-scheme dam
On-scheme canal
On-scheme pipe
Irrigation farm
On-farm dam
On-farm pipe/canal
In-field irrigation system
TABLE 1: FOUR LEVELS OF WATER MANAGEMENT INFRASTRUCTURE
.
WATER BALANCE FRAMEWORK
SYSTEM COMPONENT (BASED ON
INFRASTRUCTURE)
INFLOW OF WATER INTO SYSTEM
COMPONENT
Dam/reservoir
Total volume of water released from
storage
River bulk conveyance system
(from on-river dam to scheme/farm edge)
(if applicable)
Total volume of water entering the river
Canal bulk conveyance system
(from on-river dam to scheme/farm edge)
(if applicable)
Total volume of water entering the main
canal
On-scheme surface storage
Total volume of water entering a scheme
dam
Shared (scheme-level) groundwater aquifer
compartment
Total aquifer recharge
On-scheme canal distribution system
(if applicable)
Total volume of water entering the
on-scheme canal distribution system
On-scheme pipe distribution system
(if applicable)
Total volume of water entering the
on-scheme pipe distribution system
On-farm surface storage
Total volume of water entering a farm dam
On-farm distribution system
Total volume of water entering the on-farm
pipelines or canals
In-field system (from field edge to root
zone). Intended destination of the water
released
Total volume of water entering the
irrigation system (gross irrigation
requirement [GIR] plus precipitation)
TABLE 2: WATER BALANCE FRAMEWORK ALLOCATION OF TYPICAL IRRIGATION
SYSTEM COMPONENTS.