| Summary: | In the Lower Vaal and Riet Rivers, changing irrigation water quality has raised concern about the long-term
sustainability of irrigation due to reduced yields of certain crops and the withdrawal of some very profitable
crops.
The main aim of this study is to develop and apply models to determine the long-term financial and economic
viability of irrigation farming in the Lower Vaal and Riet Rivers, with specific aims to: evaluate the relationship
between changing water quality, soil conditions and crop production; determine the impact on yield, crop choice,
agronomic and water management practises, expected income and costs; develop models for typical farms in
different river reaches, and apply these models to test the outcome of alternative scenarios regarding internal
water quality management practises and external policy measures.
Five case study farmers were selected, one from each of the different sub-areas of the OVIB study area. The
case study farmers were representative of their sub-areas with regards to the hectares of irrigation water rights
held, and jointly, also sufficiently representative of the OVIB region.
With the contradicting aims of improved water use efficiency and increased leaching for salinity management,
the importance of a financial optimisation model was evident to solve the apparent paradox between saving
water due to itâs scarcity value and âwastingâ water to leach out salts that build up in soils through the process of
irrigation.
SALMOD was constructed using GAMS and consists of a simulation and optimisation section that calculate the
optimal crop enterprise, management and resource use combination that maximises farm returns under different
water quality, management and policy scenarios.
The management options built into SALMOD are the appropriate leaching fraction to implement and crop yield
to accept for the optimal crop / resource combination calculated. The fixed capital management options included
in SALMOD are the installation of artificial drainage, the change of irrigation system and the building of on-farm
storage / evaporation dams for return-flow management.
The % reduction in TGMASC from the long-term average ECiw (74 mS/m) to the worst expected Vaal River
ECiw as predicted by Du Preez et al, (2000) for 2020 (159 mS/m), is 84% and 58% for the small farmers from
Bucklands and Atherton respectively, between 13% and 16% for the Olierivier farmer, depending on whether
the Vaal River of the Riet River has the major impact, 1% for the large and financially strong Vaallus farmer and
3% for the small yet resource strong New Bucklands farmer (see Table 5.38). These results clearly show that
the small and resource poor farmers will be the most affected by irrigation water salinity deterioration.
Scenario results from SALMOD further show that:
- Leaching is financially viable for all case study farmers - Accepting lower yields on soils with insufficient leaching capacity is also financially viable
- For farmers with limited area of well drained soils it can be financially viable to install artificial drainage
- The option of building on-farm storage dams when returnflows are constrained to 100 mm per hectare
water rights held, is financially infeasible for all case-study farms and for all scenarios
- It is not financially viable for farmers to replace their current irrigation systems with more efficient water
saving systems, but in some instances to replace them with systems that can apply a greater leaching
fraction
- At the worst-case scenario salinity conditions, farmers with below 60 ha water rights, and who donât
grow cotton, will go out of production.
SALMOD has proved to be a valuable farm level salinity management tool. SALMOD is also potentially useful at
regional and national level for determining the farm level financial impacts of various water quality and quantity
scenarios where the farmers are affected by irrigation water salinity.
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