There is a defined need to implement an equitable and holistic approach to the management of South Africa’s scarce water resource, based on catchments as hydrological units. The impacts of land use change and potential development, in particular those associated with reductions in streamflows from afforested areas, have led to contention between competing water sector users. In the Pongola-Bivane catchment, downstream irrigators of sugarcane perceive that water utilisation by upstream afforestation has adversely affected the water resource of the catchment. This has resulted in: (a) a moratorium on any further commercial afforestation; and (b) the impetus to construct a large storage dam, the Paris Dam, which would provide a guarantee against critical low flows in the lower Pongola-Bivane catchment, and additionally supply rural communities with water.
Sensitivity studies, using the ACRU agrohydrological modelling system, were performed to simulate and evaluate the impacts of increasing areal extents of both afforestation and irrigation on streamflows from a relatively moist and a relatively dry subcatchment within the Pongola-Bivane catchment. The results revealed that both afforestation and irrigation have greater impacts in a drier than in a moister subcatchment, with (in both subcatchments) greater impacts in dry years than wet years. Different land uses impact differently on available water resources. All other things being equal, a greater extent of afforestation, than that of irrigated land use, sustains a comparable reduction in streamflows. In a year with median annual streamflows, and when for both cases the land use is increased to the extent of reducing streamflows by 10%, the moist subcatchment investigated supports an areal extent of afforestation 10.2 times greater than that of irrigation. This implies that to sustain a 10% streamflow reduction, the additional water use of every 10.2 ha of afforestation is equivalent to that of 1 ha of supplementary irrigation of sugarcane, if the water was extracted from streamflows. In a year of median annual streamflows, the dry subcatchment investigated supports an areal extent of afforestation 15.2 times greater than that of irrigation, for a streamflow reduction of 10%. In wet years, and when for both cases the land use is increased to the extent of reducing streamflows by up to 20%, the relative impacts of irrigated land use on water resources are less than those of afforestation. However, in dry years the relative impacts of afforestation on water resources are less than those of irrigated land use.
Scenario studies, again using the ACRU agrohydrological modelling system, were performed to simulate impacts of: (a) present upstream land use; (b) proposed upstream afforestation; and (c) other potential development (including anticipated RDP abstractions and additional irrigation abstractions for downstream use) on the performance of the proposed Paris Dam. The results revealed that, if no additional irrigation abstractions were drawn from the Dam, present upstream land use and proposed afforestation as well as anticipated RDP abstractions has relatively little impact on the performance of the Dam. The Dam is filled by stormflows from heavy rainfall events, with such stormflows occurring irrespective of land use. The Dam’s storage level is shown generally not to depend on low flows.
This dissertation has illustrated the extent to which agrohydrological modelling can be applied to determine the level(s) at which changes in land use and potential development become critical in the utilisation of catchment water resources. Agrohydrological modelling, therefore, plays a key role in the decision-making processes required for hydrological conflict resolution.
|Subject||Environmental sciences and Ecology|
|Subject 2||Environmental sciences and Ecology|
|Degree Type||Masters degree|