A simple model for assessing utilisable streamflow allocations in the context of the Ecological Reserve

A simple model is proposed that simulates the water balance in smallto medium-sized water resource systems (without major storage) and displays the results as flow duration curves so that they can be compared with the standard information available for the Ecological Reserve requirements. The model is designed to account for the impacts of streamflow reduction activities (afforestation), small farm dams and run-of-river abstractions under both present-day and future scenario situations. The paper presents the operation of the model and its data requirements, as well as discussing possible sources of uncertainty associated with either the model approach or the quality of the data inputs. The majority of the uncertainty in the ability of the model to generate reliable results is expected to be associated with the quality of the input data, and specifically the information available on existing water use. It should be noted, however, that any water resource systems model is faced with the same uncertainties and this highlights the need to improve the South African database on existing water use. Without such information, future planning of water allocations will be extremely difficult and highly uncertain.