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

D.a Hughes

doi:10.4314/wsa.v32i3.5267

Abstract

A simple model is proposed that simulates the water balance in small- to 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. Water SA Vol.32 (3) 2006: pp.411-417

Highlights

The concept of assurance of water supply becomes extremely important in highly variable flow regimes where there is a great deal of competition for the limited amounts of water that are available during dry periods
The approach to defining environmental water requirements in South Africa recognises the natural variability of flow regimes and that some of that variability needs
If the length of time of zero flow for the present-day or future scenario flow duration curves (FDCs) is greater than the zero flow duration of the natural FDC, it is clear that the specified abstractions are not possible, regardless of the Reserve requirements

Summary

Introduction

System yield models available within South Africa have been designed to account for a wide variety of basin and water-use configurations including streamflow reduction activities (SFRA – afforestation, for example), reservoirs (and associated abstractions and releases), inter-basin transfers, distributed small farm dams, run-of-river abstractions, return flows (from agriculture and wastewater treatment works) and the Ecological Reserve (Janse van Rensburg and Görgens, 2001). This paper proposes a simple approach that is based on the best available information and that does not require either a great deal of time or technical expertise to obtain an approximate assessment of the feasibility of granting a new licence for river water abstractions or an SFRA.

Results

Conclusion