Aspects of the water resources management practice with emphasis on nutrients control in the Chivero Basin, Zimbabwe

Abstract

This paper summarises the results of a study on qualitative, quantitative and environmental aspects of water resources management in the Lake Chivero basin, which is the main source of water supply of the City of Harare, Zimbabwe and is in advanced stage of eutrophication. In terms of water quality, an integrated database has been developed, combining existing monitoring data about natural water quality, effluent discharges and urban storm drainage, and data from research investigations during the period 1995–2000. Background pollution in the basin varied from 0.1 to 0.3 mg/l and from 0.1 to 0.4 mg/l for nitrates and phosphates (as total P), respectively. Spatial variations along the major rivers showed a steady trend of increase in nutrient levels with a peak in 1998. At Marimba River confluence the annual median values recorded were 3.5 and 4.4 mg/l for ammonia and phosphates, respectively, thus exceeding the effluent discharge regulations 7–9 times. The major nutrient sources contributing to this status are associated with operational problems of the treatment facilities and diffuse sources of pollution from pastures irrigated with effluent, as well as from urban storm water. In environmental terms a first step was undertaken towards the development of a biological water quality monitoring system, by evaluating the habitat and a-biotic characteristics of the pristine regions of the basin. As to water quantity, it was found that the existing infrastructure is capable to satisfy present water demand, but the abstraction amounts to 77% of the water generated in the basin, which could be considered as an upper limit. It is not yet clear how some provisions of the new Water Act, such as the recognition of the environment as a legitimate water user, will be implemented. With regard to urban water management, the research focused on the development of a rainfall runoff model for the composite catchment area of the Marimba river basin, a sub-urban micro catchment of the Lake Chivero basin. The paper discusses the above findings, and suggests integrated solutions, considering the provisions of the new Water Act. The extensive amount of data accumulated and analysed during this study could form a sound basis for the development of a Management Information System of the basin, based on a GIS. Such a system could be useful for the catchment councils involved.