ASSESSING UPSTREAM AND DOWNSTREAM INTERACTIONS IN CHALIMBANA RIVER CATCHMENT, ZAMBIA

Abstract

Chalimbana river originates from forest reserve number 27 east of Zambia’s capital city, Lusaka. From its headwaters to the confluence with Chongwe river, Chalimbana river meanders through the farming and rural community over a distance of approximately 51 kilometers. The extent of the catchment is 680 km2 and it lies between latitudes 15o 19 ́ and 15o 32 ́ south and longitudes 28o 21 ́ and 28o 45 ́ east. Chalimbana catchment has good arable soils for agriculture and receives mean annual rainfall of 832.5mm. Its proximity to Lusaka city has attracted settlers, peasant and commercial farmers. Over the years, agricultural development in the catchment has led to the construction of hydraulic structures (weirs and dams) in the upper and middle parts of the catchment. Currently there are 9 privately owned dams on Chalimbana river. This development on one hand has contributed to food security for the local community, surrounding areas and the city of Lusaka. Agricultural produce such as horticultural products are also exported to the international market. Agriculture has therefore contributed to the livelihood for the local people within and outside the catchment. On the other hand, construction of hydraulic structures in the catchment has changed the flow regime of the river from a perennial to an intermittent river especially in the downstream section of the catchment. Furthermore, water demands for both commercial farmers and downstream community has increased significantly over the years resulting in conflicts among themselves as well as with the downstream community. This study was therefore carried out to assess the upstream and downstream interactions in terms of water demands. The data inputs to this study included historical hydrological and climatic data, irrigation and domestic water demand and area-capacity curves for the reservoirs. A spread sheet river basin simulation model called Water Allocation and Flow Model in Excel (WAFLEX) was developed for the catchment to evaluate different water demand alternatives. This was done to determine water demand satisfaction levels for both upstream and downstream water demands. The main water demand management alternatives evaluated in this study based on current water demand were; management of catchment as a complete system, expansion of irrigation areas and improvement of irrigation efficient system. The study concludes that runoff and the storage in the reservoirs is able to sustain both irrigation and domestic water demand. Furthermore, the simulated results indicate that management of the catchment as a complete system is an initial step to the resolution of upstream-downstream water conflicts. Water availability for the downstream community and environmental flows also improves under this management option. The study also concludes that the current water demand is sustainable if the water resources are managed in an integrated manner. Expansion of irrigation area by 30% is also sustainable if irrigation efficient system is improved. The study thus recommends that downstream water demand must be provided for in the water allocation system and manage the catchment as a complete system. For optimal water use, efficiency in irrigation system must be improved.