MODELLING MULTIPLE RESERVOIR OPERATION SYSTEM FOR OPTIMUM PRODUCTIVE WATER USE IN THE SAFARI- IGAVA IRRIGATED AREA IN MARONDERA DISTRICT IN ZIMBABWE

Abstract

Zimbabwe undertook a land reform in 2000, where some large scale commercial farms were sub-divided into smaller plots and allocated to new farmers. Reservoirs owned and managed by farmer consortiums were taken over by a state enterprise, the Zimbabwe National Water Authority. A case of one such resettlement area is in the Wenimbi River basin, where 20 large scale commercial farms were sub-divided and allocated to over 600 plot holders. In 2004, a new and larger reservoir, located upstream, was commissioned, mainly to supply water to riparian farmers and a near by town. The changes have brought new water management and farming practices, hence new water supply and demand characteristics, more water users, more competition, shortages and some conflicts. This research sought to establish the amount of water available, productive water use levels, allocations and underlying causes of shortages and conflict in the Safari-Igava irrigated farms after a major policy changes. A spreadsheet-based simulation model was developed and used in analysis of reservoir operation to help in formulation of water management strategies. The research methods used included gathering of quantitative and qualitative data from the national water authority, government departments and farmers, conducting canals topographic surveys and unstructured questionnaire surveys. Data collected was on water available, water allocation, consumptive water use, crops, irrigation practices, reservoir operations and geographical data of the farms served by the river system. Results showed that annual water allocations quantities for productive use were not changed for the new setup. Farmers are expanding area under irrigation, on a gravity fed canal system, water measurement structures and reservoirs are not in use and generally in a disrepair condition. Upstream farmers are fully supplied with water while downstream farmers face shortages. Records on water inflow and release are kept at one major reservoir, while there are no records on abstractions from four reservoirs, the river and the canals. Records on current urban water supply, land use and crop production showed that demand is rising, but it is below the maximum allocations. The set up of water abstraction regulations by farmers for the shared water infrastructure was done without outside intervention, checks or controls. The supervision and regulations of water access and distribution by stakeholders like Zimbabwe National Water Authority, the Sub- Catchment Council and government departments of Irrigation and Agricultural Extension are weak. Analysis of different scenarios of water supply by the simulation model showed that there is enough water to meet all demands at current water use levels, but there may not be enough water to meet peak demand. In order to enhance equity and efficiency, new reservoir operation and reservoir management strategies are required as well as accurate recording of water abstractions for individual users. Also, strict enforcement of water abstraction rules, proper operation and maintenance of infrastructure, accurate billing and management of water supply and distribution are required in order to minimize shortages and occurrence of conflict.