Water productivity in medium and small reservoirs in the Upper East Region (UER) of Ghana

Abstract

Improving water productivity (WP) is one important strategy for addressing future water scarcity, which is driven particularly by population growth and potential climate changes. Although an understanding of WP is required to develop improved water management strategies, little is known about WP in irrigated systems in sub-Saharan Africa (SSA) and the Upper East Region (UER), Ghana, in particular. To address this problem, this study was conducted in the UER during the dry season to describe the multiple uses of reservoir water, estimate WP for crop, livestock and fishery water uses, and to assess the factors contributing to the value of irrigation water for tomato farming. Two sites were selected: a medium reservoir at Tono and a small reservoir at Dorongo.<br /> The multiple uses of reservoir water were described using onsite field observations and secondary data. Climatic, soil, irrigation supply, and drainage data were collected. Reference crop evapotranspiration and crop water use were estimated from the climatic data using the FAO-Penman Monteith approach. Soil data were analysed for physical and hydraulic properties. The SWAP model was used to estimate soil water balances at field scale using farmer-managed sample plots. Soil hydraulic properties and parameters of the pedotransfer functions in SWAP were estimated from physical soil properties using the Rosetta model. The parameters were optimized with PEST. At farm and scheme scales, the soil water balance analysis was determined conventionally based on the measurements of irrigation water inflows and outflows. The soil water balance analysis was used to assess physical WP of irrigated crops at field (plot), farm and scheme scales. Crop yield data were collected and the WP was estimated as the ratio of crop yield to the water balance components: transpiration, evapotranspiration, and irrigation water. A residual imputation method was used to determine the value of water (economic WP) for tomato irrigation using data collected from a questionnaire survey adminstered to a sample of farmers. The gross margin approach was used to estimate the value of water for livestock and fishery based on production inputs and the volume of reservoir water depleted by the livestock or required for fisheries maintainance. The multiple linear regression analysis was used to assess the contribution of the production factors to the value of irrigation water for tomato farming.<br /> The soil water balance analysis shows that plots were over-irrigated by 11 to 70%. Over-irrigation contributed to a significant water loss and very low water use efficiency at Tono. Physical WP was higher at Dorongo than at Tono. The WP values at both sites were not higher than the few WP estimates available for SSA. However, WP values were below those reported for outside SSA. The contribution of irrigation water to the total value of tomato production was high, suggesting that water plays a key role in dry season farming. The imputed value of water differs significantly among the indicators used within sites, but was not significantly different between sites. This value was found to be significantly influenced by location, crop yield and the price of the commodity. Although the value of water for livestock and fishery uses were lower than that for irrigated tomato farming, it provide important information for seasonal planning of reservoir water resources management under multiple water uses and demonstrate the importance of water allocation for the sectors given competing uses.<br /> The study concludes that WP in the study area is low, and that a potential for improvement exists within the reach of the management agencies of reservoir water resources and the farmers. The study also underscores the role of government support in ensuring that a secure market exists for perishables as an essential incentive for the farmers to invest in strategies to improve WP.