Irrigation externalities and agricultural sustainability in South-eastern Nigeria

Agricultural intensification by irrigation is increasingly regarded as the key to solving food supply problems in Sub-Saharan Africa. Conversely, mounting empirical evidence suggests that irrigation externalities might preclude long-term sustainability of arable agriculture. Choosing between intensive irrigation schemes and less intensive farming systems is therefore, problematic. The paper examines the implications of irrigation systems intensification in south-eastern Nigeria using adjacent rain-fed farms as the counterfactual. The analyses found mixed results. When first introduced, the irrigation scheme increased marginal factor productivity and gross margins but this has subsequently declined to the extent that the marginal factor product of land has become negative. The annual yields of the irrigated farms were also less stable than those of the less intensive rain-fed farms. These results indicate the dilemma that irrigation externalities present to sustainable agricultural policy and suggest a need to look again at the potential for developments in rain-fed systems.