Positive externalities and water service management in rice-based irrigatio- n systems of the humid tropics

The principles and practices of service oriented management are considered for rice-based systems of the humid tropics. It is shown that the classical service oriented approach, aimed at improving irrigation management performance, faces many obstacles in this environment. These are linked to important externalities generated by irrigation due to the permanent presence of water throughout the gross command area. The isolation of the service to specific users (plots and/or farmers) is prevented by surface and subsurface lateral transfers of water. Accordingly, this makes it difficult to apply the fundamental "user-payer" principle. In a case study from Sri Lanka, crop water consumption amounted to only 23% of the net water available within the scheme (irrigation plus rainfall); while other uses of water (externalities) are estimated at 61% of the same value. The remaining fraction of 16% is accounted for by real losses. The externalities are mostly of positive benefit and are mainly generated by evapotranspiration of perennial non-crop vegetation (trees or grass) of high value to the local community. Water bodies within the scheme (tanks and canals) also account for a significant share of water consumption through direct evaporation; this use can be considered as a positive externality (fisheries, domestic supply). With diverse and important externalities it is difficult to apply a service oriented management strategy, linking specific services with charging procedures for each user within the gross command area. This study considers, as an alternative, the idea of the entire community living within the gross command area as the user or beneficiary of water management practices. The principle of charging the population as a whole is therefore examined as a possible option.