Modeling the impacts of volumetric water pricing in irrigation districts with conjunctive use of surface and groundwater resources

Water pricing has been identified as a generally valid water supply policy to help solve problems of water scarcity and competition. As for the non-agricultural sectors, in the last three decades water pricing has been widely discussed in and promoted with regard to the irrigation management, though in the actual practice its effectiveness is quite controversial. This is particularly true in semi-arid regions, where conjunctive use of collective facilities and on-farm groundwater pumps may cause conflicts and mismanagement of water resources. Under such circumstances, irrigation water pricing policies are not easy to deploy and implement effectively, due to potential occurrence of side and unintended effects. In this framework, the present work aims at investigating the impact at the district scale of water pricing policies, on both surface water (SW) and groundwater (GW) resources. In this regard, a model which deals with the analysis of farmers' decision concerning water source selection is proposed. The analysis is carried out keeping capital asset as given, also with the aim to elicit the relevance of on-farm irrigation water cost on resources use during the irrigation season. Reference is made to an intensive agricultural district in Southern Italy, conjunctively supplied by collective schemes managed by the local irrigation board and on-farm individual groundwater pumping systems. The proposed model was built along with local stakeholders, in order to (i) underline the relationship between the water tariff applied for collective supply service and the irrigation source selection during the irrigation season; and (ii) the relevance of the conjunctive use of GW based on pumping cost convenience and service standards needed to fulfill the irrigation requirements. The results have been then integrated into a quantitative water balance model, and a scenario analysis used to show the potential side impacts that a restrictive SW tariff policy applied during drought periods may have on the GW state, in different hydrological conditions.