<italic>Ad valorem</italic> tariff and spatial equilibrium models

The objectives of this article are to illustrate theoretically and empirically how to incorporate an <italic>ad valorem</italic> tariff in spatial equilibrium models and show the equivalence of three approaches (primal, dual and MCP) of spatial equilibrium trade modelling. The article lays out in detail the theory, empirical applications and the results for all three approaches so that trade modellers can easily follow and apply them in their work. The primal approach is commonly used by spatial equilibrium modellers, though the dual approach corresponds to the textbook depiction of consumer surplus, producer surplus and gains from trade. The Mixed Complementarity Problem (MCP) is not commonly used by spatial equilibrium modellers, though it is relatively simpler if the modeller knows the equilibrium conditions. This article also presents advantages and disadvantages of each approach.