ABSTRACTInput‐output models are frequently used to estimate impacts, benefits or damages from some event. These analytic models and the questions they are designed to answer are usually based on political definitions of regions. However the true impacts propagate according to the actual spatial pattern of the regional economy. Because of the divergence between the political regions used for analysis and the economic regions on the ground, the economic impacts which spill over political boundaries can sometimes become analytically important. This paper applies these concepts to a case study of allocating irrigation water from the Pecos River in Texas and New Mexico. The U.S. Supreme Court has ruled that New Mexico used water belonging to Texas. Our analysis suggests that the spillover benefits to Texas from New Mexico's use of the water might equal or exceed the benefits which Texas would have gotten from using the water itself. Texas might be better off because New Mexico took its wate