First, concerns about the physical decline of aquifer systems include changes in both groundwater quantity and groundwater quality available (including salinity), as well as the potential for irreversible land subsidence (e.g. Konikow, 2013). Second, interactions between groundwater and surface water systems have also been a major driver for changes in groundwater policy. For example, concerns over stream depletion have led to the introduction of regulations on groundwater use in a number of transboundary river basins in the United States, including the Pecos River (between Texas and New Mexico), Arkansas River (Kansas and Colorado), and the Republican River (Kansas, Nebraska, and Colorado), as well as in other countries such as the Guadalquivir Basin in Spain. The adverse effects of stream depletion on instream habitat and endangered species have also led to regulatory action, for instance in multiple US states.