What is the purpose of this excerpt from the article "What Is GIS?"? GIS also can be used for complex modeling to answer a wide range of "what if" and ecosystem simulation questions. These may be cartographic models designed to document the co-occurrence or interrelationship of multiple data layers or they may be hypothetical research models designed to mimic natural ecological systems. Similarly, modeling with GIS can be used to predict the impacts that one set of parameters will have on another. For example, wetlands, soils, hydrography, climatology and elevation data can be combined to model flooding within a river system. Upstream changes in land use within the same system can be modeled to determine the potential impact of conversion of a forested floodplain to residential development or to agriculture. As a result, both natural system responses to storm events and the impact of human land use decisions can be assessed prior to the proposed action.