Hydrology is the science of water on Earth. Water is studied in its gas state (e.g. atmospheric water vapor), liquid state (e.g. water in lakes, rivers, and oceans) or solid state (e.g. land and sea ice). Hydrology has evolved over time in response to the need to understand both the complex water systems of the Earth and how these systems are changing over time. Recently, hydrological sciences have evolved to include simulations and future projections of complex water systems.