Because the complex interactions of human activity in concentrated settlements with air, water and land must be taken into account, urban hydrology is a distinctive branch of the broad field of hydrology. As opposed to conventional hydrology, because urban development everywhere has been in continuous states of expansion and flux, urban hydrology contends with the dimension of dynamic change. Also, urban water management utilizes the social and biological sciences as well as the physical sciences. The fact that the term urban hydrology gained currency less than two decades ago reflects the lag in the recognition that America became an urban nation over half a century ago.