Hydrological models are simplified, conceptual representations of a part of the hydrologic cycle. Drainage basins are the principal hydrologic unit considered in fluvial geomorphology. Rangeland health classification reflects soil and site stability, hydrologic function, and biotic integrity. Conceptual models represent hydrologic systems using physical concepts. Conservative solutes are often used as hydrologic tracers for water movement and transport. Infiltration is a component of the general mass balance hydrologic budget. This entails selecting the best levee designs for the hydrologic conditions. These chemicals eventually entered to natural hydrologic systems. Caesium-137 has been used in hydrologic studies analogous to those with tritium. It was not until the 17th century that hydrologic variables began to be quantified. Global warming has a direct connection with the hydrologic cycle. These were considered part of the encompassing hydrologic unit. Its maximum range in ideal hydrologic conditions is about . The HIF also tests, evaluates, repairs, calibrates, and develops hydrologic equipment and instruments. The office also has a fire weather and hydrologic program.