• As a branch of earth science, it is concerned with the water in streams and lakes, rainfall and snow fall, snow and ice on the land and water occurring below the earth’s surface in the pores of the soil and rocks.
• In general, hydrology is a very broad subject of an inter-disciplinary nature drawing support from allied sciences, such as meteorology, geology, statistics, chemistry, physics and fluid mechanics.
• Hydrology is basically an applied science. To further emphasise the degree of applicability, the subject is sometimes classified as
  1. Scientific hydrology—the study which is concerned chiefly with academic aspects.
  2. Engineeringor applied hydrology—a study concerned with engineering applications
• In a general sense engineering hydrology deals with
  1. estimation of water resources,
  2. the study of processes such as precipitation, runoff, evapotranspiration and their interaction
  3. the study of problems such as floods and droughts, and strategies to combat them.

HYDROLOGY: The term hydrology is from Greek: hydōr, "water"; and logos, "study" • Hydrology means the science of water. It deals with the occurrence,(circulation)movement, distribution, and quality of water on the Earth, including the hydrologic cycle, water resources and environmental watershed sustainability