Hydrology emerged as a science in response to the need to understand the complex water systems of the Earth and assist to resolve water problems.
It is the science that comprises of the occurrence, circulation, movement, and characteristics of the oceans and rivers of the globe and their association with the environment within each stage of the hydrologic cycle.
Surface-water hydrology: is an area of specialization that encompasses the surface waters of the entire world (overland flows, rivers, lakes, wetlands, estuaries, oceans, seas, etc.) It correlates the dynamics of surface water flow. This includes discharge measurements; the statistical variability of flow at a determined location, floods; drought susceptibility, and the advancement of the levels of inundation danger and the dynamics of surface waters.
Engineering Hydrology: is concerned mainly with engineering applications. It is concerned with, amongst others, to figure out the maximum likely flood occurrence at particular dam sites and the maximum withdraw over a spillway, flow in a Culvert, waterways or urban stormwater drainage system. It contributes greatly to assess the necessary water reservoir capacity/volume to guarantee sufficient water for irrigation or municipal water supply in drought condition.
Moreover, engineering hydrology studies the catchment water yield and the relationship between surface water and groundwater resources. The analysis of processes such as runoff, precipitation, challenges such as inundations, droughts and the approach to address them.