Surface Water Hydrology

Introduction Hydrology is the science dealing with the properties, occurrence, distribution, and circulation of water on and below the earth surface and in the atmosphere. Although water covers about 75% of the earth surface, a small percentage is used directly for water supply. This is because about 96.5% of the earth water is found in the oceans as saltwater. If the earth were a uniform sphere, the oceans would be sufficient to cover it totally to a depth of about 2.6 km. Of the earth freshwater, about two thirds is polar

common practice is that hydrologists assess these processes quantitatively by empirical or rational equations, because the path that a drop of water will follow is apparently difficult to describe analytically.

Water surface runoff is typically discharged through the lowest point at the outlet of a specific area called drainage basin, watershed, or catchment (Figure 2.2). The basin constitutes a unit of hydrologic system where all the input precipitation is discharged through the outlet as streamflow unless it is lost by abstraction processes such as infiltration. Thus the basin may be used as a reference area for calculating the availability of water for a specific purpose. The boundary of a basin is separated topographically by local geographical divides such as a ridge, hill, or mountain that can be traced on a map. No specific limits have been established for the size of a drainage basin as the area can be very small or