Hydrology is often defined science which deals with the physical properties, occurrence and movement of water in the atmosphere, on the surface of and in the outer crust of the earth. This is an all inclusive and somewhat controversial definition for there are individual bodies of science dedicated to study of various element contained within this definition. Meteorology, oceanography, geohyrolog, among others are typical.

Hydrologist have been studying the flow or runoff of water over land for many decades and some rather sophisticated theories have been proposed to describe the process. Unfortunately most of these attempts have been only partially successful not only because of the complexity of the process and the many interactive factors involved, but also because of the stochastic nature of rainfall, snoemelt and other sources of water.

Most of the factors and parameters that influence surface runoff have been defined, but for many complete functional descriptions of their individual effect exist only in empirical formula. Extensive field data, empirically determined coefficient and sound judgement and experience are required for their quantitative analysis.

By application of the principles and methods of modern hydrology, it is possible to obtain solutions which are functionally acceptable and form the basis for the designed of highway bridges and drainage structures . It is the purpose of this manual to present some of these principles and techniques and to explain their uses by illustration.

Hydrologist cycle

Hydrologist cycle is a description of the transformation of water from one phase to another and it's motion from one location to some other. Hence it represents the complete life cycle of water on and near the surface of the earth.

Hydrologic cycle take place through different pathways and at different rates but the concepts remain the same. Water evaporates from the ocean, and condensation takes place, some of the water evaporate as it is falling and the rest moves over ground or is intercepted by building, trees and other vegetation. The intercepted water evaporate directly back to the atmosphere thus completing a part of the cycle. The remaining precipitation falls to the ground surfaces or onto the water surface of rivers lakes, ponds and oceans.

It is with the study of these different aspects that allow hydrologists to carryout a lot of things such as calculate water budgets. Hydrologists also study pollution by looking at the sources, transportation mechanisms, and the ultimate fate of the pollutant.