Basic Hydrology Concept

Basic Hydrology Concept

Introduction

Water is vital for all living organisms on Earth.

For centuries, people have been investigating where water comes from and where it goes, why some of it is salty and some is fresh, why sometimes there is not enough and sometimes too much. All questions and answers related to water have been grouped together into a discipline.

The name of the discipline is hydrology and is formed by two Greek words: "hydro" and "logos" meaning "water" and "science".

What is Hydrology?

• It is a science of water.
• It is the science that deals with the occurrence, circulation and distribution of water of the earth     and earth’s atmosphere.
•  A good understanding of the hydrologic processes is important for the assessment of the water resources, their management and conservation on global and regional scales.
• In general sense engineering hydrology deals with
• Estimation of water resources
• The study of processes such as precipitation, evapotranspiration, runoff and their interaction
• The study of problems such as floods and droughts and strategies to combat them

Hydrologic Cycle

Water exists on the earth in all its three states, viz. liquid, solid, gaseous and in various degrees of motion.

Water, irrespective of different states, involves dynamic aspect in nature.

The dynamic nature of water, the existence of water in various state with different hydrological process result in a very important natural phenomenon called Hydrologic cycle.

Evaporation of water from water bodies, such as oceans and lakes, formation and movement of clouds, rain and snowfall, stream flow and ground water movement are some examples of the dynamic aspects of water. 

                       

• Evaporation from water bodies
• Water vapour moves upwards
• Cloud formation
• Condensation
• Precipitate
• Interception
• Transpiration
• Infiltration
• Runoff–streamflow
• Deep percolation
• Ground water flow

The hydrologic cycle has importance influence in a variety of fields agriculture, forestry, geography, economics, sociology, and political scene.

Engineering application of the knowledge are found in the design and operation of the projects dealing with water supply, hydropower, irrigation & drainage, flood control, navigation, coastal work, various hydraulic structure works, salinity control and recreational use of water.  

Water Budget Equation

Catchment area

• The area of land draining in to a stream or a water course at a given location is called catchment area / drainage area / drainage basin / watershed.
• A catchment area is separated from its neighbouring areas by a ridge called divide / watershed.  
• A watershed is a geographical unit in which the hydrological cycle and its components can be analysed. The equation is applied in the form of water-balance equation to a geographical region, in order to establish the basic hydrologic characteristics of the region. Usually a watershed is defined as the area that appears, on the basis of topography, to contribute all the water that passes through a given cross section of a stream.

If a permeable soil covers an impermeable substrate, the topographical division of watershed will not always correspond to the line that is effectively delimiting the groundwater.

Watershed characteristics

For a given catchment, in an interval of time ∆t, the continuity equation for water in its various phases can be given as:
                Mass inflow – Mass outflow = change in mass storage
If the density of the inflow, outflow and storage volumes are the same:

V_i - Inflow volume in to the catchment, V_o - Outflow volume from the catchment and ∆S - change in the water volume

Therefore, the water budget of a catchment for a time interval ∆t is written as:

                P – R – G – E – T = ∆S

P = Precipitation, R = Surface runoff, G = net ground water flow out of the catchment, E = Evaporation, T = Transpiration, and ∆S = change in storage 

The above equation is called the water budget equation for a catchment

               

NOTE: All the terms in the equation have the dimension of   volume and these terms can be expressed as depth   over the catchment area.




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