21. Basic Hydrology Concept 1.1. IntroductionWater 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".
3What 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.
4In general sense engineering hydrology deals with Estimation of water resourcesThe study of processes such as precipitation, evapotranspiration, runoff and their interactionThe study of problems such as floods and droughts and strategies to combat them
51.2 Hydrologic CycleWater exists on the earth in all its three states, viz. liquid, solid, gaseous and in various degrees of motion.
6Hydrologic cycle….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.
7Hydrologic Cycle 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.HydrologicCycle
8Hydrologic cycle….Evaporation from water bodiesWater vapour moves upwardsCloud formationCondensationPrecipitateInterceptionTranspirationInfiltrationRunoff–streamflowDeep percolationGround water flow
9Hydrologic cycle….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.
101.3 Water Budget EquationCatchment areaThe 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.
111.3 Water Budget Equation Catchment area…. 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.
12Watershed and watershed divide Watershed/ catchmentWatershed/ catchmentWatershed divide
13Catchment area….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.
15Mass inflow – Mass outflow = change in mass storage Water Budget EquationFor 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 storageIf the density of the inflow, outflow and storage volumes are the same:Vi - Inflow volume in to the catchment, Vo - Outflow volume from the catchment and ∆S - change in the water volume
16Water Budget Equation… Therefore, the water budget of a catchment for a time interval ∆t is written as:P – R – G – E – T = ∆SP = Precipitation, R = Surface runoff, G = net ground water flow out of the catchment, E = Evaporation, T = Transpiration, and ∆S = change in storageThe above equation is called the water budget equation for a catchmentNOTE: All the terms in the equation have the dimension of volume and these terms can be expressed as depth over the catchment area.
17Components of hydrologic cycle Evapo transpirationPrecipitationStream flow(Runoff)Inter flowInfiltrationBase flowGroundwater flow
181.3 World Water BudgetTotal quantity of water in the world is estimated as 1386 M km3M km3 of water is contained in oceans as saline waterThe rest 48.5 M km3 is land water13.8 M km3 is again saline34.7 M km3 is fresh water10.6 M km3 is both liquid and fresh24.1 M km3 is a frozen ice and glaciers in the polar regions and mountain tops
241.4 Application in Engineering Hydrology finds its greatest application in the design and operation of water resources engineering projectsThe capacity of storage structures such as reservoirThe magnitude of flood flows to enable safe disposal of the excess flowThe minimum flow and quantity of flow available at various seasonsThe interaction of the flood wave and hydraulic structures, such as levees, reservoirs, barrages and bridges