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Hydrology & Runoff

What is Hydrology??

  • A science dealing with the properties, distribution and circulation of water.
  • Engineering hydrology involves the application of  the knowledge of hydrologic science for practical uses, the solution of engineering problems and the betterment of mankind.
  • The availability of water to sustain life is perhaps the most recurrent constraint in human history and will remain so in the foreseeable future.
  • The engineering hydrologist, or water resources engineer, is involved in the planning, analysis, design, construction and operation of projects for the control, utilization and management of water resources.
  • Hydrologic calculations are estimates because mostly the empirical and approximate nature of  methods are used to describe various hydrological processes.
  • It uses hydrologic principles in the solution of engineering problems arising from human exploitation of water resources of the earth.

Hydrology Cycle

  • Journey of water from the ocean to atmosphere and back to the earth and ultimately to the ocean through the processes of evaporation, precipitation, percolation, runoff and return to the ocean is called hydrologic cycle.
  • Hydrological cycle describes the continuous movement of water on, above and below the surface of the earth.
  • It forms the basis for the study of hydrology.
  • The cycle has no exact beginning or ending point but  may be, conveniently, assumed to start with evaporation from oceans.
  • Sun is the main source of energy for hydrologic cycle.
  • Let us consider the cycle begins with oceans.

Scope of Hydrology

  • The maximum probable flood
  • The water yield from a basin
  • The ground water development
  • The maximum intensity of storm and its frequency

Application of Hydrology

  • Irrigation schemes
  • Dams and hydroelectric power projects
  • Water supply projects
  • Designing dams for water supply or hydroelectric power generation.
  • Designing bridges.
  • Designing sewers and urban drainage system.
  • Predicting geomorphologic changes, such as, erosion or sedimentation.
  • Assessing the impact of natural and anthropogenic environmental change.
  • It tells us what hydrologic hardware and software are needed for real-time flood forecasting.

Precipitation / Rainfall

  • Precipitation is defined as liquid or solid condensation of water vapor falling from clouds or deposited from air onto the ground.
  • Precipitation is measured as the amount of water that reaches horizontal ground or the horizontal plane of the earth’s surface, and is expressed as a vertical depth of water or the water equivalent of solid precipitation (snow, ice).
  • The unit of precipitation: millimetre, inch.

Why do we need to measure rainfall??

  • Agriculture- What to plant in certain areas, where and when to plant, when to harvest.
  • Hotriculture / Viticulture- How and when to irrigate.
  • Engineers- To design structures for runoff control i.e. storm-water drains, bridges, etc.

Forms of Precipitation

  • Dew
  • Mist
  • Fog
  • Rain
  • Hail
  • Snow

What is Run-Off??

  • Runoff can be defined as the portion of the precipitation that makes it’s way towards rivers or  oceans, etc as surface or subsurface flow.
  • Portion which is not absorbed by the deep strata.
  • A run off is when water flows over land areas and mixes with soil, minerals, and other contents.
  • This is a major part of the water cycle. When this runoff across certain surfaces and reaches a channel, it is known as non point source.
  • Water that drains from the land.
  • This water can come from rain or from snowmelt.
  • Every time it rains the water that falls to the ground and makes its way to reeks, streams, rivers, lakes and oceans.

Classification of Runoff

  • Surface Runoff- Water flows over the land and is first to reach the streams and rivers which ultimately discharge the water to the sea.
  • Inter flow or subsurface slow- A portion of rainfall infiltrates into surface soil and depending upon the geology of basins, runs as subsurface runoff and reaches the streams and rivers.
  • Groundwater flow or base flow- It is that portion of rainfall which after infiltration, percolates down and joins the ground water reservoir which is ultimately connected to the ocean.

Factors affecting Runoff

  • Precipitation characteristics.
  • Shape and size of the catchment.
  • Topography
  • Geological characteristics
  • Character of the catchment surface.
  • Storage characteristics of the catchment.

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