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Introduction to Hydrology

Introduction

  • Definition and Uses of Hydrology
    ‘Hydrology’ is originated from two words ‘hydro’, which means water and ‘logos’, which means study. Hence in simple way hydrology is the study of water. Hydrology deals with occurrence, distribution and movement of water on earth, which includes water of atmosphere and subsurface of the earth.
    As per US research council hydrology is defined as: “Hydrology is the science, that treats waters of the earth, their occurrence, circulation and distribution, chemical and physical properties, their reaction to environment and relation to living beings.”
    Hydrology generally comes together with meteorology, which is the science of the study of climate in the atmosphere of earth. Hydrology is the interdisciplinary subject related to the meteorology, soil mechanics, and geology, geomorphology, hydraulics, physics, chemistry, biology, mathematics etc. hydrology can be classified as Scientific and Applied (or Engineering) hydrology. Here we are dealing with the later one.
  • Scope, Importance, and Application of Hydrology in the Field of Engineering
    Hydrology is the science that covers a wide range of study but the scopes of engineering hydrology are confined to:
    1. For the estimation of available water resources to use water from those resources to fulfill water supply demand, hydropower generation, irrigation etc.
    2. For the study of hydrological processes: evaporation, transpiration, precipitation, run off and infiltration which causes the direct effect to the quantity of available water (discharge) in the resources.
    3. For the study of the water-related calamities like flood and drought and to minimize their effects.
  • Applications of Hydrology in the Field of Civil Engineering are
    1. For the correct assessment of the flow in hydropower, irrigation, water supply, and drainage projects.
    2. To estimate the maximum expected discharge for the design of Dam, Reservoir, Barrage, Spillway, Culverts, Drainage system etc.
    3. To determine the minimum storage capacity of the reservoir so that sufficient water is always available for Hydropower, Irrigation and Water supply Projects.
    4. To analyze the effects that will cause by the construction of a hydraulic structure in the river.
    5. To compute the total volume of water (Discharge) that is available in the water body from its catchment area.
    6. For the dimensioning of the navigation channel.
    7. For the feasibility study of Run of River (ROR) hydropower stations and their operation with poundage.
    8. To control erosion and minimize the sedimentation in river channel which will cause erosion of hydraulic structures.
    9. For the prevention of stream pollution.

Hydrological Cycle and Water Balance Equations

  • Hydrological Cycle
    Hydrological Cycle
    Hydrological Cycle
    Hydrological cycle is a continuous endless cycle comprising of the processes of evaporation, transpiration, precipitation, infiltration and runoff. This cycle is the earths water circulatory system, which transfers moisture from water bodies to atmosphere by solar radiation and atmosphere to earth in the form of precipitation. Finally, the precipitated water is conveyed to ocean and lake by river and stream and again back to atmosphere by evaporation. Hydrological processes are described below in brief:
    1. Evaporation
      Evaporation is the process by which water in liquid state is changes to the vapour state by the absorption of solar radiation. The process of conversion of solid water to vapour is considered as sublimation.
    2. Transpiration
      The water contained in plant tissues also vaporized and ejected in the atmosphere, the process is known as transpiration. Infiltrated water is available for plants roots which get vaporizes and returns to the atmosphere by transpiration whereas the intercepted and runoff water returns to the atmosphere by evaporation. So evaporation and transpiration come together and it is referred as evapotranspiration, which is the hydrological loss.
    3. Precipitation
      Evaporated water is lifted up in the atmosphere until it condenses and finally falls in the earth surface in the form (solid or liquid) of rain, snow, hail, drizzle, sleet etc. This process of fall of moisture to earth’s surface from the atmosphere is known as precipitation.
    4. Infiltration
      The rain water is first absorbed by the soil in earth’s surface and gets into the ground. This process of movement of water into the ground from the surface is called infiltration. Infiltrated water is stored in soil as soil moisture.
    5. Run off
      Runoff is that part of the precipitation which gets into stream channel following the path above or below the ground surface is called runoff. When runoff reaches stream channel, it is called stream flow. The flow of infiltrated water below ground surface to get into the stream channel is called subsurface runoff. The flow of water in the stream channel before the precipitation occurred is called as base flow.
    6. Interception
      The part of the precipitation that returns to the atmosphere without contributing to runoff is called interception. It is considered as the hydrological loss. The water is intercepted by vegetation, build up areas and other objects. Interception and infiltration are commonly called as initial losses.
  • Water Balance Equation
    Water balance equation is the law of conservation of mass. This equation is also known as continuity equation or conservation equation. Water balance equation states that the water input in a basin is equal to the total outflow and change in storage in the basin.
    Mathematically,
    Inflow – outflow = change in storage------------(1)
    I – O = DS
    i.e. = Inflow – outflow
    Sources of inflow are precipitation and sources of outflow are evaporation, transpiration, runoff interception. But these parameters involved in the above equation are not accurately computable.
    For time interval of Dt, water budget equation can be given as,
    P – ( R + ET ) = DS--------------(2)
    Where, P = precipitation
    R = Surface runoff + subsurface Runoff
    ET = Evapotranspiration
    DS = Change in storage
    Unit of the parameters in the equation is units of volume which can be changed into the unit of depth by dividing the volume by area of the basin.
    In the case of storage basin or reservoir, water balance equation can be written as,
    (P + I) – (R + ET) = DS ----------------(3)
    Where, I = Inflow
    Considering a long time period, i.e. for a year, change in storage of the basin is equal to zero. Then,
    P – R = ET
  • World Water Balance
    96.5% water in the earth is in the form of ocean water, 1.7% in the form of polar ice and 1.7% in the form of groundwater. Only 0.1% of water is on the surface and in the atmosphere. Ocean water is completely saline and 1% of the water in the ground is also saline. Only 2.5% of ground water is fresh water. About two third of the fresh water is in the polar ice and ground water. Only 0.006% of fresh water is carried by rivers.
  • Development of Hydrological Study in Nepal
    • Systematic recording of hydrological data in Nepal began in 1962 as a program of USAID and Nepal government. The organization was a subsection under Department of Electricity which evolved and took the present shape of Department of Hydrology and Meteorology (DHM) in 1988.
    • The Scientific hydrological study was started in Nepal when Government of India initiated Koshi project in the 1940s.
    • In 1947 hydrological stations at Koshi at Barahachhetra, Sunkoshi at Kampughat, and Tamor at Mulghat were established.
    • In 1956, meteorological observation stations were established with the support of Government of India.
    • As a section under Department of electricity, Nepal started hydrological activities in an organized way from Karnali basin in 1962.
    • In 1966 publication of hydro-meteorological data was started.
    • DHM was merged with the Department of Irrigation in 1972.
    • DHM was again separated from Department of Irrigation in 1988.
    • In 1993, nationwide hydro-meteorological data management project was started.
    • Water resource strategy was prepared in 2002 and National Water Plan was published in 2005.
    • Currently, in the major basins of Nepal (Koshi, Narayani, Karnali, Mahakali, Bagmati, West Rapti, Kankai, and Kamala) there are 120 hydrological stations and 282 meteorological stations.

 

 

 

 


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