Precipitation & Its Measurement Civil Engineering (CE) Notes | EduRev

Engineering Hydrology

Civil Engineering (CE) : Precipitation & Its Measurement Civil Engineering (CE) Notes | EduRev

The document Precipitation & Its Measurement Civil Engineering (CE) Notes | EduRev is a part of the Civil Engineering (CE) Course Engineering Hydrology.
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Precipitation & General aspects of Hydrology

Precipitation & Its Measurement Civil Engineering (CE) Notes | EduRev

  1. Index of Wetness
    (i) Index of wetness = (rainfall in a given year at a given place /average annual rainfall of that place) x 100
    (ii) % Rain deficiency = 100 - % index of wetness
  2. Aridity index
    A.I = (PET - AET / PET) x 100
    Where, A.I = Aridity index
    PET = Potential Evapo- transpiration
    AET = Actual Evapotranspiration
    (i) AI ≤ 0 → Nonarid
    (ii) 1 ≤ A.I ≤ 25 → Mild Arid
    (iii) 26 ≤ A.I ≤ 50 → Moderate arid
    (iv) A.I > 50 → Severe Arid
  3. Optimum Number of rain Gauge: (N)
    N = (CV / ∈)2
    Precipitation & Its Measurement Civil Engineering (CE) Notes | EduRev
    where, Cv = Coefficient of variation,
    ∈ = Allowable % Error,
    σ Standard deviation of the data, n = Number of stations,
    Precipitation & Its Measurement Civil Engineering (CE) Notes | EduRev mean of rainfall value
  4. Estimation of missing rainfall data
    (i)
    Precipitation & Its Measurement Civil Engineering (CE) Notes | EduRev
    If N1, N2…Nn < 10% of Nx
    where, N1, N2,…Nx..Nn are normal annual precipitation of 1, 2,…x…n respectively.
    P1, P2…Pn are rainfall at station 1, 2,…. N respectively.
    And Px is the rainfall of station x.
    Case: A minimum number of three stations closed to station ‘x’
    Precipitation & Its Measurement Civil Engineering (CE) Notes | EduRev
    (ii)
    Precipitation & Its Measurement Civil Engineering (CE) Notes | EduRev
    If any of N1, N2, N3
    Nn > 10% of N­x
  5. Mean rainfall Data
    To convert the point rainfall values at various into an average value over a catchment the following three methods are in use:
    (i) Arithmetic Avg Method: when the rainfall measured at various stations in a catchment area is taken as the arithmetic mean of the station values.
    Precipitation & Its Measurement Civil Engineering (CE) Notes | EduRev
    Where, P1, P2…Pn are rainfall values
    Of stations 1, 2…n respectively.
    In practice this method is used very rerely.
    (ii) Thiessen Polygon Method: in this method the rainfall recorded at each station is given a weightage on the basis of an area closest to the station.
    Precipitation & Its Measurement Civil Engineering (CE) Notes | EduRev
    Where, P1, P2…Pn are the rainfall data of areas A1, A2…An. The thiessen-polygon method of calculating the average precipitation over an area is superior to the arithmetic average method.
    (iii) Isohyetal Method: An isohyet is a line joining points of equal rainfall magnitude. The recorded values for which areal average P is to be determined are then maked on the plot at appropirate stations. Neighbouring stations outside the catchment are also considered.
    Precipitation & Its Measurement Civil Engineering (CE) Notes | EduRevPrecipitation & Its Measurement Civil Engineering (CE) Notes | EduRev 
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