Precipitation & Its Measurement | Engineering Hydrology - Civil Engineering (CE) PDF Download

Precipitation & General aspects of Hydrology

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 = (C_V / ∈)²
   
   where, C_v = Coefficient of variation,
   ∈ = Allowable % Error,
   σ Standard deviation of the data, n = Number of stations,
    mean of rainfall value
4. Estimation of missing rainfall data
   (i)
   
   If N₁, N₂…N_n < 10% of N_x
   where, N₁, N₂,…N_x..N_n are normal annual precipitation of 1, 2,…x…n respectively.
   P₁, P₂…P_n are rainfall at station 1, 2,…. N respectively.
   And P_x is the rainfall of station x.
   Case: A minimum number of three stations closed to station ‘x’
   
   (ii)
   
   If any of N₁, N₂, N₃…
   N_n > 10% of N­_x
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.
   
   Where, P₁, P₂…P_n 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.
   
   Where, P₁, P₂…P_n are the rainfall data of areas A₁, A₂…A_n. 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.