Precipitation and General Aspects of Hydrology

By Vishwajeet Sinha|Updated : January 7th, 2017

Precipitation & General aspects of Hydrology

Index of Wetnesso

  • index of wetness =


  • % Rain deficiency = 100 - % index of wetness

Aridity index


Where, A.I = Aridity index

PET = Potential Evapo- transpiration

AET = Actual Evapotranspiration

  1. AI ≤ 0 → Nonarid
  2. 1 ≤ A.I ≤ 25 → Mild Arid
  3. 26 ≤ A.I ≤ 50 → Moderate arid
  4. A.I > 50 → Severe Arid

Optimum Number of rain Gauge: (N)


where, Cv = Coefficient of variation,

∈ = Allowable % Error,

σ Standard deviation of the data, n = Number of stations,

image004 mean of rainfall value

Estimation of missing rainfall data


where, N1,N2,…Nx..Nn are normal annual percipitation 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’



If any of N1,N2,N3

Nn > 10% of x

Mean rainfall Data

To convert the point rainfall values at various into an averaga value over a catchment the followoing three methods ar in use

(i) Arthmetic 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…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.


Where, P1,P2…Pn are the rainfall data of areas A1,A2An. The thiessen-polygon method of calculating the average percipitation 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 detarmined are then maked on the plot at appropirate stations.Neighbouring stations outside the catchment are also considered.





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