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**Precipitation & General aspects of Hydrology**

**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**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**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**Estimation of missing rainfall data****(i)**

If N_{1}, N_{2}…N_{n}< 10% of N_{x}

where, N_{1}, N_{2},…N_{x}..N_{n}are normal annual precipitation of 1, 2,…x…n respectively.

P_{1}, P_{2}…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 N_{1}, N_{2}, N_{3}…

N_{n}> 10% of N_{x}**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, P_{1}, P_{2}…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, P_{1}, P_{2}…P_{n}are the rainfall data of areas A_{1}, A_{2}…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.

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