Precipitation and General Aspects of Hydrology

• % 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, 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

where, N₁,N₂,…N_x..N_n are normal annual percipitation 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’

If any of N₁,N₂,N₃…

N_n > 10% of N­_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…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 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.