Short Notes: Precipitation & General aspects of Hydrology | Short Notes for Civil Engineering - Civil Engineering (CE) PDF Download

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Short Notes on Engineering Hydrology 
 
Precipitation & General aspects of Hydrology 
Index of Wetness 
• index of wetness = 
 
• % Rain deficiency = 100 - % index of wetness 
Aridity index 
 
Where, A.I = Aridity index 
PET = Potential Evapo- transpiration 
AET = Actual Evapotranspiration 
a. AI = 0 ? Nonarid 
b. 1 = A.I = 25 ? Mild Arid 
c. 26 = A.I = 50 ? Moderate arid 
d. A.I > 50 ? Severe Arid 
Optimum Number of rain Gauge: (N) 
 
where, Cv = Coefficient of variation, 
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Short Notes on Engineering Hydrology 
 
Precipitation & General aspects of Hydrology 
Index of Wetness 
• index of wetness = 
 
• % Rain deficiency = 100 - % index of wetness 
Aridity index 
 
Where, A.I = Aridity index 
PET = Potential Evapo- transpiration 
AET = Actual Evapotranspiration 
a. AI = 0 ? Nonarid 
b. 1 = A.I = 25 ? Mild Arid 
c. 26 = A.I = 50 ? Moderate arid 
d. A.I > 50 ? Severe Arid 
Optimum Number of rain Gauge: (N) 
 
where, Cv = Coefficient of variation, 
? = Allowable % Error, 
s Standard deviation of the data, n = Number of stations, 
 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 Nx 
Mean rainfall Data 
To convert the point rainfall values at various into an average value over a 
catchment the following three methods ar 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 …Pn are rainfall values 
Of stations 1,2…n respectively. 
In practice this method is used very rerely. 
Page 3


Short Notes on Engineering Hydrology 
 
Precipitation & General aspects of Hydrology 
Index of Wetness 
• index of wetness = 
 
• % Rain deficiency = 100 - % index of wetness 
Aridity index 
 
Where, A.I = Aridity index 
PET = Potential Evapo- transpiration 
AET = Actual Evapotranspiration 
a. AI = 0 ? Nonarid 
b. 1 = A.I = 25 ? Mild Arid 
c. 26 = A.I = 50 ? Moderate arid 
d. A.I > 50 ? Severe Arid 
Optimum Number of rain Gauge: (N) 
 
where, Cv = Coefficient of variation, 
? = Allowable % Error, 
s Standard deviation of the data, n = Number of stations, 
 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 Nx 
Mean rainfall Data 
To convert the point rainfall values at various into an average value over a 
catchment the following three methods ar 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 …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,A2…An. 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.