Page 1
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,
Page 2
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.
Page 4
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.
Infiltration, Run off and Hydrographs
Infiltration
Infiltration is the flow of water into the ground through the soil surface.
• Horton’s Equation: Horton expressed the decay of infiltration capacity with
time as an exponential decay given by
Where,
fct = infiltration capacity at any time t from start of the rainfall
fco = initial infiltration capacity at t = 0
fct = final steady state value
td = Duration of rainfall
kh = constant depending on soil.
Infiltration indices
In hydrological calculations involving floods it is found convenient to use a constant
value of filtration rate for the duration of the storm. The defined average infiltration
rate is called infiltration index and two types of indices are in common use
Page 5
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.
Infiltration, Run off and Hydrographs
Infiltration
Infiltration is the flow of water into the ground through the soil surface.
• Horton’s Equation: Horton expressed the decay of infiltration capacity with
time as an exponential decay given by
Where,
fct = infiltration capacity at any time t from start of the rainfall
fco = initial infiltration capacity at t = 0
fct = final steady state value
td = Duration of rainfall
kh = constant depending on soil.
Infiltration indices
In hydrological calculations involving floods it is found convenient to use a constant
value of filtration rate for the duration of the storm. The defined average infiltration
rate is called infiltration index and two types of indices are in common use
(i) W-index: In an attempt to refine the f-index the initial losses are separated
from the total abstractions and an average value of infiltration rate, called W-index,
is defined as
Where, P = Total storm precipitation (cm)
R = Total storm runoff (cm)
Ia = initial losses (cm)
te = Duration of rainfall excess
W-index = Avg. rate of infiltration (cm/hr)
(ii) f-index: The f index is the average rainfall above which the rainfall volumes is
equal to the runoff volume. The f index is derived from the rainfall hyetograph with
the edge of the resulting run- off volume.
? - ?nd?x =
I - R
24
Where, R = Runoff in cm from a 24- h rainfall of intensity I cm/day
Runoff
Runoff means the draining or flowing off of precipitation from a catchment area
through a surface channel. It thus represents the output from the catchment in a
given unit of time.
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