Engineering Hydrology
Engineering Hydrology
Engineering Hydrology
Forms of Precipitation:
* Saturation (es)
Saturation deficit = es – e
es : saturation of certain temperature
e: actual vapour pressure .
* Condensation occurs if vapour pressure > es
e
* Relative humidity = H = es * 100 %
* Dew point :
It the temperature at which the saturation vapour pressure is equal to
the existing vapour pressure .
It is possible to mesure the actual vapour pressure as
( ew – e ) = Y ( t - t s )
where
ew : vapour pressure at tw
e : actual vapour pressure .
tw : temperature of wet thermometer
t: temperature of dry thermometer
Y: device constant
= 0.485 if e in mm Hg
= 0.66 if e in mm bar
The atmosphere
1. Ozone
Most oxygen in the atmosphere is present in the form of O2 (diatomic)
and a small portion is present in the form of O3 (triatomic), which is
called Ozone.
Ozone is found at high attitudes (20 – 70 km) above the earth’s surface
What is the important of Ozone?
Ozone prevents harmful ultra violet radiation from reaching down the
biological life on earth surface.
P1 P2
17.27 T
es = 6.11 exp ( 234.3 + T )
Where:
es : in mbar
T: Temperature in Cْ
Example:
Find the saturation vapor pressure at 20 Cْ
17.27 * 20
es = 6.11 exp ( 234.3 + 20 )
345.4
= 6.11 exp ( 257.3 )
= 6.11 * 3.828
= 23.39 mb
P= P
RT
The gas constant for water vapor is 1.6 times the gas constant, so P for the
vapour is:
e
PV = ----- 1
1.6 RT
P e
Pm = RT ( 1 – 0.375 P )
From which we can note that the density of wet air is less than the density
of dry air.
From the figure
1 and 2
Increasing the vapor pressure by adding vapor to air while the
temperature is constant to reach Ps (T, es) where the air becomes
saturated: further addition of vapor will cause condensation.
1 and 3
Air is cooled without changing its pressure to reach Ps (Td, e), here the air
becomes sutured and further decreasing of temperature will cause
condensation.
The temperature Td is called the dew point.
1 and 4
Vapor is added to air without controlling temperature. By increasing
vapor content, air temperature will decrease (Tw). This temperature is
called the wet bulb temperature, which means the temperature to which
the original air can be cooled by increasing its vapor content. This
principle is used in measuring the relative humidity of air.
In general the different between es and e is called saturation deficit.
So, saturation deficit = (es – e)
Example:
Determine the density in kilograms per cubic meter of:
a. dry air at 30 Cْ and pressure of 900 mb
= 1.035 Kg/m3
b- Relative humidity = h
e
.7 = es * 100
e = .7es
17.27 Tc
es = 6.11 exp ( ) ( Tin co )
234.3 + Tc
= 42.43 mb
e = 0.7 * 42.43
= 2907 mb
P e
Pm = RT ( 1 – 0.375 P )
29.7
= 1.035 ( 1 – 0.375 * )
900
= 1.022 kg / m3
Example:
For an atmosphere with relative humidity of 80 % and temperature of 30
Cْ determine the following:
Solution
27.3 + Tc
237.2 * Tc
1. es = 6.11 exp
= 42.4 mbar
e
2. h = 80 % es = .8
e = .8 * es
= 33.94 mb
Wind velocity
Air in motion is called wind.
Wind velocity is measured by anemometer.
Wind velocity can be calculated at any height using the formula
V Z
= ( )
Vo Zo
Where:
Vo – the observed velocity at Zo
V = the velocity at a height Z
Example:
If a wind velocity was estimated to be 5 m / second at 10 m above
ground, what will be the wind velocity at 2 m above the ground?
Solution:
Vo = 5 m / sec
Zo = 10 m
Z=2M
V=?
V 2
5 = ( 10 ) .15
V = 78 * 5 = 3.9 m / sec
* Rain gauges
Are used to measure rain fall. They must be distributed on the following
basis:
1. Catchment area
2. dominant storms
3. type of dominant rain type (rain or snow)
4. topography
5. canopy (trees height, density…)
ε = 10%
Example:
Station A B C D E F
Rainfall cm: 82.6 102.9 180.3 110.3 98.8 136.7
29.54 2
N= ( 10 ) 8.7, say g stations
Nx PA PB PC
Px = 3( + + )
Na NB NC
Or Px = PA + PB + Pc it PA , PB , Pc
3
Example:
A B C D
17.27 T
es = 6.11 exp . ( 237.3 + T ) …(2.1)
where es is in millibars and T is in oC .
T es T es T es T es T es
o o o o o
in C in mb in C in mb in C in mb in C in mb in C in mb
0 6.1078 10 12.272 20 23.373 30 42.430 40 73.777
1 6.5662 11 13.119 21 24.861 31 44.927 41 77.802
2 7.0457 12 14.017 22 26.430 32 47.551 42 82.015
3 7.5753 13 14.969 23 28.086 33 50.307 43 86.423
4 8.1294 14 15.977 24 29.831 34 53.200 44 91.034
5 8.7192 15 17.044 25 31.671 35 56.236 45 95.855
6 9.3465 16 18.173 26 33.608 36 59.422 46 100.89
7 10.013 17 19.367 27 35.649 37 62.762 47 106.16
8 10.722 18 20.630 28 37.796 38 66.264 48 111.66
9 11.474 19 21.964 29 40.055 39 69.934 49 117.40
100e
h = es …(2.2)