PRECIPITATION
-
Is any liquid or frozen water that forms in the atmosphere and
falls to the earth
All forms of water that reach the earth from the atmosphere
To form it the atmosphere needs moisture, moist air undergoes
lifting and resultant cooling, phase change due to condensation
Forms of precipitation:
1.
2.
-
Drizzle – light steady rain in fine drops
0.5 mm, intensity: <1mm/hr
Rain – condensed vapor, falls in drops
> 0.5 mm from clouds; max 6 mm
Type
Light Rain
Moderate Rain
Heavy Rain
Intense Rain
Torrential Rain
Intensity, mm/hr
Trace to 2.5
2.5-7.5
7.5-15
15-30
>30
3.
4.
5.
6.
7.
8.
9.
Glaze – freezing of drizzle or rain (contact with cold objects
Sleet – frozen rain drops (cold air)
Snow – ice crystals due to sublimation (water vapor to ice)
Snowflakes – ice crystals fused together
Hail – small lumps of ice (> 5mm diameter)
Due to freezing and melting
Dew – moisture condensed in small drops upon cool surfaces
Frost – feathery deposit of ice formed in the ground or surface
of exposed objects
10. Fog – thin cloud by condensation of atmospheric vapor
11. Mist – thin fog
Classification of Precipitation
1. Thermal Convection (convective lifting)
- Form of local whirling thunder storms
2.
3.
4.
5.
-
Warm air rises due to its low density then cools to a cauliflower
shaped cloud
Bursts into a thunder storm
Called tornadoes when accompanied by destructive winds
Conflict between two air masses (frontal lifting)
2 air masses with contrast in temperature, precipitation will
occur
Orographic lifting
Mechanical lifting of moist air over mountain barriers
Heavy precipitation on the windward side
Cyclones (cyclonic precipitation)
Due to lifting of moist air converging into a low-pressure belt
(due to pressure differences - unequal heating of earth)
Winds blow spirally inward counterclockwise in the northern
hemisphere and clockwise in the southern hemisphere
Tropical Cyclone (hurricane or typhoon)
300 to 1500 km (small, high wind velocity, heavy precipitation)
Extra-tropical cyclone (anti-cyclone) – large diameter (3000
km), causes wide spread frontal type precipitation
Rainfall Measurement
Rain gauge – measures rainfall rate in a certain period of time
1.
-
Non-recording type
Cylindrical vessel with enlarged base
Symons rain gauge
Measured 3 to 4 times in a day
Measured by a graduated measuring glass inside the receiving
bottle
2. Recording type
- Automatic mechanical arrangement (clockwork, drum with
graph paper, and a pencil point)
a. Weighing Bucket Type Rain Gauge
- Most common self-recording rain gauge
-
Certain weight of water makes a pen move on a chart wrapped
around a clock-driven drum.
b. Floating or Natural Syphon Type Rain Gauge
- Has a float that rises on top of the container as level rises in the
container
c. Tipping Bucket Type Rain Gauge
- Circular rain gauge consisting of a sharp-edged receiver and
provided with a funnel inside
- 0.25 mm rainfall tips the bucket and is discharged into the
container
EVAPORATION
-
Transition from liquid to gas below the temperature at which it
boils
Water to water vapor
Commonly expressed in mm/hr or in/hr
Evaporation depends on:
1. Vapor pressure at the water surface and air above
Dalton's law: evaporation rate, E, controlled by two factors, the
windspeed and the saturation deficit:
E = u ( es-ea )
u: function of windspeed
ea : current vapor pressure
es: saturation pressure at that temperature
2.
3.
4.
5.
Air and water temperature
Windspeed
Quality of water
Size of the water body
Evapotranspiration (Total Evaporation) (ET)
-
Evaporation from soil, water bodies and loss of water from
plants.
ET = T + E
Evaporimeters:
-
Water-containing pans which are exposed to the atmosphere
and the loss of water by evaporation measured in them at
regular intervals
Lake Evaporation = Cp x Pan Evaporation
Cp = Pan coefficient
Values of pan coefficient:
Types of Pan
Average Value
Class A Land Pan
0.7
ISI Pan (modified
0.8
class A)
Colorado Sunken
0.78
Pan
USGS Floating Pan
0.8
-
Range
0.6 – 0.8
0.65 – 1.10
0.75 – 0.86
0.70 – 0.82
We usually use Class A and USGS
As per range (they also usually use the maximum range but to
be safe, use the mean amount – av. value)
Water molecules are tightly held by intermolecular forces – energy
is consumed in evaporation to move the molecules away
Energy = latent heat if vaporization (λ) = 2.5 MJ/kg *megajoules
per kg
λ = 2.501 – 0.00236T (T = temperature in Celsius)
Water to vapor – water vapor pressure is low (vaporization) =
evaporation
Water vapor to liquid (condensation)
If the rate of vaporization equal condensation, the air is saturated
and evaporation stops
Difference of Saturated vapor pressure (eS) and vapor pressure in
the air (ea) determines the rate of evaporation
**Bigger difference = more evaporation
Factors that affect evaporation in open water
a. Solar radiation
b. Temperature of water and air (ex. Clothes)
c. Difference in vapor pressure between water and the overlying
air
d. Wind speed across the lake
The amount of water evaporated from water surface is estimated
by:
a. Evaporimeter
b. Empirical formula
c. Analytical Formula
Practice Problem
A reservoir with a surface area of 250 hectares had the following
average values of climate parameters during a week:
a. Water temp – 20°C
b. Relative humidity – 40%
c. Wind velocity at 1 m above ground surface – 16 km/h
Estimate the average daily evaporation from the lake by using
Meyer’s Formula
RAINFALL-RUNOFF RELATION
STORM HYDROGRAPH (FLOOD HYDROGRAPH)
- A graph showing the relationship between rainwater and
discharge in a river
- Mainly used to observe discharge for a given storm event
- Shows how storm has affected a river
IMPORTANCE OF STORM HYDROGRAPH
Flood control flood control structures on flood plains such as
 Dikes (levees)
 Drainage canal (divergent)
For flood forecasting
Concepts:
Peak stream flows are a result of storm rainfall
Components:
AB - Baseflow recession
BC - rising limb how engineers know what to forcast gives indication
of how fast water is reaching the channel (represents level of water)
CD - falling limp shows the river as its level flows
DE - baseflow recession
BASEFLOW SEPARATION METHOD
A. STRAIGHT LINE METHOD
- Drawing a horizontal line from
the point at which the surface
runoff begins to intersect with
the recession limb
- For ephemeral streams
B. FIXED BASE METHOD
- Surface runoff is assumed to
end a fixed time “N” after the
hydrograph peak
- Base flow before surface runoff
began is projected to the time
of peak, and then projected on
the recession limb at time “N”
after peak
C. VARYING/VARIABLE
SLOPE
METHOD
- The base flow curve before
surface runoff is extrapolated
forward to the time of peak
discharge and then the base
flow curve after surface runoff
ceases
is
extrapolated
backward to the point of
inflection on the recession
limb. A straight line is used to
connect the endpoint of the
extrapolated curves
Main timing aspects of the storm hydrograph:
1. Duration of rainfall excess (D):
- Time from start to finish of
rainfall excess
2. Lag time (tp):
- Time from the center of mass of
the rainfall excess to the peak of
the hydrograph
3. Time of rise (TR):
- Time from the start of rainfall
excess to the peak of the
hydrograph
4. Time of concentration (tc):
- Time for a wave of water to propagate from the most distant
point in the watershed to the outlet. One estimate is the time
from the end of net rainfall to the inflection point of the
hydrograph
5. Time base (Tb):
- Total duration of the DRO (Direct Runoff) hydrograph
PROBLEM SOLVING
1. Determine the DRH (direct runoff hydrograph) assuming an
initial baseflow of 400cfs, the ∅-index, and the ERH from the
observed rainfall and streamflow data given in the table. The
watershed is 7.03 sq.mi.
Unit Hydrograph
States that “basin outflow resulting from 1.0 inch (1.0 mm) of direct
runoff generated uniformly over the drainage area at a uniform
rainfall rate during a specified period of rainfall duration.”
Procedure for analysis:
1. Analyze the hydrograph and separate base flow.
2. Measure the total volume under the DRH and convert this to
inches (mm) over the watershed.
3. Convert total rainfall to rainfall excess through infiltration
methods, such that rainfall excess = DRO, and evaluate duration
D of the rainfall excess that produced the DRO hydrograph.
4. Divide the ordinates of the DRH by the volume in inches (mm)
and plot these results as the unit hydrograph
Problem solving:
Following are the ordinates of a storm hydrograph of a river draining
a catchment area of 423 km2 due to a 6-h isolated storm. Derive the
ordinates of a 6-h unit hydrograph for the catchment.
t = Time (Fv) start of storm (h)
Q = Runoff discharge (m3/s)
t
Q
t
Q
-6
10
54
39
0
10
60
31
6
30
66
26
12
18
24
30
87.5 115.5 102.5 85
72
78
84
90
21.5 17.5
15
12.5
36
71
96
10
42
48
59 47.5
102
10