PRECIPITATION - asep sapei 's blogasepsapei.staff.ipb.ac.id/files/2011/07/Hpart2.pdf · 2011. 12. 11. · Hydrology Asep Sapei 18 2.0 649 3.0 806 4.0 883 5.0 909 5.8 917 = Froms of

Hydrology Asep Sapei

16

PRECIPITATION

= Precipitation :

- Is a process where the water vapor in the air

precipitate to the earth surface as rain or snow

- Most of precipitation in tropical area (like Malaysia) is

as rain precipitation ≈ rainfall

= Precipitation formation:

- Moisture is always present in the atmosphere, even on

cloudless day But require some cooling

mechanism to form precipitation

- Cooling mechanism

Is achieve by lifting the air:

- Convective : result from unequal radiative heating and cooling of the earth’s surface and atmosphere

- Convergence : caused by orographic barriers (mountaints)

- Condensation and freezing nuclei

- Nuclei are small particles ranging from 0.1 – 10 µm in diameter

- Condensation nuclei consists of: products of

combustion, oxides of nitrogen and salt particles

Salts particles are most effective, and may result

in condensation with RH 75 %


17

- Freezing nuclei consists of: clay minerals (such as

kaolin)

- Growth of water droplets and ice crystals

o Through diffusion of water vapor

o Grow in a second at the beginning, but become slow

thereafter

o Produce fog or cloud elements, generally < 10 µm o For precipitation to occur, cloud elements must

increase in size at least about 440 µm o Clouds with water content ≥ 4 g/m3 produce

rainfall that reach the ground

o Throught collision and coalescence due to

different falling speeds form larger particle

up to 6 mm

o Large drop may break up due to air

resistance

= Terminal velocity :

- Maximum falling speed

Diameter

(mm)

Terminal velocity

(cm/s)

0.5 206

1.0 403

1.5 541


18

2.0 649

3.0 806

4.0 883

5.0 909

5.8 917

= Froms of precipitation

- Drizzle or mist.

o Consists of tiny liquid water droplets

o Diameter : 0.1 – 0.5 mm

o Intensity < 1 mm/h

- Rain

o Water droplets > 0.5 mm

o Rain intensities in US:

- Light : Intensity < 2.5 mm/h

- Moderate: Intensity 2.8 – 7.6 mm/h

- Heavy: Intensity > 7.6 mm/h

- Glaze : is the ice coating, specific gravity 0.8-0.9

- Rime : is opaque deposit of ice granules, specific

gravity 0.2-0.3

- Snow : is composed of ice crystals. specific gravity 0.1

- Hail : is the precipitation in the form of balls of ice

= Types of precipitation


19

According to the factor mainly responsible for the lifting

which cause it

1. Convective precipitation :

- Is caused by the rising of warmer, lighter air in

colder, denser surrounding

- Spotty

2. Orographic precipitation :

- Result from mechanical lifting over mountain

barriers

3. Cyclonic precipitation :

- Result from the lifting of air converging into a low

pressure area


20

= MEASUREMENT OF PRECIPITATION

o Rainfall parameters:

� Amount

� Intensity

� Time of beginning and ending of precipitation

� Raindrop-size distribution (rare)

o On the basis of the vertical depth of water that would

accumulate on a level surface if the precipitation

remained where it fell

o Precipitation gages

� Non-recording type: for daily rainfall (at 7:00

AM)

� Recording type: record by a pen trace on a chart,

punched tape recorder or electronic (data logger)

a. Tipping bucket type

b. Weighing type


21

c. Float recording

(a) (b) (c)

� Mesurement errors:

- instrument : friction, water creeping up the

stick, initial moisten the funnel etc.

- Environment: wind windbreak which is no

higher than twice their distance

� Precipitation gage network

- Minimum densities of precipitation network:

� For flat regions of temperate,

Mediterranean and tropical zone: 600 –

900 km2 per station


22

� For mountainous regions of temperate,

Mediterranean and tropical zones : 100 –

250 km2 per station

� For small mountainous islands with

irregular precipitation : 25 km2 per

station

� For arid and polar zones : 1500 – 10000

km2 per station


23

o Other instrument:

� Radar measurement: use electromagnetic energy


24

� Satellite : for area where gage network are

inadequate or nonexistent, such as over the ocean.

= INTERPRETATION OF PRECIPITATION DATA

o Estimating missing precipitation data

- Many stations have short breaks in their record

because of absences of the observer or instrument

failure

- Is estimated from observations at three stations as

close to and as evenly spaced around the mentioned

station � If the normal annual precipitation at each of the stations

is with in 10 % estimated rainfall is arithmetic

average of the precipitation at the other station

� If the normal annual precipitation at each of the stations

is more than 10 % use the normal ratio method

++++++++==== C

C

xB

B

xA

A

xx P

N

NP

N

NP

N

NP

3

1

P : rainfall at X, A, B and C

N : the normal annual precipitation at X, A, B and C

o Double mass analysis

- Change in gauge location, exposure, instrumentation

or observational procedure A relative change in

precipitation catch need consistency tests

- By comparing its accumulated annual or seasonal

precipitation with the concurrent accumulated value

of mean precipitation for a group of surrounding

station


25

A change gage location in

June 1961

To make the prior record

comparable with that for

the more recent location

adjust by ratio of slopes of

two segment (0.74/1.19)

o Average precipitation over area

- Arithmetically mean � Good for flat country and the gages are uniformly

distributed - Thiesen method

� For nonuniform distribution of gage

� By providing a weighting factor for each gage


26

� The stations are plotted on a map, and the effective area

are determined


27

- Isohyetal method � The most accurate method � Use contours of equal precipitation (isohyets)

o Depth-Area-Duration Analysis

- To determine the maximum amount of precipitation

within various durations over areas of variation sizes

-


28

= Variation in Precipitation

o Geographic variation

� Precipitation is heaviest near the equator and

decreases with increasing latitude

� Precipitation tends to be heavier near coastlines

� Amount and frequency of rainfall are generally

greater on the windward side of mountain barrier

o Time variation

� Seasonal distribution : wet period and dry period


29

INFILTRATION

= Infiltration :

Rain falls upon the ground

It wets vegetation (interception) and surface soil

(depression and retention storage)

Subsequent rain must penetrate the surface layers

(infiltration) and or runoff the surface towards a stream

channel

= Factors influencing infiltration:

- Soil condition

o Provides a large number of passageways for water

to move into the surface

o Depends on

• The size of the particles that make up the soil

• The degree of aggregation between individual

particles

• The arrangement of the particles and

aggregates


30

- Vegetation

o Reduce surface sealing

o Organic debris forming a sponge-like surface

o Root system provide passageways to the subsoil

- Others

o Land slope : not significant for slope steeper than 2

%

o Antecedent soil moisture : water causes some of

colloids in the soil to swell reduce infiltration

rate

o Water temperature : not significant

viscosity

= The variation of infiltration

= Infiltration equation (empirical, base on time):

1. Kostiakof

batf ==== BAtF ====

where f : infiltration capacity or the maximum rate at which soil

under a given condition can take water through its

surface, cm/h

F : accumulated infiltration, cm


31

a, b, A and B : constants

t : elapse time, h

2. Phillip

5.0−−−−++++==== dtcf

Where c and d are constants

3. Horton

(((( )))) ktcc effff −−−−−−−−++++==== 0

Where fc : the constant infiltration capacity as t approaches

infinity, cm/h

fo : infiltration capacity at the onset of infiltration,

cm/h

k : constant

Infiltration capacity determination

1. Double ring infiltrometer


32


33

Example :

� Data t (min) Accumulated infiltration F

(cm)

2 1.66

5 2.37

10 3.59

20 4.87

30 5.92

45 7.84

60 9.75

90 11.14

120 14.71

o Diameter: inner ring 28-33 cm

Outer ring : 50-60 cm

o hight : 40 cm

o Ring push into soil up to the

depth of 15 cm

o The ring are flooded to a depth

of 10 cm

o The decreasing of water

surface are observed several

times

o Water is added if the water

surface decreased 2.5 cm


34

180 16.69

240 18.82


35

� Calculation: t

(min)

F

(cm)

∆ t

(min)

∆ F

(cm)

f=∆F/∆t

(cm/min)

2 1.66

5 2.37 3 0.71 0.24

10 3.59 5 1.22 0.24

20 4.87 10 1.28 0.13

30 5.92 10 1.05 0.11

45 7.84 15 1.92 0.13

60 9.75 15 1.91 0.13

90 11.14 30 1.39 0.05

120 14.71 30 3.57 0.12

180 16.69 60 1.98 0.03

240 18.82 60 2.13 0.04

a. Kostiakof

batf ====

1

1

++++

++++====∫∫∫∫==== bt

b

afdtF

(((( )))) tbb

aF log1

1loglog ++++++++

++++==== linear


36

- Plot F Vs t on double logarithmic paper

- Determine b:

b + 1 = 5/9.4 = 0.53 b = - 0.47

- Determine a :

log (a/(b+1)) = 1.01

a/0.53 = 2.74 a = 1.45

- So the infiltration equation is: 47.045.1 −−−−==== tf

c. Phillip

5.0−−−−++++==== dtcf ctdtF ++++==== 5.02

15.0

11 2 ctdtF ++++==== 2xt 2125.0

121 2 tcttdttF ++++====


37

25.0

22 2 ctdtF ++++==== 1xt 1215.0

212 2 tcttdttF ++++====

15.0

225.0

11221 22 tdttdttFtF −−−−====−−−−

(((( ))))15.0

225.0

1

1221

2 tttt

tFtFd

−−−−

−−−−====

To determine d, chose two t, exp.: t1 = 10 min and t2 =

120 min

(((( )))) 52.092.2692

7.283

10120120102

1071.1412059.35.05.0

========−−−−

−−−−====

xxx

xxd

Substitute c to one of the equation F:

15.0

11 2 ctdtF ++++====

cxx 101052.0259.3 5.0 ++++==== c = 0.03

Infililtration equation : 5.052.003.0 −−−−++++==== tf


38

2. Φ index

- Is as the average rainfall intensity above which the

volume of rainfall equals the volume of runoff

- Rainfall and runoff are measured shaded

area

- Unshaded area infiltration

- Depends on rainfall characteristics

determine average Φ index

0.00

0.20

0.40

0.60

0.80

1.00

1.20

2 10 30 60 120

240

t (menit)

f (c

m/m

en

it)

Kostiakof

Phillip

Data


39

-


40

SURFACE RUNOFF

= Surface Runoff :

· It is portion of the precipitation that makes its way

toward stream channels, lakes or oceans as surface

flow

· It will occur only when the rate of precipitation

exceeds the rate at which water may infiltrate into

the soil

= Factors affecting runoff:

1. Rainfall :

o Intensity

Advance rainfall less runoff

Delayed rainfall much runoff

If rainfall intensity > infiltration rate fi = fa

If rainfall intensity < infiltration rate

fa=rainfall

intensity


41

o Duration

rainfall duration ↑ runoff ↑

o Area distribution

Significant if the cathment is large

2. Catchment, watershed

· Is the area where runoff from rainfall on that area

flow out through an outlet

· The catchment boundary is as ridge line

infiltration infiltration

rainfall rainfall

runoff


42

· Watershed factors affecting runoff:

a. Size. Size ↑ runoff ↑ b. Shape

A : elongated shape

B : fan shape

C : radial

Long, narrow

lower

runoff


43

3. Topography : slope of upland area and gradient of

channels

Slope ↑ runoff ↑ 4. Geology or soil : infiltration, permeability, soil layer

5. Surface culture

= Flow parameters (from a catchment)

a) Amount of flow : measured at the outlet at a certaint

period of time and then divided by area unit of

depth (mm, cm, etc.)

b) Flow discharge : Depth of flow divided by time. Unit:

mm/h


44

= Runoff estimation

1. Rational method

- For predicting a peak runoff

- Suumption: small area, uniform rainfall with high

intensity and short duration - CiAQ 0028.0====

Where Q : peak runoff rate, m3/s

C : runoff coefficient

i : rainfall intensity for the design period and

for duration equal to the time of

concentration (tc), mm/h

A : catchment area, ha

385.077.0 1000195.0 −−−−==== Sxltc (Kirpich, 1940)

tc : time of concentration, min

l : maximum flow length, m

S : average watershed gradient, m/m


45

2. Soil Conservation Service Method

- The peak flow AQqq u====

q : peak runoff rate, m3/s

qu: unit peak flow rate, m3/s per ha/mm of runoff (from

graph)

A : watershed area, ha


46

Q : runoff depth, mm

(((( ))))[[[[ ]]]] [[[[ ]]]]5.07.0

8.0 )(4407/9100gc S

NLT −−−−====

Tc : time of concentration, hrs

L : longest flow length, m

N : runoff curve number

Sg : average watershed gradient, m/m

(((( ))))SI

SIQ

8.0

2.02

++++−−−−

==== (((( )))) 254/25400 −−−−==== NS

I : rainfall, mm

S : max. potential difference between rainfall and runoff,

mm

N : curve number (between 0 – 100)


47

= STREAMFLOW

o Runoff collected in the stream streamflow


48

o Expressed by streamflow discharge

Unit : m3/s, l/s, etc

o Water stage

- Is the elevation above some arbitrary zero datum of

the surface at a station

- Instrument for measuring water stage

� Manual gage : Staff gage

� Recording gage


49

• Record on continuous strip chart or

punched tape or data logger

o Discharge measurement

� The discharge at a section is derived from point

measurement of velocity

Q = AV

Q : discharge, A: cross section area and V: flow

velocity

� Velocity measured by a current meter

• Type of current meter : cup type and propeller


50

type

Cup type propeller type

• Velocity (V) is a function of propeller revolution

speed (N)

bNaV ++++==== ; a and b : constants

• Velocity may be measured at:

o One depth : at the depth of 0.6D

For shallow flow

o Two depths : at the depth of 0.2D and 0.8D (((( )))) 2/8.02.0 vvvaverage ++++====

o Three depths : at the depth of 0.2D, 0.6D

and 0.8D. For deep flow (((( )))) 3/8.05.02.0 vvvvaverage ++++++++====

• Measurement of wide stream or river

o Divide the stream into 20 to 30 vertical

sections. No section include more than 10 %

of the total flow

o Measure the velocity of each section at 1, 2

or 3 depths, then determine the average

velocity of each section

o Measure the area of section

o Calculate the discharge of each section

o Add the increment of discharge


51

= Rating curve

o Periodic measurements of flow and simultaneous

stage observation provide data for making a rating

curve

o such a curve is approximately parabolic


52

HYDROGRAPH

� The curve that relates the streamflow and time

o Components of streamflow hydrograph :

- Overland flow or surface runoff , is that water which travels over the ground surface to a channel

- Interflow or subsurface flow, is a portion of infiltration water which move laterally through the upper

soil layer until it enters a stream channel - Ground water flow or base flow, is a portion of

groundwater which discharge into stream. This flow cannot fluctuate rapidly


53

For convenience the total flow to be divided into two

parts:

- Direct runoff, consists of surface runoff and a substantial portion of the interflow

- Base flow, is considered to be largely groundwater

o A Typical hydrograph consists of : rising limb, crest,

falling limb (recession)

- The point of inflection on the falling side to

mark the time at which surface inflow to the

channel system ceases

- Recession curve represents withdrawal of water

from storage within the basin

o Hydrograph separation or hydrograph analysis

- Base on time of direct runoff remains relatively

constant from storm to storm

- Separation method:


54

� Provided by terminating the direct runoff at

a fixed time (time base N) after the peak of

hydrograph

2.08.0 AN ====

N : time base, days

A : area, km2

� And then N is evaluated, too short or too long

� Extent the recession existing before the

storm to a point under the peak

� From this point, a straight line is drawn to

the hydrograph at a point N days after peak

� Separation also can be made by draw AC line

� Unit hydrograph


55

- Is a typical hydrograph where the volume of runoff

from a storm of specified duration under the

hydrograph is commonly adjusted to 1 unit (cm or mm)

equivalent depth over the catchment

- Since the physical characteristics of the basin –

shape, size, slope etc- are constant, might expect

similarity in the shape of hydrographs from storm of

similar rainfall characteristics

- Influence by storm characteristics (duration, time-

intensity pattern, area distribution of rainfall and

amount of rainfall)

- Unit hydrograph principle:

• Different intensity of rain of same duration

o the same period of runoff

o different ordinates of hydrograph

• Superposition applicable

- Unit hydrograph parameters

Snyder introduces 3 parameters:


56

• Basin lag (tp)

Is the time

between mass

centre of unit rain

of tr h duration and

runoff peak flow

3.0)( LLCt catp ====

tp : basin lag, h

Ct : a coefficient.

Between 1.8 – 2.2

Lca : distance from gauging station to centroid of

catchment, measured along main stream channel, mile

L : distance from station to catcment boundary measured

along

the main stream channel, mile

• Peak flow (qp)

p

ppt

Cq640

====

qp : in cubic feet per second per square mile of area

Cp : a coefficient. Between 0.56 – 0.69

• Unit hydrograph base length (T, in days)

)24

(33pt

T ++++====


57

- Synthetic unit hydrograph

a) Can be derived from streamflow hydrograph

b) Make hydrograph separation (direct runoff and

base flow)

c) Calculate volume of direct runoff

d) Calculate ordinates of unit hydrograph by dividing

volume of direct runoff with total flow depth

- Unit hydrographs of various duration

o Changing a short duration unitgraph to a longer

duration unitgraph (t2>t1)


58

o Changing a long duration unitgraph to a shorter

duration unitgraph (t2<t1)

� S curve


59

� Example : 4 h unit hydrograph is changed to 3 h

unit hydrograph. Catctment area is 300 km2

Qe = (2.78x300)/4= 208 m3/s or cumecs


60

Documents

PRECIPITATION - asep sapei 's blogasepsapei.staff.ipb.ac.id/files/2011/07/Hpart2.pdf · 2011. 12. 11. · Hydrology Asep Sapei 18 2.0 649 3.0 806 4.0 883 5.0 909 5.8 917 = Froms of