bfc 32002 hydrology chapter 3. evaporation, transpiration
TRANSCRIPT
BFC 32002 Hydrology
Chapter 3. Evaporation, Transpiration &InfiltrationZarina Md Ali
Based on BFC 32002 Hydrology Module Email: [email protected]
Phone Nu: 074564359 / 0197722315
BFC32002_Ch3/ZARINA'S 1
Learning OutcomesAfter completing this chapter, the students should be able to :
• simulate the rate of evaporated and transpired water over time in modeling conceptual.
• define the infiltration process and estimate the infiltration rate.
BFC32002_Ch3/Z
ARIN
A'S
2
Introduction
BFC32002_Ch3/Z
ARIN
A'S
3Evaporation is the process by which water istransformed from the liquid phase to vapourphase (transferred from the land and watermasses of the earth to the atmosphere) (1)
Evapotranspiration (ET)is the sum ofevaporation and planttranspiration from theearth's land surface toatmosphere. (3)
Transpiration isthe processwhere plantsabsorb waterthrough theroots and thengive off watervapour throughpores in theirleaves. (2)
Infiltration is theprocess by whichprecipitation orwater soaks intosubsurface soilsand moves intorocks throughcracks and porespaces (4)
Transpiration
BFC32002_Ch3/Z
ARIN
A'S
4
Transpiration is the process by which water moves through plants and
evaporates through leaf stomata, which are small openings in the leaves.
The important factors affecting transpiration are: atmospheric vapour
pressure, temperature, wind, light intensity and characteristic of the plant,
such as the root and leaf systems.
All types of plantations need water for life. Each plantation differs to
consume water. Only small water remains in its body, and most of water is
evaporated through the leaf. In the field condition, it is difficult to
differentiate between evaporation and transpiration process, those
processes are interconnected, therefore it is commonly called
evapotranspiration (ET).
Estimating Transpiration
BFC32002_Ch3/Z
ARIN
A'S
5
Example 3.4Determine the monthly consumptive use of an alfalfa crop
grown in southern California in July if the average monthly
temperature is 72oF, the average value of daytime hours in
percentage of the year is 9.88, and the mean montly
consumptive use coefficient for alfalfa is 0.85.
BFC32002_Ch3/Z
ARIN
A'S
6
Solution:
Using equation:
100
ktpu
= 0.85 x 72 x 9.88/100= 6.05 in of water.
BFC32002_Ch3/Z
ARIN
A'S
7
BFC32002_Ch3/Z
ARIN
A'S
8
Example 3.5Determine the seasonal consumptive use of a tomato crop grown in
New Jersey if the mean monthly temperature for May, June, July
and August are 61.6, 70.3, 75.1 and 73.4 o F, respectively and the
percent daylight hours for the given months are 10.02, 10.8,10.22
and 9.54 as percent of the year, respectively.
BFC32002_Ch3/Z
ARIN
A'S
9
Solution:
Consumptive use coefficient is 0.65 to 0.70 (tomatoes & 4 months).
Since New Jersey is a humid area Ks = 0.65.
Evapotranspiration (ET) Evapotranspiration (ET) is a term used to describe the sum ofevaporation and plant transpiration from the earth's land surface toatmosphere
Potential evapotranspiration (PET @ ETP) is a representation of theenvironmental demand for evapotranspiration and represents theevapotranspiration rate of a short green crop, completely shading theground, of uniform height and with adequate water status in the soilprofile.
Evapotranspiration is said to equal potential evapotranspiration whenthere is ample water.
BFC32002_Ch3/Z
ARIN
A'S
10
Accordingly, good estimates of evapotranspiration are a requisites for
hydrologic modeling. There are based on two concepts:
1. Potential Evapotranspiration , Etp measure of the ability of the
atmosphere to remove water from the surface through the
processes of evaporation and transpiration assuming no control
on water supply
2. Actual Evapotranspiration, ETa is the quantity of water that is
actually removed from a surface due to the processes of
evaporation and transpiration.
BFC32002_Ch3/Z
ARIN
A'S
11
Basically, there are three major approaches :
a. Theoretical, based on physics of the process.
b. Analytical (logical), based on energy or water budgets.
c. Empirical (observation)
Estimating Evapotranspiration (a) Empirical Formula - The Thornhwaite Method
• defines potential evaporation by assuming soil storage is not
depleted.
• The Thornthwaite – Holzman equation is a good example of a mass
transfer equation that has often been employed for this purpose.
• An equation for estimating evapotranspiration potential:
BFC32002_Ch3/Z
ARIN
A'S
12
BFC32002_Ch3/Z
ARIN
A'S
13
Estimating Evapotranspiration (b) The Penman Method
• a method to combine the mass transport and energy
budget theories, & one of the more reliable approaches to
estimating ET rates using climatic data.
• this equation gives good result for evaporation rate of free
surface water, E0 if at that place there is no observation by
pan evaporation or water balance study.
BFC32002_Ch3/Z
ARIN
A'S
14
BFC32002_Ch3/Z
ARIN
A'S
15
BFC32002_Ch3/Z
ARIN
A'S
16
BFC32002_Ch3/Z
ARIN
A'S
17
BFC32002_Ch3/Z
ARIN
A'S
18
Example 3.6Using the Penman method, estimate ET, given the following data :
temperature at water surface = 22oC, temperature of air = 33oC,
relative humidity = 45%, wind velocity = 1.5 mph (36 mi/day). The
month is June at latitude 33o north, r = 0.07 and n/D = 0.70.
BFC32002_Ch3/Z
ARIN
A'S
19
Solution:
1. Given the data for temperature, the values of eo and ea can be
determined. Using Figure 3.7, the saturated vapor pressures are found to be
20.02. For a relative humidity of 45%, ea = 38.04 x 0.45 = 17.12. Then,
BFC32002_Ch3/Z
ARIN
A'S
20
eo = 20.02 mm Hg and ea = 38.04 mm Hg.
BFC32002_Ch3/Z
ARIN
A'S
21
2. The value of B =17.69 from Table 3.5for temp =33oC.
3. R = 16.56using Figure 3.6for month of Juneat latitude 33onorth,
BFC32002_Ch3/Z
ARIN
A'S
22
4. The value of Δ = 1.2
Δ = 1.2, RA= 16.56 and B = 17.69, n/D = 0.70 ea = 38.04 x 0.45 =
17.12, r = 0.07
BFC32002_Ch3/Z
ARIN
A'S
23
Δ = 1.2, H = 6.38, Eo = 1.73,
Example 3.7Estimate the monthly potential evapotranspiration for June. The
mean monthly temperatures are shown in the Table below. The
average relative humidity is 50%. The wind speed is 130 mi/day.
Assume that n/D = 70%, γ = 0.27, and r = 25% at 50O latitude.
BFC32002_Ch3/Z
ARIN
A'S
24
Penman method
BFC32002_Ch3/Z
ARIN
A'S
25
Solution:
Month: JuneLatitude: 50O
Given: n/D = 70% and r = 25%
BFC32002_Ch3/Z
ARIN
A'S
26
Temp: 24.2OC or 75.5OC
4aT = B = 15.7 mm/day
eo = 22 mm HgRH (h) = ea/eo ,
ea = 0.5 22= 11 mm Hg.
BFC32002_Ch3/Z
ARIN
A'S
27
Infiltration
• this analogy can be simplified in two
important aspects, which is:
• maximum rate at which the ground
can absorb water is called as the
infiltration rate.
• volume of water that ground can hold
is known as the field capacity.
BFC32002_Ch3/Z
ARIN
A'S
28
• the flow of water into the ground through the soil surfaceand the process can be easily understood through asimple analogy.
• infiltration rate influence the timing of overland flow inputsto the channel systems.
Infiltration Capacity
BFC32002_Ch3/Z
ARIN
A'S
29
f0 = initial infiltration capacity,
cm/hr or mm/hr
fc = final constant infiltration
capacity, cm/h or mm/h
The relationship between infiltration capacity and time which is known as
Infiltration Capacity Curve. The infiltration capacity of a soil is assumed
highly at the beginning of a storm and has an exponential decay as the time
elapses. It is continues in decreasing until it is reach at the constant level
(saturated).
Factors Affecting InfiltrationThree main factors:
(a) Characterictics of Soil
• texture, structure, permeability, under drainage and type of
soil.
• a soil with a good underneath drainage would obviously
have a higher infiltration capacity. dry soil can absorb more
water than one whose has full pore.
• land use has a significant influence on fc , for instance, a
forest soil which is rich with organic matter will have much
higher value of constant infiltration rate that the similar types
of soil in an urban area where is subjected to compaction.
BFC32002_Ch3/Z
ARIN
A'S
30
Factors Affecting Infiltration(b) Soil Surface
• At the soil surface, the impact of raindrops causes the
fines in the soils to be displaced and these in turn can clog
the pore spaces in the upper layers. This is an important
factor affecting the infiltration capacity.
• Thus a surface covered by grass and other vegetation
which can reduce this process has a pronounced influence
on the value of fc.
• Viessman and Lewis (2003) stated that infiltration rate for
bare-soil is 2.5 mm/h - 25 mm/h.
• However, soil with grass cover tends to increase the
values by a factor between 3 and 7.5. BFC32002_Ch3/Z
ARIN
A'S
31
Factors Affecting Infiltration(c) Fluid Characteristics
• Water infiltrating into the soil will have many impurities,
both in solution and suspension.
• The turbidity of water, especially the clay and colloid
content is an important factor as suspended particles
block the fines pores in the soil and reduce its infiltration
capacity.
• The temperature of the water is also a factor in the sense
that it affects the viscosity of the water which in turn
affects the infiltration rate.
• Besides that, contamination of the water by dissolved
salts also affects the soil structure and then the infiltration
rate.
BFC32002_Ch3/Z
ARIN
A'S
32
Infiltration Measurement
BFC32002_Ch3/Z
ARIN
A'S
33
• Infiltration characteristics of soil can be obtained by
conducting controlled experiment on small areas.
• The experiment set-up is called an infiltrometer, which
are flooding type infiltrometer and rainfall simulator.
Infiltration Measurement
BFC32002_Ch3/Z
ARIN
A'S
34
(a) Flooding Type Infiltrometer (single)
• consist a metal cylinder and open at both
ends (30 cm dia & 60 cm long), planted into
the ground to a depth of 50 cm.
• water is poured to a depth of 5 cm and pointer is set to mark the
water level.
• add water to keep the water level at the tip of the pointer as
infiltration proceeds, and may take 2 to 3 hours till reach uniform
rate.
• experiments are continued is obtained, surface of the soils is
usually protected by a perforated disk to prevent formation of
turbidity and its settling on the soil surface.
• Disadvantage of simple ring: infiltered water spreads at the outlet
from the tube, and can’t be figured as area in which infiltration
takes place.
Infiltration Measurement
BFC32002_Ch3/Z
ARIN
A'S
35
(a) Flooding Type Infiltrometer (double)
• double ring is used to overcome problem
of area.
• the rings are inserted in to the ground and
water is maintained on the soil surface to a
common fixed level.
• the outer ring provides a water jacket to the infiltering water of the
inner ring and hence, prevents the spreading out of the water from
the inner tube.
• the measurement of water volume is done in the inner ring only.
• main disadvantages of flooding type infiltrometer are:
1. The raindrop effect is not simulated.
2. The driving of the tube or rings disturbs the soil structure.
3. The results of the infiltrometer depend to some extent on their
size with the larger meters give less rates than the smaller
ones and this is due to the border effect.
Infiltration Measurement
BFC32002_Ch3/Z
ARIN
A'S
36
(b) Rainfall Simulator
• this instrument give low values than
flooding type infiltrometers, due to the
rainfall effect and turbidity of the surface
soil
• consist a small plot of land (about 2 m x 4 m size), series of
nozzles and measures apparatus.
• the nozzles produce raindrops fall a height of 2 m and capable in
producing various intensities of rainfall.
• Using the water budget equation involves volume of rainfall,
infiltration and runoff, infiltration rate and its variation with time can
be calculated.
• If the rainfall intensities is higher than the infiltration rate, infiltration
capacity values are obtained.
Infiltration Methods(a) Horton Model
The infiltration process was thoroughly studied by Horton in early
1930s. An outgrowth of his work proposed the following empirical
equation to describe the decline in the potential infiltration rate, fpas a function of time
BFC32002_Ch3/Z
ARIN
A'S
37
Infiltration Methods
In cases where water is not continuously ponded above the soil
column, the potential infiltration fp can be expressed in terms of
the cumulative infiltration, F by implicit relationship BFC32002_Ch3/Z
ARIN
A'S
38
The temporal variation in
infiltration rate is applicable
when the water is continuously
ponded above the soil column;
the functional form of this
equation is illustrated in Figure
3.13.
Infiltration Methods
Both equations form an implicit relationship between the
cumulative infiltration, F and the potential infiltration rate, f
where t is simply a parameter in the relationship
BFC32002_Ch3/Z
ARIN
A'S
39
Example 3.8A catchment soil has Horton infiltration parameters: fo = 100 mm/h,
fc = 20 mm/h and k = 2 min-1. What rainfall rate would result in
ponding from beginning of the storm? Is this rainfall rate is
maintained for 40 minutes, describe the infiltration as a function of
time during the storm.
BFC32002_Ch3/Z
ARIN
A'S
40
Solution:
The potential infiltration rate varies between a maximum of 100
mm/h (fo) and minimum of 20 mm/h (fc). Any storm in which the
rainfall rate exceeds 100 mm/hr during the entire storm will cause
ponding from the beginning of the storm. Under these
circumstance, the infiltration rate, f as a function of time is given as
equation as
Example 3.9An initial infiltration was recorded as 5.5 cm/hr during 10 hours of
rainfall. Given that fc and k is 0.4 cm/hr and 0.32 respectively,
determine;
a. Infiltration at 5 hours.
b. Total infiltration within first 8 hours.
c. Total infiltration between 5 and 10 hours from rainfall begin.
BFC32002_Ch3/Z
ARIN
A'S
41
Solution:
fo = 5.5 cm/hr, fc = 0.4 cm/hr dan k = 0.32 h-1
a) Infiltration at 5 hours.( )( ) kt
c o cf f f f e
hr/cm43.1e)4.05.5(4.0f )5(32.05
BFC32002_Ch3/Z
ARIN
A'S
42
Solution:
b) Total infiltration within the first 8 hours.
c) Total infiltration between 5 and 10 hours from rainfall begin.
dt)t(fF
105
)kt(coc )]e1(
K
)ff(tf[F
cm56.4F
e132.0
1.5)5(4.0)e1(
32.0
1.5)10)(4.0(F 5x32.010x32.0
Infiltration Methods(b) Green-Ampt Model
The Green – Ampt model sometimes called the delta
function model is today one of the most realistic models
of infiltration available to the engineer in designing a
storm water management systems.
BFC32002_Ch3/Z
ARIN
A'S
43
Infiltration Index
In hydrological calculation, it is convenient to use a constant
value of infiltration rate for the duration of the storm. The
average infiltration rate is called infiltration index
Infiltration Index• it is convenient to use a constant value of infiltration rate for
the duration of the storm
• average infiltration rate is called infiltration index (Φ)
• this index is the average rainfall above which the rainfall
volume is equal to runoff volume.
• the Index is derived from the rainfall hyetograph with the
edge of the resulting runoff volume.
• The initial loss is also considered as infiltration.
• The Φ value is found by treating it as a constant
infiltration capacity.
• If the rainfall intensity is less than Φ, then the infiltration
rate is equal to the rainfall intensity.
BFC32002_Ch3/Z
ARIN
A'S
44
BFC32002_Ch3/Z
ARIN
A'S
45
• if the rainfall intensity is larger
than Φ the difference between
rainfall and infiltration in an
interval of time represents the
runoff volume as shown as in
figure.
• the amount of rainfall in excess of the index is called rainfall
excess.
• the Φ Index thus accounts for the total abstraction and
enables runoff magnitudes to be estimated for a given
rainfall hyetograph
Example 3.10A storm with 10 cm rainfall produced a direct runoff of 5.8 cm. Table
below show the time distribution of the storm, estimate the Φ index.
BFC32002_Ch3/Z
ARIN
A'S
46
Solution:
Time (hour) 1 2 3 4 5 6 7 8Rainfall (cm/h) 0.4 0.9 1.5 2.3 1.8 1.6 1.0 0.5
Total rainfall, P = 0.4 (1) + 0.9 (1) + 1.5 (1) + 2.3 (1) + 1.8 (1) + 1.6
(1) + 1(1) + 0.5 (1) = 10 cm
Total runoff, R = 5.8 cm
Assume te is 8 hours,
cm/h525.08
8.510
t
R-P Index
e
But this value of Φ makes the rainfall of the first hour and eight hour
ineffective as their magnitude is less than 0.525 cm/h. The value of
te is need to modified.
Then, assume te is 6 hours.
Total rainfall, P = 10 - 0.4 – 0.5 = 9.1 cm
Then,
This value of Φ is satisfactory and by calculating the rainfall excess
Total rainfall excess = 5.8 cm = total runoff BFC32002_Ch3/Z
ARIN
A'S
47
cm/h55.06
8.51.9
t
R-P Index
e
Time
(hour)1 2 3 4 5 6 7 8
Rainfall
excess
(cm)
0 0.35 0.95 1.75 1.25 1.05 0.45 0
.
Example 3.11The rainfall intensity in the 50 hectar of catchment area is given
table below. If volume of surface runoff is 30000 m3, estimate Φ
index for the catchment area and sketch the circumstances in form
of hyetograph.
BFC32002_Ch3/Z
ARIN
A'S
48
Solution: Time
(hour)
Rainfall intensity
(mm/hour)
1 5
2 10
3 38
4 25
5 13
6 5
7 0
mm/h66
6096
t
R-P Index
e
Runoff, R = (3x104)/(0.5x1000x1000)
= 60 mm
Total rainfall = (5+10+38+25+13+5)(1)
= 96 mm
Then, te = 6 hours
But this value of Φ makes the rainfall of the first hour and six
hour ineffective as their magnitude is less than 6 mm/h.
BFC32002_Ch3/Z
ARIN
A'S
49
Then, te = 4 hours
Sketch in form of hyetograph
mm/h5.64
60)55(96
t
R-P Index
e
:
0 2 4 6 8
10 12 14 16 18 20 22 24 26 28 30 32 34
1 2 3 4 5 6 7 Hours (h)
ø = 6 mm/h
ø = 6.5 mm/hj
Rainfall Intensity (mm/h)
Rainfall Intensity versus Time