Engineering Hydrology Chapter III Abstraction From Precipitation Macaspac, Jerrol R.
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Engineering Hydrology
Chapter III
Abstraction From Precipitation
Macaspac, Jerrol R.
3.1 IntroductionAbstraction from Precipitation
Surface runoff is the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, melt water, or other sources flows over the land.
Abstraction From Precipitation:
Evaporation
Transpiration
Infiltration
Surface
Detention
Storage
3.2 Evaporation ProcessAbstraction from Precipitation
Evaporation is the process by which water is converted from its liquid form to its vapor form and thus transferred from land and water masses to the atmosphere.
Vapour Pressure - Rate of evaporation is proportional to the difference between the saturation vapour pressure (SVP) at the water temperature ( ew ) and the actual vapour pressure in the air ( ea ).
John Dalton’s Law of EvaporationEL = C ( ew - ea )
where:EL – rate of evaporation in mm/day
C – constantew & ea – are in mm Hg.
• Evaporation occurs till ew = ea.
• If ew > ea condensation takes place.
Factors affecting Evaporation Rate
Temperature - Rate of evaporation increases with an increase in water temperature.
Wind - Wind helps to remove the evaporated water vapour from the zone of evaporation, thereby creating greater scope for evaporation.
Atmospheric Pressure – Other factors remaining the same, a decrease in atmospheric pressure (as in high altitude areas) increases the evaporation rate.
Factors affecting Evaporation Rate
Soluble salts – When a solute is dissolved in water, the vapour pressure of the solution is less than that of pure water and hence it causes reduction in the rate of evaporation. Under identical conditions evaporation from sea water is about 2-3% less than that from fresh water.
Heat storage in water bodies – Deep water bodies have more heat storage capacity than shallow water bodies. The effect of heat storage is to change the seasonal evaporation rates and the annual evaporation remains more or less unaltered.
Factors affecting Evaporation Rate
Estimating the rate of Evaporation
Evaporation Process
3.3 EvaporimetersAbstraction from Precipitation
Evaporimeters are water– containing pans which are exposed to the atmosphere and loss of water by evaporation from these pans are measured at regular intervals (daily).
Types of EvaporimetersClass A Evaporation Pan
A pan of diameter 1210mm and depth 255mm
Depth of water is maintained between 18 and 20cm
The pan is made of unpainted GI sheet
The pan is placed on a wooden platform of height 15cm above ground level to allow free air circulation below the pan
Evaporation is measured by measuring the depth of water in a stilling well with a hook gauge.
Types of EvaporimetersISI Standard Pan ( Modified Class A Pan)
A pan of diameter 1220mm and depth 255mm
The pan is made of copper sheet 0.9mm thick, tinned inside and painted white outside
The pan is placed on a square wooden platform of width 1225mm and height 100mm above ground level to allow free air circulation below the pan
A fixed point gauge indicates the level of water
Types of EvaporimetersColorado Sunken Pan
920mm square pan made of unpainted GI sheet, 460mm deep, and buried into the ground within 100mm of the top
Main advantage of this pan – its aerodynamic and radiation characteristics are similar to that of a lake
Disadvantages – difficult to detect leaks, expensive to install, extra care is needed to keep the surrounding area free from tall grass, dust, etc.
Types of EvaporimetersUS Geological Survey Floating Pan A square pan of 900mm sides
and 450mm deep. Supported by drum floats in the
middle of a raft of size 4.25m x 4.87m, it is set afloat in a lake with a view to simulate the characteristics of a large body of water
Water level in the pan is maintained at the same level as that in the lake, leaving a rim of 75mm.
Diagonal baffles are provided in the pan to reduce surging in the pan due to wave action
Disadvantages – High cost of installation and maintenance, difficulty in making measurements.
Pan Coefficient, Cp
Pan Coefficient – Evaporation Pan are not exactly models of large reservoirs and the following drawbacks:
1. They differ in the heat – storing capacity and heat transfer from the sides and bottom.
2. The height of the rim in an evaporation pan affects the wind action over the surface.
3. The heat – transfer characteristics of the pan material is different from that of the reservoir.
The evaporation observed from a pan has to be corrected to get the evaporation from a lake under similar climatic and exposure conditions. Thus:
Lake evaporation = Cp x Pan Evaporation
Pan Coefficient, Cp
Evaporation Station
Arid Zones – 1 station for every 30,000 sq.km.
Humid Temperate Zones – 1 station for every 50,000 sq.km.
Cold regions – 1 station for every 1,00,000 sq.km.
A typical hydrological station contains the following:
Ordinary Rain gauge, Recording Rain gauge, Stevenson Box with maximum and minimum thermometer and dry and wet bulb thermometers, wind anemometer, wind direction indicator, sunshine recorder, thermo hydrograph and pan evaporimeter.
1. A class A pan was set up adjacent to a lake . The depth of water in the pan at the beginning of a certain week was 195mm. In that week there was a rainfall of 45 mm and 15 mm of water was removed from the pan to keep the water level within the specified depth range. If the depth of the water in the pan at the end of the week was 190mm calculate the pan evaporation. Using a suitable pan coefficient estimate the lake evaporation in that week.
Problem
2. A canal is 80 km long and has average surface width of 15 m. If the evaporation measured in a class A pan is 0.5 cm/day, what is the volume of water evaporated in a month?
Problem
3.4 Empirical Evaporation Equation
Abstraction from Precipitation
Most of the available empirical equations for estimating lake evaporation are a Dalton type equation of the general form:
EL = K f(u) ( ew - ea )
EL – lake evaporation in mm/day
1. Meyer’s Formula (1915)EL = KM ( ew - ea )( 1 + )
- monthly mean velocity in km/hr. at about 9m above the ground.
KM – 0.36 for large deep waters; 0.5 for small, shallow waters.
Empirical Evaporation Equation
2. Rohwers’s Formula (1931) - considers a correction for the effect of pressure in addition to
the wind speed effect.
Pa – mean barometric reading in mm Hg.
Uo – mean wind velocity in km/hr. at ground level, w/c can be taken to be the velocity at 0.6 m height above ground.
= wind velocity at height h above the
ground C = Constant
Empirical Evaporation Equation
a) A reservoir with a surface of 250 hectares had the following average values of climate parameters during a week : Water Temperature = 20 o C, Relative Humidity = 40%, Wind Velocity at 1.0 m above ground surface = 16 km/h. Estimate the average daily evaporation from the lake by using Meyer’s Formula.
b) An ISI Standard evaporation pan at the site indicated a pan coefficient of 0.80 on the basis of calibration against controlled water budgeting method . If this pan indicated an evaporation of 72 mm in the week under question, i.) estimate the accuracy if Meyer ‘s Method relative to the pan evaporation measurements. ii.) Also, estimate the volume of water evaporated in the lake in that week.
Problem
3.5 Analytical Methods of Evaporation
Abstraction from Precipitation
1.Water – Budget Method
2.Mass – Transfer Method
3.Energy – Balance Method
Analytical Methods of Evaporation
Water – Budget MethodWater – Budget Method – simplest but the least reliable. If the unit of time is kept very large, estimates of evaporation will be more accurate.
= daily precipitationVis = daily surface inflow into the lake
Vig = daily groundwater inflow
Vos = daily surface outflow from the lake
Vog = daily seepage outflow
EL = daily lake evaporation
= increase in lake storage in a dayTL = daily transpiration loss
P, Vis,Vos and ∆ can be measured.𝑆Vig, Vog and TL can only be estimated.
Energy – Budget Method
Energy – Budget Method – is application of law of conservation of energy. The energy available for evaporation is determined by considering the incoming energy, outgoing energy and energy stored in the water body over a known time interval.Fig. 3.4 Energy Balance in a
Water Body
Energy – Budget MethodH n = Ha + He + Hg + Hs + Hi
H n = net energy received by the water surface.
= Hc(1 – r) - Hb
Hc(1 – r) = incoming solar radiation into a surface of reflection coefficient(albedo) r.Hb = back radiation (long wave ) from water body.
Ha = sensible heat transfer from water surface to air.
Hc = heat energy used up in evaporation.
= Hg = heat flux into the ground.
Hs = heat stored in a water body.
Hi = net heat conducted out of the system by water flow
( advected energy)
Energy – Budget Method
Ha can be estimated as:
Pa = atmospheric pressure in mm Hg.
Tw = temperature of water surface in oC
Ta = temperature of air in oC
EL can be evaluated as
3.6 Reservoir Evaporation and Methods for its Reduction
Abstraction from Precipitation
Reservoir Evaporation and Methods for its Reduction
The Water Volume lost due to evaporation from a reservoir in a month is calculated as:
VE = A Epm Cp
VE = volume of water lost in evaporation in a month ( m3)A = average reservoir area during the month (m2)Epm = pan evaporation loss in metres in a month (m)
Cp = relevant coefficient.
Methods to Reduce Evaporation Losses Reduction of Surface
AreaAs the area increases
the rate of evaporation also increases.
Mechanical CoversPermanent roofs over
the reservoir, temporary roofs and floating roofs such as rafts and light-weight floating particles.
Chemical FilmsApplication of Cetyl
Alcohol (hexadecanol) and Stearyl alcohol (octadecanol) which forms layers on water surface to prevent water molecules to past them.
3.7 TranspirationAbstraction from Precipitation
TranspirationTranspiration is the process by which water leaves the body of a living plant and reaches the atmosphere as water vapor. The water is taken up by the plant-root system and escapes through the leaves.
The important factors affecting transpiration are :- atmospheric vapor pressure, - temperature, - wind, light intensity and characteristics of the plant, such as the root and leaf systems.
3.8 EvapotranspirationAbstraction from Precipitation
Evapotranspiration
Evapotranspiration - takes place at the land where plants exist; also lose moisture by the evaporation of water from soil and water bodies.
Potential Evapotranspiration (PET) - It is is defined as the amount of evaporation that would occur if a sufficient water source were available.
Actual Evapotranspiration (AET) - Actual evapotranspiration is the quantity of water that is actually removed from a surface due to the processes of evaporation and transpiration.
Field Capacity - is the maximum quantity of water that the soil can retain against the force of gravity. Any higher moisture input to a soil at field capacity simply drains away.
Permanent Wilting Point - is the moisture content of the soil at which the moisture is no longer available in sufficient quantity to sustain the plants.
Evapotranspiration Terms:
3.9 Measurement of Evapotranspiration
Abstraction from Precipitation
Measurement of Evapotranspiration
The measurement of Evapotranspiration for a given vegetation type can be carried out in two ways:1. Lysimeters - is a
measuring device which can be used to measure the amount of actual evapotranspiration which is released by plants, usually crops or trees. By recording the amount of precipitation that an area receives and the amount lost through the soil, the amount of water lost to evapotranspiration can be calculated.
Measurement of Evapotranspiration
The measurement of Evapotranspiration for a given vegetation type can be carried out in two ways: Field Experimental Plots - The
different elements of the water budget (other than ET) in a known interval of time are measured in special experimental plots established in the field. ET is then estimated as:
ET = Precipitation + Irrigation Input – Runoff – Increase in Soil Moisture Storage – Groundwater
Loss
3.10 Evapotranspiration Equations
Abstraction from Precipitation
Evapotranspiration Equations Penman’s Equation
PET = daily potential evapotranspiration in mm/dayA = slope of the saturation vapour pressure vs temperature curve at the mean air temperature, in mm Hg/ oC. Table 3.3Hn = net radiation in mm of evaporable water/ day.
Ea = parameter including wind velocity and saturation deficit. = psychrometric constant = 0.49 mm Hg/ oC
Evapotranspiration EquationsNet Radiation can be estimated by the following equation:
Ha = incident solar radiation outside the atmosphere on a horizontal surface, expressed in mm of evaporable water per day ( it is a function of the latitude and period of the years as indicated in Table 3.4)a = a constant depending upon the latitude and is given by a = 0.29cosb = a constant with an average value of 0.52n = actual duration of bright sunshine in hoursN = maximum possible hours of bright sunshine ( it is a function of latitude as indicated in table 3.5)
Evapotranspiration Equations r = reflection coefficient (albedo)
Stefan – Boltzman constant = 2.02 x 10-9 mm/dayTa = mear air temparature in degrees kelvin = 273 + oC
Ea = 0.35(1 + (ew - ea)
U2 = mean wind speed at 2m above ground in km/day
ew & ea were define earlier.
Surface Range of r values
Closed Ground Corps 0.15 – 0.25
Bare 0.05 – 0.45
Water Surface 0.05
Snow 0.45 – 0.95
Table 3.3 Saturation Vapour Pressure of WaterTemperature Saturation
Vapour Pressure
A
o C ew (mm Hg) (mm/ C)
0 4.58 0.30
5 6.54 0.45
7.5 7.78 0.54
10 9.21 0.60
12.5 10.87 0.71
15 12.79 0.80
17.5 15 0.95
20 17.54 1.05
22.5 20.44 1.24
25 23.76 1.40
27.5 27.54 1.61
30 31.82 1.85
32.5 36.68 2.07
35 42.81 2.35
37.5 48.36 2.62
40 55.32 2.95
45 71.20 3.66
(mm Hg)where t = temperature in oC
Table 3.4 Mean Monthly Solar Radiation at Top of Atmosphere, Ha in mm of Evaporation
Water/Day
North Latitude
Jan Feb Mar Apr May Jun Jul Aug Sept
Oct Nov Dec
0o 14.5
15.0
15.2
14.7
13.9
13.4
13.5
14.2
14.9
15.0
14.6
14.3
10o 12.8
13.9
14.8
15.2
15.0
14.8
14.8
15.0
14.9
14.1
13.1
12.4
20o 10.8
12.3
13.9
15.2
15.7
15.8
15.7
15.3
14.4
12.9
11.2
10.3
30o 8.5 10.5
13.7
14.8
16.0
16.5
16.2
15.3
13.5
11.3
9.1 7.9
40o 6.0 8.3 11.0
13.9
15.9
16.7
16.3
14.8
12.2
9.3 6.7 5.4
50o 3.6 5.9 9.1 12.7
15.4
16.7
16.1
13.9
10.5
7.1 4.3 3.0
Table 3.5 Mean Monthly Values of Possible Sunshine Hours, N
North Latitude
Jan Feb
Mar
Apr May
Jun Jul Aug
Sept
Oct Nov
Dec
0o 12.1
12.1
12.1
12.1
12.1
12.1
12.1
12.1
12.1
12.1
12.1
12.1
10o 11.6
11.8
12.1
12.4
12.6
12.7
12.6
12.4
12.9
11.9
11.7
11.5
20o 11.1
11.5
12.0
12.6
13.1
13.3
13.2
12.8
12.3
11.7
11.2
10.9
30o 10.4
11.1
12.0
12.9
13.7
14.1
13.9
13.2
12.4
11.5
10.6
10.2
40o 9.6 10.7
11.9
13.2
14.4
15.0
14.7
13.8
12.5
11.2
10.0
9.4
50o 8.6 10.1
11.8
13.8
15.4
16.4
16.0
14.5
12.7
10.8
9.1 8.1
Table 3.5 Mean Monthly Values of Possible Sunshine Hours, N
North Latitude
Jan Feb Mar Apr May
Jun Jul Aug Sept
Oct Nov Dec
0o 12.1
12.1
12.1
12.1
12.1
12.1
12.1
12.1
12.1
12.1
12.1
12.1
10o 11.6
11.8
12.1
12.4
12.6
12.7
12.6
12.4
12.9
11.9
11.7
11.5
20o 11.1
11.5
12.0
12.6
13.1
13.3
13.2
12.8
12.3
11.7
11.2
10.9
30o 10.4
11.1
12.0
12.9
13.7
14.1
13.9
13.2
12.4
11.5
10.6
10.2
40o 9.6 10.7
11.9
13.2
14.4
15.0
14.7
13.8
12.5
11.2
10.0
9.4
50o 8.6 10.1
11.8
13.8
15.4
16.4
16.0
14.5
12.7
10.8
9.1 8.1
Empirical Formulae for Evapotranspiration
Blaney – Criddle Formula. Assumes that PET is related to the hours of sunshine and temperature (these are measures of solar radiation in an area).PET (in cm) in a crop growing season.
ET = PET in crop season in cm
K = an empirical formula, depends on the type of the crop and stage of growth
F = sum of monthly consumptive use factors for the periodPh = monthly percent of annual day-time hours, depends on
the latitude of the place ( table 3.6) = mean monthly temperature in oF
Empirical Formulae for Evapotranspiration
Thornthwaite Formula. Uses only mean monthly temperature along with an adjustment for day length.
ET = monthly PET in cm
La = adjustment for the number of hours of daylight and days in the month, related to the latitude of the place (table 3.8) = mean monthly air temperature in oC. = the total 12 monthly valuesof heat index = where i = 1.514
a = an empirical constant
Estimate the PET of an area for the season November to February in which wheat is grown. The area is in North India at a latitude of 30o N with mean monthly temperature as below:
Problem
The End
Reporter: Macaspac, Jerrol R.Course/Section: BSCE-4DCollege: Technological University of the Philippines – Manila.S.Y. : 2nd Semester 2011 – 2012Subject : HydrologyInstructor: Engr. Juanito H. NericReference Book: Engineering Hydrology by K. Subramanya.
INTERCEPTION
Interception
When it rains over a catchment, not all the precipitation falls directly onto the ground, a part of it may be caught by the vegetation and subsequently evaporated. The volume of water so caught is called interception. Three possible routes of intercepted precipitation:
Interception loss- retained by the vegetation as surface storage and returned to the atmosphere by evaporation
Throughfall- drip off the plant leaves to ground surface or the surface flow
Stemflow- rainwater may run along the leaves and branches and down the stem to reach the ground surface
Interception
For a given storm, the interception loss is estimated as Where:
= interception loss in mm = interception storage whose varies from
0.25 to 1.25 mm depending on nature of vegetation = ratio of vegetal surface area to its
projected area = evaporation rate in mm/h during the
precipitation = duration of rainfall in hours
Interception
Coniferous trees have more interception loss than deciduous ones.
Dense grasses have nearly same interception losses as full grown trees.
Agricultural crops in their growing season also contribute to high interception losses.
Interception process has a very significant impact of ecology of the area related to silvicultural aspects.
However, in hydrological studies dealing with floods interception loss is rarely significant and is not separately considered.
DEPRESSION STORAGE
Depression Storage
When the precipitation of a storm reaches the ground, it must fill up all depressions before it can flow over the surface. The volume of water trapped in this depressions is called depression storage.
Depression Storage
Depression storage depends on a vast number of factors the chief which are: The type of soil Condition of the surface reflecting the amount
and nature of depression Slope of the catchment The antecedent precipitation, as a measure of
soil moisture
INFILTRATION
Infiltration Is the flow of water into the ground through the
soil surface such as rainfall, snowmelt or irrigation into the soil.
The infiltration process is a component in the overall unsaturated redistribution process.
Infiltration4 Moisture Zones Zone 1: At the top, a thin
layer of saturated zone is created.
Zone 2: Beneath zone 1, there is a transition zone.
Zone 3: Next lower zone is the transmission zone where the downward motion of the moisture takes place.
Zone 4: The last zone is the wetting zone. The moisture content in this zone will be at or near field capacity & the moisture content decreases with the depth.
Infiltration
An infiltration model:
Infiltration Capacity
The maximum rate at which a given soil at a given time can absorb water is defined as the infiltration capacity.
Designated as and is expressed in units of cm/h Actual rate of infiltration is expressed as
where: = intensity of rainfall
Infiltration Capacity The infiltration capacity of a soil is high at the
beginning of a storm and has an exponential decay as the time elapses.
The infiltration capacity of an area is dependent on a large number of factors, chief of them are: Characteristics of the soil (texture, porosity
and hydraulic conductivity) Condition of the soil surface Vegetative cover Current moisture content Soil temperature
Characteristics of Soil
The type of soil, viz. sand, silt or clay, its texture, structure, permeability and underdrainage.
When the soils occurs in layers, the transmission capacity of the layers determines the overall infiltration rate.
The land use has a significant influence on
Infiltration Capacity
Infiltration CapacitySurface of Entry
At the soil surface, the impact of raindrops causes the fines in the soil to be displaced and in turn can clog the pore spaces in the upper layers of the soil.
A surface covered with grass and other vegetation which can reduce this process has a pronounced influence on the value of .
Fluid Characteristics Water infiltrating into the soil will have many
impurities, both in solution and in suspension. Turbidity, temperature, contamination of water.
Measurement of Infiltration
Infiltration characteristics of soil can be estimated by: Using flooding type infiltrometers Measurement of subsidence of free
water in a large basin or pond Rainfall simulator Hydrograph analysis
Measurement of InfiltrationFlooding Type Infiltrometer are experimental devices used to obtain data
relating to variation of infiltration capacity with time
Two commonly used types: Tube type or Simple Infiltrometer Double Ring Infiltrometer
Measurement of Infiltration
Simple (Tube Type) Infiltrometer Consists of a metal cylinder,
30 cm in diameter and 60 cm long, open at both ends.
The cylinder is driven into the ground to a depth of 50 cm
Knowing the volume of water added during time intervals, the plot of infiltration capacity vs. time is obtained.
A major objection to the simple infiltrometer is that infiltered water spreads at the outlet from the tube
Measurement of Infiltration
Double- Ring Infiltrometer This commonly used
infiltrometer is designed to overcome the basic objection of the tube infiltrometer.
Two sets of concentrating rings with diameters of 30 cm and 60 cm and of a minimum length of 25 cm.
The two rings are inserted into the ground and water is applied into both rings to maintain a constant depth of about 5.0 cm.
Measurement of Infiltration
Rainfall Simulator In this a small plot of land, of
about 2m 4m size. Is provided with a series of nozzles on the longer side with arrangements to collect and measure the surface runoff rate.
The specially designed nozzles produce raindrops falling from a height of 2m and are capable of producing various intensities of rainfall.
The simulator give lower values than flooding type infiltrometers.
Measurement of Infiltration
Hydrograph Analysis Reasonable estimation of
the infiltration capacity o a small watershed can be obtained by analyzing measured runoff of hydrographs and corresponding rainfall records.
In this the evapotranspiration losses are estimated by knowing the land cover/ use of the watershed.
Modeling Infiltration Capacity
The figure shows a typical variation of infiltration with time.
Cumulative infiltration capacity is defined as the accumulation of infiltration volume over time period since the start of the process and is given by
Thus the curve vs. time is the mass curve of infiltration. It may be noted that
Modeling Infiltration Capacity Equations
HORTON’S EQUATION (1933) Horton expressed the decay of infiltration capacity with
time as an exponential decay given by for
Where: infiltration capacity at any time from the start of the rainfall initial infiltration capacity at final steady state infiltration capacity occurring at . Also known as constant rate or ultimate infiltration capacity Horton’s decay coefficient which depends upon soil characteristics and vegetation cover
Modeling Infiltration Capacity Equations
PHILLIP’S EQUATION (1957) Phillip’s two term model relates as
Where: a function of soil suction potential and called
as sorptivity Darcy’s Hydraulic conductivity
By the above equation infiltration capacity should be expressed as
Modeling Infiltration Capacity Equations KOSTIAKOV EQUATION (1932)
Kostiakov model expresses cumulative infiltration capacity as
where and are local parameters with and . The infiltration capacity would now be expressed as
GREEN- AMPT EQUATION (1911) Green and Ampt proposed a model based on
Darcy’s Law as
Where: porosity of soil capillary suction at the wetting front Darcy’s hydraulic conductivity
Estimation of Parameters of Infiltration Models
HORTON’S MODEL
Plot against and fit the best straight line through the plotted points. The intercept gives and the slope of the straight line is
PHILLIP’S MODEL
Plot the observed values of against on an arithmetic paper graph paper. The best fitting straight line through the plotted points gives as the intercept and as the slope of the line.
Estimation of Parameters of Infiltration Models
KOSTIAKOV MODEL
The data plotted as vs. and the best fit plotted points gives as intercept and the slope is.
GREEN AMPT MODEL
Values of are plotted against and the best fit straight line is drawn through the plotted points. The intercept and the slope of the line are coefficients and respectively.
Modeling Infiltration Capacity Example 1:
Infiltration capacity data obtained in a flooding type infiltration test is given below:
a. For this data plot the curves of i. Infiltration capacity vs. time ii. Infiltration capacity vs. cumulative infiltration, and iii. Cumulative infiltration vs. time
b. Obtain the best values of the parameters in Horton’s infiltration capacity equation to represent the data set.
Time since start (min)
5 10 15 25 45 60 75 90 110 130
Cumulative infiltration depth (cm)
1.75
3.0
3.95 5.5 7.25
8.3 9.3 10.2
11.28 12.36
Modeling Infiltration Capacity
Example 2:The infiltration capacity in a basin is
represented by Horton’s equation as
Where is in cm/h and is in hours. Assuming the infiltration t take place at capacity rates in a storm of 60 minutes duration, estimate the depth of infiltration in (i) the first 30 minutes and (ii) the second 30 minutes of the storm.
Classification of Infiltration Capacities
In this method, the soils are considered divided into four groups known as hydrologic soil groups. The steady state infiltration capacity, being one of the main parameters in this soil classification, is divided into four infiltration classes.
Infiltration Class
Infiltration Capacity (mm/h)
Remarks
Very Low 2.5 Highly Clayey Soils
Low 2.5 to 25.0 Shallow soils, Clay Soils, Soils low in organic matter
Medium 12.5 to 25.0 Sandy loam, Silt
High 25.0 Deep sands, well drained aggregated soils
Infiltration Indices In hydrological calculations involving floods I is
found convenient to use a constant value of infiltration 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.
- INDEX Is the average rainfall above which the rainfall
volume is equal to the runoff volume.
Duration of rainfall:
Total rainfall:
Infiltration Indices Example 3:
A storm with 10 cm of precipitation produced a direct runoff of 5.8 cm. The duration of the rainfall was 16 hours and its time distribution is given below. Estimate the - index of the storm.
Time from start (h)
0 2 4 6 8 10 12 14 16
Cumulative rainfall (cm)
0 0.4
1.3 2.8
5.1
6.9 8.5 9.5 10.0
Infiltration Indices- INDEX
Where total storm precipitation (cm) total storm runoff (cm) initial losses (cm) duration of the rainfall excess, i.e. the total
time in which the rainfall intensity is greater than (in hours) and
defined average rate of infiltration (cm)
The End
Reporter: Dinglasa, Sherylyn Joy.Course/Section: BSCE-4DCollege: Technological University of the Philippines – Manila.S.Y. : 2nd Semester 2011 – 2012Subject : HydrologyInstructor: Engr. Juanito H. NericReference Book: Engineering Hydrology by K. Subramanya.
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