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5316 Environmental Hydrology
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Presentation
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Definition:
In
hydrology precipitation is
any form of water like rain,
snow, hail and sleet derived
from atmospheric vapor,
falling to the ground.
OR What ever water reaches to earth from the atmosphere is called precipitation.
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Precipitation is one of the most important events of hydrology.
Floods and droughts are directly related to the occurrence of
precipitation. Water resources management, water supply schemes,
irrigation, hydrologic data for design of hydraulic structures and
environmental effects of water resources development projects are
related to precipitation in one way or the other. So it is important to
study various aspects of precipitation.
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• Factors determining precipitation or the amount of atmospheric moisture over a region
a. Climate
b. Geography
• Ocean surfaces is the chief source of moisture for precipitation
Formation of Precipitation
• Precipitation is the basic input to the hydrology.
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Precipitation
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Drizzle:
• Drizzle are the minute particles of water at start of rain.
• Drizzle has diameter under 0.02 inch.
• Intensity is usually less than 0.4 inch / hour.
• Its speed is quite slow and they are normally evaporated rather
than flowing on the surface.
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Rain:• Rain is the most common type of precipitation.• The size of drops is more than 0.02 inch and less than 0.25 inch
in diameter.• Flow is generally produced on the ground by it if the rate of
rainfall is more than the rate of infiltration of soil.Glaze:• It is the ice coating formed on the drizzle or rain drops as it
comes in contact with the cold surfaces on the ground.Sleet:• It is the frozen rain drops cooled to the ice stage while falling
through air at subfreezing temperatures
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Snow:• Snow is the precipitation in the form of ice crystals resulting from
sublimation i.e. from water vapor to ice directly• Often the warm surface of earth melts the freshly fallen snow.• However, if the Earth’s surface is cold, the snow can accumulate on
the groundSnowflake:• It is made up of a number of ice crystals fused togetherHails:• Precipitation in the form of balls or lumps of ice over 0.2 inch
diameter• Formed by alternate freezing and thawing as the particles are carried
up and down in highly turbulent air currents• Their impact is also more as compared to other forms of precipitation.
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Precipitation is formed from water vapor in the
atmosphere.
Factors influencing precipitation formation.
i. Mechanism of cooling
ii. Condensation of water vapors
iii.Growth of droplets
iv.Accumulation of moisture
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Mechanism of Cooling:
• There reduction in pressure when air ascends from the surface of
earth to upper levels in the atmosphere is the only mechanism
capable of producing the degree and rate needed to account for
heavy rainfall.
• The capacity of a given volume of air to hold a certain amount of
water vapors is lowered due to cooling.
• Super saturation is known to occur in the atmosphere so the excess
moisture over saturation condenses through the cooling process
which ultimately results in precipitation.
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Condensation of Water Vapors:
• It normally occurs when there is 100% relative humidity and a
condensation nuclei having an affinity for water is present.
• Sources of these condensation nuclei are the particles of sea salt,
carbon dioxide and the sulfurous and nitrogenous oxides emanating
from surface of the earth into the atmosphere.
• There appears to be always sufficient nuclei present in the
atmosphere.
• Condensation will always occur in air the lower atmosphere is
cooled to saturation, often before the saturation point is reached.
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Growth of Droplets:It is required so that the liquid particles present in the clouds can reach the groundThere are two process regard as most effective for droplet growth.
i. Coalescence of droplets through collision.
ii. Co-existence of ice crystals and water droplets.i. Coalescence of droplets through collision:
• Coalescence of droplets through collision due to difference in speed
of motion between larger and smaller droplets results in the growth
of the droplets.
• The growth of the droplets increases their weight as a result of
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which their rate of fall is also increased whereby more collision
with other droplets and more growth of droplets takes place.
ii.Co-existence of ice crystals and water droplets.• The growth of droplets is also achieved by their co existence with
the ice crystals.• This generally happens in a temperature range of 10oF to 20oF• Bergeron’s theory.
When ice crystals and water droplets co exist in a cloud, an imbalance is caused due to lower saturation vapor pressure over ice as compared to water this results in the evaporation of water droplets and condensation of much of these droplets on ice crystals also causing their growth and ultimate fall through clouds.
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• Accumulation of moisture:• Heavy rainfall amount over a river basin exceed by far the amount of
water vapor at the atmospheric volume vertically above the basin at the beginning of the rainfall.
• Convergence:The net horizontal influx of air per unit area is called convergence.
• Clearly there must be a large net horizontal inflow of water vapor into the atmosphere above the basin area.
• The moisture added to the atmosphere over a basin may be transported very large distance in the lowest layer of the atmosphere. When this moist current reaches a region of active. Vertical motion it rises thousands of feet and loses much of its contained water vapor in just a few hours.
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i. Convectional precipitation
ii. Orographic Precipitation
iii. Cyclonic Precipitation
Convectional precipitation:-
• Earth becomes heated due to solar energy.
• Air when comes to contact with heated earth becomes lighter than the air around it.
Causes of precipitation
Causes of precipitation is classified into following types based upon the lifting mechanism. Cycloni
c
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• Its Creates atmospheric instability and the laps rate increase near the
earth surface increase rapidly.
• Lighter air rises by convection , potentially causing convective
precipitation.
• In convectional precipitation the main element is thermal convection
of the moisture laden air.
• Source of heat is only the solar radiation and it heats the major
portion of the earth.
• Air when comes into a low pressure atmospheric system also creats convection precipitation.
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gy Causes of precipitation
Orographic Precipitation:-• Moisture laden air masses are lifted by contact with Orographic
barriers and it occurs expansion and condensation.
• Orographic precipitation is most pronounced on the windward side of mountain range, generally heaviest precipitation occurs where favorable Orographic effects are present.
• Heaviest precipitation due to south easterlies in the subcontinent
occurs along the Southern slopes of Himalaya and its other ranges.
• Monsoon rainfall (June to October) decreases gradually as the
distance from the line of heaviest rainfall increases.
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Cyclonic Precipitation:-
• Precipitation in plain regions is generally cyclonic in character and
depending upon whether they occur within or beyond the tropics it
is divided into further two types.
• Tropical
• Extra tropical
• Precipitation in the Indo-Pak subcontinent are of tropical variety.
• In ward maritime air-mass of low latitude in high temperature
tropical cyclones are violent storms formed. These are known as
typhoons or cyclones.
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• In the center of the cyclonic storm there is small low pressure air.
The isobars around such a low pressure are very nearly circular in
shape and generally greater than the extra tropical cyclones.
• Tropical cyclones have a average diameter of over 300 to 400 miles
and the wind speed around cyclones may be as high as 60 to 90
miles per hour.
• In September, October and November these storms are very
destructive in Bangladesh and cause considerable loss of life and
property over the coastal districts.
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gy Distribution of precipitation over different types of surfaces
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gy Precipitation data and its analysis
• Precipitation Data
• Precipitation data are necessary for most land use plans and for hydrologic planning like water for human, agriculture, disposal of waste water and the control of excess rainfall .
• In agriculture precipitation data can be indicate when and where a lack or a surplus of water for crops may be expected.
• State and federal government have collected and published precipitation data and it is available in some libraries, in electronic tapes or CD’s and now it is also available on the Web.
• Special reports are published on special occasions like flood events of major importance, rainfall rate duration frequency or droughts.
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• Analysis of precipitation data
• Point Data Analysis
• Point precipitation data refers to precipitation of a station, data
could be in form of hourly record, daily record, monthly
precipitation or annual precipitation.
• Depending upon the nature of catchment and its area, there could be
as many gauging stations as feasible. Before using records from a
rain gauge check its continuity and consistency.
• Record may not be continuous and consistent due to many reasons
• Now we will discuss the checking consistency of data and hence its
adjustment accordingly.
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Estimation Of Missing Precipitation Record• The precipitation record should be complete but due to absences of
the observer or because of instrumental failures Some precipitation stations may have short breaks in the records.
• U.S. Weather Bureau, estimates the missing precipitation of a station from the observations of precipitation at some other stations as close to and as evenly spaced around the station with the missing record as possible.
• The station whose data is missing is called interpolation station and gauging stations whose data are used to calculate the missing station data are called index stations.There are two methods for estimation of missing data.
i. Arithmetic mean method
ii. Normal ratio method
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• If the normal annual precipitation of the index stations lies within 10% of normal annual precipitation of interpolation station then arithmetic mean method applies otherwise the normal ratio method is used for this purpose .
Consider that record is missing from station “X”
Now let
N= Normal annual precipitation.
(Mean of 30 years of annual precipitation data)
P = Storm Precipitation.
Let Px is the missing precipitation for station “X” and Nx is normal annual precipitation of station this station, Na, Nb & Nc are normal annual precipitations of near by three stations, A, B and C respectively and Pa, Pb, Pc are the storm precipitation of that period for these stations.
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Now we have to compare Nx with Na , Nb and Nc separately. If difference of Nx – Na, Nx - Nb, Nx – Nc is with in 1/10% of Nx then we use, simple arithmetic mean method otherwise the normal ratio method
Simple Arithmetic Mean Method According to the arithmetic mean method the missing precipitation is
given as Px = where n is number of nearby stations.
In case of three stations 1, 2 and 3 Px = (P1 + P2 + P3)/3 and naming stations as A, B and C instead of 1, 2 and 3
Px = (Pa + Pb + Pc)/3
Pi
ni
1i
n
1
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Normal Ratio Method
According to the arithmetic mean method the missing precipitation
is
Px =
where Px is the missing precipitation for any jth period at the
interpolation station “X”. Pi is the precipitation. for the same period
at the “ith” station of a group of index stations and “Nx” and Ni are
the normal annual precipitation values for the X and ith stations e.g.
Px =
PN
Nn i
ni
i i
x
1
1
PN
Nn i
ni
i i
x
1
1
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gy Consistency of Precipitation Data
• Using precipitation in the solution of hydrologic problems, it is necessary to ascertain that time trends in the data are due to meteorological changes.
• Quite frequently these trends are the result of the changes in the gauge location are not disclosed in the published record, changes in the intermediate surroundings such as construction of buildings or growth of trees, etc. and changes in the observation techniques.
• The consistency of the record then is required to be determined and the necessary adjustments be made. This can be achieved by the method called the double mass curve technique.
• The double mass curve is obtained by plotting the accumulated precipitation at the station in question along X-axis and the average accumulated precipitation of a number of other nearby stations which are situated under the same meteorological conditions along Y-axis.
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If the curve has a constant slope, the record of station “X” is
consistent. However, if there is any break in the slope of the curve,
the record of the station is inconsistent and has to be adjusted by the
formula
Pa = (Sa / So) x Po
Where
Pa = Adjusted precipitation.
Po = Observed Precipitation .
Sa =Slope prior to the break in the curve
So =Slope after the break in the curve.
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gy Precipitation Events
• A storm is described by several key parameters and total amount of
precipitation or depth, usually in inches and millimeters or
centimeters.
• Duration:-
• The time from the beginning of the storm until the end of the storm is
called duration.
• The average rate of precipitation or intensity, is found by dividing the
amount of precipitation during a given period by the length of that
period and it is measured in inches, millimeter or centimeter per
hour.
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Precipitation events can be divided into three types
i. Geographical And Seasonal Variation
ii. Historic Time Trends
iii.Storm Area Patterns
Geographical And Seasonal Variation
Annual precipitation is different in the different countries
For Example:
In contiguous U.S average annual precipitation is 30 in.(75cm) but there is great spatial variation amounts and seasonality across the country due to availability of moisture , temporal variance , difference of precipitation mechanism.
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U.S. Annual Precipitation
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In the figure mountains of
extreme pacifc northwest,
mean annual mountains of the
extreme pacific mean annual
precipitation is upto 140 in. is
moving wet and cool air
masses, midlatitude cyclones,
and orographic lifting over
mountains.
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Historic Times Trends:-
Climate tends to fluctuate in cycles even now there is concern about
long term global warming with unknown regional effects on
precipitation. Over's the past few the past few hundred years
precipitation has tended fluctuate in cycles of about 3, 7, 15 to 20 and
100 or so years. Storm Area Patterns:-
Rainfall amounts, duration, and intensities vary spatially within the
area covered by a given storm. Large area storms such as large frontal
system, tend to be more uniform in distribution and have longer
durations.
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Amount of Precipitation:-The amount of precipitation means the vertical depth of water that would accumulate on a level surface, if the precipitation remains where it falls. The amount of precipitation is measured in length units (inches, ft., cm, etc.).
Intensity of precipitation:-Amount of precipitation per unit time is called the intensity of precipitation.
Both the amount and rate or intensity of precipitation are important in hydrologic studies.
The precipitation is measured by rain gauges. Types of rain gauges:-i. Non-recording rain gauge. (Standard rain gauge)ii. Recording rain gauge
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Non-recording rain gauge
• In non recording or standard rain gauges observer has to take readings and he has to record the time also for calculation of intensity of rain fall.
• The standard gauge of U.S. Weather Bureau has a collector of 8 inch diameter.
• Rain passes from a collector into a cylindrical measuring tube inside the overflow can.
• Its cross sectional area is 1/10th of the collector, so that 0.1 inch rain fall will fill the tube to 1 inch depth.
• A measuring stick is inside it, which measures up to 0.001 inch.
• When snow is expected the collector and tube are removed. The snow collected in the outer container or over flow can is melted, poured into the measuring tube and then measured.
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Recording rain gauge
Recording rain gauges gives the rain recorded automatically with
respect to time, so intensity of rain fall is also known. Now these rain
gauges are also used it is of several types
Types of recording rain gauges.
i. Float type
ii. Weighing type
iii.Tipping bucket type
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Float type • This type of rain gauge has a receiver and a float chamber along
with some recording arrangement.
• In this type the rain is led into a float chamber containing a light.
• The vertical movement of the float as the level of water rises is recorded on a chart with the help of a pen connected to float. T
• he chart is wrapped around a rotating clock driven drum.
• To provide a continuous record for 24 hours some automatic means are provided for emptying the float chamber quickly when it becomes full, the pen then returning to the bottom of the chart.
• Siphoning arrangement activates when the gauge records a certain fixed amount of rain (mostly o.4 inches of rainfall.). Snow can not be measured by this type of rain gauge.
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Weighing type• The weighing type rain gauge consists of a receiver, a bucket, a
spring balance and some recording arrangement.
• The weighing type gauge weighs the rain or snow which falls into a bucket which is set on a lever balance.
• The weight of the bucket and content is recorded on a chart by a clock driven drum.
• The record is in form of a graph one axis of which is in depth units and the other has time.
• The records show the accumulation of precipitation. Weighing type gauges operate from 1 to 2 months with out stop. But normally one chart is enough only for 24 hours. The receiver is removed when snow is expected. Snow can be measured by this type of rain gauge.
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Tipping bucket type• This type of gauge used at some weather bureau first order stations
is equipped with a remote recorder located inside the office which is away from the actual site.
• The gauge has two compartments pivoted in such a way that one compartment receives rain at one time.
• This type of gauge is not suitable for measuring snow without heating the collector. Plotting is similar to that of other recording rain gauges.
TIPPING BUCKET TYPE RAIN GUAGE
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gy Average Precipitation over an Area
• Conversion of point precipitation of various gauging stations into average precipitation of that area a great experience and skill is required.
• There are three methods to find average precipitation over a basin.
• Accuracy of estimated average precipitation will depend upon the choice of an appropriate method.
• Methods to determine average precipitation.
i. Arithmetic Mean Method
ii. Thiessen Polygon Method
iii.Isohyetal method
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Arithmetic Mean Method• In this method the precipitation over area is the arithmetic average
of the gauge precipitation values.
• Data is taken from only those stations which are within the boundary.
• This is the simplest method but only be applicable when
• Basin area is flat
• All stations with in practical limits are uniformly distributed over the area.
• The rainfall is also nearly uniformly distributed over the area.
P (average) =
n
iiPn 1
1 Where I = Station
Pi = Precipitation N = Number of gauges
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Thiessen Polygon Method:- The following steps are used in Thiessen Polygon Method.
• Draw the given area according to a certain scale and locate the stations where measuring devices are installed.
• Join all the stations to get a network of non-intersecting system of triangles.
• Draw perpendicular bisectors of all the lines joining the stations and get a suitable network of polygons, each enclosing one station. It is assumed that precipitation over the area enclosed by the polygon is uniform.
• Measure area of the each polygon.
• Formula for the Average precipitation of the whole basin.
P (average) = (P1 A1 + P2 A2 + ...+ Pn An)/A
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Where
• P1 = Precipitation at station enclosed by polygon of area A1
• P2 Precipitation. at station enclosed by polygon of area A2
and so on
• Pn = Precipitation at station enclosed by polygon of area An
• A represents the total area of the catchment.
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Isohyetal method:-
• Draw the map of the area according to a certain scale.
• Locate the points on map where precipitation measuring
instruments are installed.
• Write the amount of precipitation for stations.
• Draw isohyets (Lines joining points of equal precipitation).
• Measure area enclosed by every two isohyets or the area enclosed
by an isohyet and boundary of the catchment.
• Average precipitation formula
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P(average) = (P1 A1 + P2 A2 + ...........+ Pn An)/A
Where
P1= Average precipitation of two isohyets 1 and 2
A1= Area between these two isohyets.
P2 = Average precipitation of two isohyets 2 and 3
A2 = the area b/w these two isohyets.
And so on
Pn = Average precipitation of isohyets n-1 and n
An = the area b/w these two isohyets.
Note:
The last and first areas mentioned Should be between an isohyet and boundary of the catchment.
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Rainfall Frequency Distribution:-Hydrologist need to estimate the probality that a given rainfall event will occur to assistant planners in determining the likelihood of the success or failure of a given project
Parametersi. Durationii. Intensityiii.Return period i. Duration
The time from the beginning of the storm to the end is called duration.
ii. IntensityAmount of precipitation per unit time is called the intensity of precipitation.
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iii. Return period:-
The return period is the average period of he time in years expected either between high intensity storm or between very dry periods.
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Figure from weather web for frontal rainfall
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gy Precipitation and Flow
Types Of Flow
i. Ground Water Flow
ii. Shallow Subsurface
Flow
iii. Horton Overland
Flow
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Water movement in wet and dry grains
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gy Groundwater Zones
Vertical Zones Of Subsurface Water:-
There are three vertical zones of subsurface water.
i. Soil water Zone:-
ii. Vadose Zone:-
iii. Capillary Zone:-
Soil water Zone:-
• Extends from the ground surface down through the major root
zone.
• Its thickness is usually a few but varies with soil type and
vegetation.
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Vadose Zone:-
• Vadose zone is also called unsaturated zone.
• Extends from the surface to the water table through the root zone ,
intermediate zone and capillary zone.
Capillary Zone:-
• Capillary zone extends from the water table up to the limit of
capillary rise.
• It varies Inversely to the pore size of the soil and directly with the
Surface tension.
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Water Table:-
The level to which water will rise in a well drilled into the saturated
zone.
Saturated Zone:-
Occurs beneath the water table where porosity is direct measures of
the water contained per unit volume.
Porosity:-
Porosity average about 25% to 35% for most aquifer system.
Expressed as the ratio of the volume of voids to the total volume.
n = Vv/V
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Unconfined aquifer:-An aquifer where the water table exists under atmospheric as defined by levels in shallow wells.
Confined Aquifer:-An aquifer that is overlain by a relatively impermeable unit such that the aquifer is under pressure and the water level rises above the confined unit.
Potentiometric Surface:-
In a confined aquifer, the hydrostatic pressure level of water in the
aquifer, defined by the water level that occurs in a lined penetrating
well
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Leaky confined aquifer:-represents a stratum that allows water to flow from above through a leaky confining zone into the underlying aquifer.
Perched aquifer:-Occurs when an unconfined water zone sits on top of a clay lens, separated from the main aquifer below.
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gy Hydrologic Equation
• The hydrologic equation is a statement among components of
Hydrologic Cycle.
• It states that “Rate of Inflow minus the Rate of Outflow is equal to
the Rate of change of storage”
I-O = Δs/Δt
where I = Rate of Inflow,
O = Rate of Outflow,
Δs/Δt = Rate of Change of Storage
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Components of Inflow:• Precipitation• Import of water by channeling it into any given area• Groundwater Inflow from an adjoining area
Components of Outflow:• Surface runoff outflow• Water channeled out of an area for Irrigation etc.• Evaporation• Transpiration• Interception
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Change in Storage: This occurs as change in
• Groundwater
• Surface Reservoir water and Depression storage
• Detention storage
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• Calculations regarding the incoming, outgoing and changes in water
quantities inside a catchment show its water budget.
• This can be done by applying the Hydrologic equation to a
catchment area.
• Inflow can be the precipitation “P” on the ground surface
• Outflow consists of Interception Losses “Li”, Surface Runoff “R”
and Evaporation “E”
• The storages are Infiltration “F” and Depression Storage “D”
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• The Hydrologic equation can thus be expressed as
P – (Li +R + E) = D + F
or R = P – (Li+ E + D + F)or R = P – L
where “L” represents all losses• If all quantities on the right hand side can be measured, the surface
runoff of a given catchment in response to known precipitation can easily be measured.
• Very difficult to measure the exact quantities so relationships are developed and on the basis of these relationships, different quantities are estimated.
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Thanks
