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HYDROLOGY
Unit 1 - Module 2 (Hydrological, Fluvial, Coastal and
Limestone Environments)
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CONTENT
The hydrological cycle
The storm hydrograph and water balance
Drainage basin characteristicsDrainage patterns and drainage density
Mapwork
2
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Concepts associated with the hydrological cycle and the river basin
Major flows and factors influencing flows within the hydrological cycle
THE HYDROLOGICAL CYCLE
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THE HYDROLOGICAL CYCLE
Describes the continuous
movement of all forms of water
(vapour, liquid and solid) on, in,and above the Earths surface.
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MAIN CONCEPTS
An area of land that is drained by a river andits tributaries is known as a drainage basin,and its boundary is marked by a watershed
a ridge of higher ground, beyond which anywater will drain into an adjacent basin.
The water balance of a drainage basin is animportant concept. Changes in the waterbalance are shown by storm hydrographsand river regimes.
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A SYSTEMS APPROACHThis is a common approach in geography and the twomain examples in this topic are:
The hydrological cycle:a closed system.
6
Both consist oftransfers, stores,
inputs of water but
the hydrological
cycle is a closed
system as no gains
or losses from
outside are added to
the system.
ga.water.usgs.gov/edu/charts/waterdistribution.gif
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A SYSTEMS APPROACH
The drainage basin system:an opensystem.
The drainage basin system is said to be
open as both inputs and outputs of energyand material occur.
All systems in their natural state aim to be in
a state of balance (dynamic equilibrium) asthis is when they function best.
Heavy rainfall, drought and human activitysuch as deforestation can easily upset the
balance. 7
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A SYSTEMATIC REPRESENTATION OF THE
HYDROLOGICAL CYCLE
8
The Atmosphere
Channel
Flow
Groundwater
FlowBaseflow
Throughflow
Surface Runoff
(Overland
Flow)
Percolation
Throughfall
Stemflow
Infiltration
Variabl
e level
(water table)
Transfer
Key:
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CHANGES OVER TIME
Only during the ice ages are there noticeabledifferences in the location of water storage onthe earth. During these cold cycles, there is less
water stored in the oceans and more in icesheets and glaciers.
It can take an individual molecule of water froma few days to thousands of years to complete
the hydrologic cycle from ocean to atmosphereto land to ocean again as it can be trapped inice for a long time.
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WORLD WATER SUPPLY BY LOCATIONOceans - 97.08%
Ice Sheets and Glaciers - 1.99%
Ground Water - 0.62%
Atmosphere - 0.29%
Lakes (Fresh) - 0.01%
Inland Seas and Salt Water Lakes - 0.005%
Soil Moisture - 0.004%
Rivers - 0.001%
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DRAINAGE BASIN PROCESSES ABOVE THESURFACEPrecipi tat ion
This may be defined as water in solid or liquid form which resultsfrom the condensation of water vapour in the atmosphere andwhich accumulates in clouds and falls to the earth as rain, snow,ice, hail or sleet.
The precipitation input is an important factor affecting how riversbehave. Depending upon the size of the drainage basin,precipitation totals will influence the input (the potential amount ofwater which can enter a system) and the output (the eventualstreamflow).
The basic understanding is that precipitation varies over space
and time. Important precipitation factors:1. Where and how much? (location and magnitude)
2. When and how often? (Regime/seasonality and frequency)
3. How heavy? (Intensity)
4. What type? (Form)
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DRAINAGE BASIN PROCESSES ABOVE THESURFACEIntercept ion
The second stage of the precipitation input to adrainage basin system begins once the water hasarrived at the land surface.
The amount of incoming precipitation which reachesthe ground surface directly depends not only upon itstype, volume, intensity and timing, but also upon thesurface cover. This may be artificial cover, such as
roads and buildings, but is mainly natural or cultivatedvegetation.
The interruption in the arrival of precipitation at thesurface is known as interception.
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THROUGHFA
LLwater
drips off theleaves and
twigs
THROUGHFLO
W - water fallsthrough the
spaces in the
vegetation
STEMFLOWwater trickles
along branches,
down the trunk,
etc
INTERCEPTION LOSS
water is held on the plant
and evaporates back into the
atmosphere
INFILTRATION
vegetation encourageswater to infiltrate the soil
NBThe type
of vegetation
cover will
determine the
interception
characteristics
(e.g.
evergreen vs.
deciduous
forests)
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DRAINAGE BASIN PROCESSES ABOVE THESURFACEEvapo ration and Transpirat ion
Some of the precipitation input does notbecome streamflow. Instead, it is lost from thesystem by the process of evaporation. Thisevaporated part of the precipitation input isreturned directly to the atmosphere.
Equally important in many environments istranspiration. Although this water has reached
and penetrated the ground surface, it has beentaken up by plants and so has not movedthrough the drainage basin.
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DRAINAGE BASIN PROCESSES ABOVE THESURFACEEvapotranspirat ion
Water lost from vegetation via both
evaporation and transpiration.
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A physical process where moisture is lostdirectly to the atmosphere from soil andwater surfaces due to the suns heat
Evaporation
A biological process where water is lostfrom stomata pores in plant leavesTranspiration
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Temperature,
wind and cloud
cover affects
evaporation
rates
Evaporation
of
interception
loss
Transpiratio
n from the
stomata
Soil moisture provides a constant supply of water all year round
Evaporation from rivers andlakes occur all year
PETthe water loss that
would occur from an area if
there was a constant supplyof water to the surface and
the transpiring vegetation.
Thus, PET is the maximum
possible water loss for a
particular environment.
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DRAINAGE BASIN PROCESSES AT AND BELOWTHE SURFACEInf i l t rat ion
Infiltration is the process whereby waterenters the soil surface.
Hydrologists usually study this watermovement by measuring the infiltration rate(how much water is passing through in a
certain time) and infiltration capacity (themaximum rate at which a particular soilunder specific conditions can absorbprecipitation) of the water.
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FACTORS INFLUENCING THE AMOUNT OF INFILTRATION
1. Intensity of
precipitationRainfall of great intensity, i.e.
Downpour, is less likely toinfiltrate then low intensity
rainfall, e.g. drizzle.
2. Vegetation coverVegetation helps to break up
the soil, increasing air space,
which the water can infiltrate.
3. Angle of slopeWater will run off a steeper
slope more easily than a
gentle slope. The quicker the
water runs off, the less likely
it is to infiltrate
4. Nature of the soil androck typeThe size of the soil and rock
particles, the amount of air
space and cracks affect
infiltration. A sandy soil has
larger particles and more air
spaces than a clay soil. Thisencourages infiltration
5. Depth of the water
tableIf the water table is near to
the surface, the soil will
become quickly saturatedand less infiltration will
6. Time
If rainfall occurs over a long
period of time, infiltration will
decrease as the soil store
fills up, i.e. High antecedentmoisture conditions
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DRAINAGE BASIN PROCESSES AT AND BELOWTHE SURFACEThroughf low
Water which does infiltrate the soil will move verticallydownwards at first.
Then movement swings progressively downslope due
to the effects of gravity, and the decrease ininfiltration capacity of the soil with increasing depth:they have fewer spaces and cracks in the lowerhorizons of the profile.
Unless the soil contains many spaces, root systemsand animal burrows, throughflow is a slow process(between 0.01mm and 1mm per minute), however,the water eventually arrives at the slope base.
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DRAINAGE BASIN PROCESSES AT AND BELOWTHE SURFACEPercolat ion
Some water will continue downwards to the
water table by the process of deep
percolation.
At the water table it becomes part of the
groundwater store
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DRAINAGE BASIN PROCESSES AT AND BELOWTHE SURFACEOverland f low
Water which cannot infiltrate collects on the groundsurface in any hollows and depressions as depressionstorage.
If these hollows become full, then the water may flow overthe ground surface in trickles, rivulets and even thinsheets as overland flow.
Vegetation-covered surfaces have a high infiltrationcapacity and, consequently, overland flow is relatively
rare under natural conditions. Human activities which result in soil compaction, e.g. the
passage of farm machinery or trampling by animals, canincrease overland flow.
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Main Concepts
Spatial and Temporal Changes
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THE STORM HYDROGRAPH AND WATERBALANCE
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THE WATER BALANCE
The water balance of a drainage basin is animportant concept.
Changes in the water balance of a basin are
shown by storm hydrographs and riverregimes.
The human impact on the hydrological cycle
can be seen in examples of deforestation,the construction of large dams and irrigation.
There are also contrasts between water usesin developed and developing countries.
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STREAMFLOW
Streamflow is generated by the outputs from thestores in the drainage basin system.
It occurs when the stores fill up or when they havesufficient water to release it steadily. The stores
release water at different rates and at different times.
Also, the processes which deliver the water to theriver channel operate at different speeds.
At times of extreme conditions, water inputs may notenter a store, but move directly to the stream byoverland flow.
As a result, streamflow fluctuates constantly.
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STREAMFLOW
We measure this varying streamflow in two ways:
Discharge(Q) is the volume of water passing a specificgauging station per unit of time. Discharge is expressed
as cubic meters of water per second (m
3
/s), oftenabbreviated as cumecs.
Runoff is the volume of water passing a gauging station,represented bas the thickness of water spread over the
drainage basin area above the gauging station. Runoff isexpressed as millimetres per month or year. Measuringrunoff allows us to compare the amount of waterdischarged by a river system with the precipitation inputsover the drainage basin.
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THE STORM HYDROGRAPH
A storm hydrograph records the discharge pattern ofa river at a specific gauging station, following a singlerainstorm event. In order to show the relationshipbetween the precipitation input and the discharge ofthe water past the gauging station, most stormhydrographs include the rainfall graph.
This relationship is important to the hydrologistbecause it determines the speed and scale of the risein discharge, and therefore the likelihood of flooding.
A hydrograph only describes what happened tostreamflow. We need to be able to interpret the graphin order to explain what happened, in turn predictingand forecasting what might happen. Drainage basins,however, change constantly over time and space.
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HYDROGRAPH TERMINOLOGY
Rising limbthe steep ascending early part of thehydrograph
Receding limbthe descending part of the hydrograph
Peakthe maximum discharge level
Lagthe delay between peak rainfall and peakdischarge
Baseflowlow water conditionswhen groundwaterfeeds the river
Quickflowthe main contributor to the rising limb
Impermeablegeological term for rocks in a drainagebasin that would probably give a hydrograph with a highpeak
Throughflowwhen water that has passed through the
soil feeds the hydrograph 28
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THE STORM HYDROGRAPH CHANGES IN
DRAINAGE BASIN CHARACTERISTICS
Stores and Flows
The antecedent moisture conditions will
influence how a drainage basin responds to
a rainfall event.
Thus, the same storm may cause a different
discharge response and hydrograph pattern
at different seasons.
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THE STORM HYDROGRAPH CHANGES IN
DRAINAGE BASIN CHARACTERISTICS
Rainfal l Intensity
Steady rainfall, even over several days, will allow thevarious water stores to fill up gradually and efficiently.
This controls the speed and volume of runoff to the
stream channels. This will be reflected in a broad, flathydrograph.
If, however, the precipitation input is intense andexceeds the soil infiltration and vegetationinterception capacities, quickflow processes e.g.overland flow, dominate, even when the basin storesare not full. Discharges rise suddenly and flooding islikelya situation identified by a flashy hydrograph.
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THE EFFECT OF DRAINAGE BASIN CHARACTER ON THE HYDROGRAPH
Basin size
The volume of runoff, the discharge(Q) and the lag time tend toincrease with the size of thedrainage basin.
This model may be applied tohumid environments such as theBritish Isles. However, in arid andsemi-arid regions, such as theSahel of Africa, runoff anddischarge volume may decreasedownstream i.e. as basin sizeabove a gauging station increases.
This is due to high evaporationrates, loss by seepage of water intothe channel bed, and the absenceof inputs from tributaries.
31
Small
drainage
basin
Medium
drainage
basin
Largedrainage
basin
D
is
c
h
a
r
g
e
(
Q)
Time
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THE EFFECT OF DRAINAGE BASIN CHARACTERON THE HYDROGRAPHBasin shape
The shape of a drainage basin will affect the
shape of the storm hydrograph. An elongated
basin will take longer to achieve a throughputof water from a rainstorm than a short, broad
basin.
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THE EFFECT OF DRAINAGE BASIN CHARACTERON THE HYDROGRAPHStream networ k character ist ics
The pattern of streams within a drainage basin influences thetransfer of water and consequently the shape of the hydrograph,i.e. the stream response to rainfall events. Two key variables areinvolved:
Stream Density = the total length of the drainage channels,divided by the drainage basin area.
Calculating stream density Dd = L/A where:
Dd = the drainage density in km per
km
2
L = the sum of the total stream inlengths in km
A = the catchment area in km2
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Stream netwo rk character ist ics (...cont in ued)
Stream order = the way the various channels in adrainage basin fit together.
The most widely used method for describing thisarrangement has been devised b y A.N. Strahler (1952)and is based on a hierarchal set of stream orders, givinga negative relationship between stream order and numberof streams.
This ratio is called Hortons law of stream numbers. Analysis of the stream order structure in a drainage basin
can help in predicting the shape of hydrographs, andhence flood forecasting.
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RIVER REGIMES
Shows the pattern of streamflow over a
longer period of time (usually a year)
Looks similar to the storm hydrograph,
except for the horizontal axis depictingmonths instead of hours
Regimes can be simple or complex
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Climatic
Physical
Biotic (human and vegetation)
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FACTORS AFFECTING DRAINAGE BASINCHARACTERISTICS
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CLIMATIC FACTORS
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Prolonged Rainfall - saturated ground, infiltrationcapacitites reached, overland flow, flooding
Intense stroms - rainfall intensity greater thaninfiltration capacity - overland flow, flash flooding
Snowfall - water is held in storage, impeded
infiltration, runoff after melting
Precipitation
If evapotranspiration rates are high, then there willbe less water available to flow into the main riverTemperature
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PHYSICAL FACTORS
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Rainfall reaches the main channel more quickly in a small drainagebasin
Circular basins have a shorter lag time (NB Newson - 1994)
In steeper sloping basins, water is more likely to reach the channelquickly
Basin size,shape and relief
Permeable (porous & pervious) rock permit rapid infiltrationtherefore little surface runoff and limited number of surfacestreams
Rock type(geology)
Controls the speed of infiltration, the amount of soil moisture storageand the rate of throughflow
Sandy soils allow rapid infiltration and do not encourage flooding
(large pore spaces) Clays have much smaller pore spaces; reduces infiltration and
throughflow but increases surface runoff and encourages floddingSoil type
Refers to the number of surface streams in a given area.
Density is higher on impermeable rocks and clays.
The higher the density, the greater the probability of flash floods
Drainage
density
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BIOTIC FACTORS
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Can prevent flooding by intercepting rainfall and storing moisture on leaves which is thenevaporated
Tropical rainforests intercept up to 80% of rainfall whereas arable land may intercept only up to10%
Seasonal changes - deciduous trees
Flooding more likely to occcur in deforested areas e.g. the increasingly frequent flooding inBangladesh is attributed tot he removal of trees in Nepal and other Himalayan areas
Deforestation can also have an important effect on local climate - increase in light intensity,temperature, wind speed and mositure (consequences - organic content decomposed faster,raindrop impact increases, ET rates decrease, overland runoff increases)
Vegetation
Increases flood risk - reduced infiltration
Reduces the distance that water must travel toreach a channel
Increases velocity (artificial channels smoother)
Urbanization
Can reduce the earth's albedo (reflectivity) by as much as 10% -a reflective sandy surface may be replaced by one with darkgreen crops
Can also cause changes in precipitation - moist soils andvegetation cover leads to increased Et rates therefore increasedrainfall e.g. Kansas, Colorado.
Irrigation
Groundwater changes - seepage leads to increased groundwater
Salinisation - occurs when groundwater levels are close to thesurface and capilllary forces bring water to the surface where it
may evaporate, leaving behind any soluble salts that it is carrying
Construction
of dams
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Characteristics
Main factors influencing drainage patterns
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DRAINAGE PATTERNS
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DRAINAGE PATTERNS
Over time, a stream system achieves a particulardrainagepatternto its network of stream channels and tributaries asdetermined by local geologic factors.
Drainage patterns or netsare classified on the basis of their formand texture.
Their shape or pattern develops in response to the localtopography and subsurface geology.
Drainage channels develop where surface runoff is enhanced andearth materials provide the least resistance to erosion.
On sloping surfacesexcess water will run off. Fewer drainagechannels will develop where the surface is flat and the soil
infiltration is high because the water will soak into the surface. The fewer number of channels, the coarser will be the drainage
pattern.
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TYPES OF DRAINAGE PATTERNS
Dendritic drainage
patternis the most
common form and looks
like the branching pattern oftree roots. It develops in
regions underlain by
homogeneous material.
That is, the subsurface
geology has a similar
resistance to weathering sothere is no apparent control
over the direction the
tributaries take. Tributaries
joining larger streams at
acute angle (less than 90
degrees).
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TYPES OF DRAINAGE PATTERNS
Parallel drainagepatterns
form where there is a
pronounced slope to the
surface. A parallel pattern also
develops in regions of parallel,elongate landforms like
outcropping resistant rock
bands. Tributary streams tend
to stretch out in a parallel-like
fashion following the slope of
the surface. A parallel patternsometimes indicates the
presence of a major fault that
cuts across an area of steeply
folded bedrock. All forms of
transitions can occur between
parallel, dendritic, and trellis
patterns.
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TYPES OF DRAINAGE PATTERNS
Trellis drainagepatterns
look similar to their
namesake, the common
garden trellis. Trellisdrainage develops in folded
topography like that found
in the Appalachian
Mountains of North
America. Down-turned folds
called synclinesformvalleys in which resides the
main channel of the stream.
Short tributary streams
enter the main channel at
sharp angles as they run
down sides of parallelridges called anticlines.
Tributaries join the main
stream at nearly right
angles.
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TYPES OF DRAINAGE PATTERNS
The rectangular drainage
patternis found in regions
that have undergone
faulting. Streams follow thepath of least resistance and
thus are concentrated in
places were exposed rock
is the weakest. Movement
of the surface due to
faulting off-sets thedirection of the stream. As
a result, the tributary
streams make shape bends
and enter the main stream
at high angles.
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TYPES OF DRAINAGE PATTERNS
The radial drainage
patterndevelops
around a centralelevated point. This
pattern is common
to such conically
shaped features as
volcanoes. Thetributary streams
extend the
headward reaches
upslope toward thetop of the volcano.
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TYPES OF DRAINAGE PATTERNS
The centripetal drainage
patternis just the opposite of
the radial as streams flow
toward a central depression.
This pattern is typical in thewestern and southwestern
portions of the United States
where basins exhibit interior
drainage. During wetter
portions of the year, these
streams feed ephemeral lakes,which evaporate away during
dry periods. Salt flats are
created in these dry lake beds
as salt dissolved in the lake
water precipitates out of
solution and is left behind
when the water evaporatesaway.
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TYPES OF DRAINAGE PATTERNS
Deranged or contorted
patternsdevelop from the
disruption of a pre-existing
drainage pattern. The figureon the right began as a
dendritic pattern but was
altered when overrun by
glacier. After receding, the
glacier left behind fine grain
material that form wetlandsand deposits that dammed
the stream to impound a
small lake. The tributary
streams appear
significantly more contorted
than they were prior toglaciation.
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