5/26/2018 Hydrology

1/48

HYDROLOGY

Unit 1 - Module 2 (Hydrological, Fluvial, Coastal and

Limestone Environments)

5/26/2018 Hydrology

2/48

CONTENT

The hydrological cycle

The storm hydrograph and water balance

Drainage basin characteristicsDrainage patterns and drainage density

Mapwork

2

5/26/2018 Hydrology

3/48

Concepts associated with the hydrological cycle and the river basin

Major flows and factors influencing flows within the hydrological cycle

THE HYDROLOGICAL CYCLE

3

5/26/2018 Hydrology

4/48

THE HYDROLOGICAL CYCLE

Describes the continuous

movement of all forms of water

(vapour, liquid and solid) on, in,and above the Earths surface.

5/26/2018 Hydrology

5/48

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.

5

5/26/2018 Hydrology

6/48

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

http://upload.wikimedia.org/wikipedia/commons/9/94/Water_cycle.png

5/26/2018 Hydrology

7/48

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

5/26/2018 Hydrology

8/48

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:

5/26/2018 Hydrology

9/48

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.

9

5/26/2018 Hydrology

10/48

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%

10

5/26/2018 Hydrology

11/48

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)

11

5/26/2018 Hydrology

12/48

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.

12

5/26/2018 Hydrology

13/48

13

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)

5/26/2018 Hydrology

14/48

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.

14

5/26/2018 Hydrology

15/48

DRAINAGE BASIN PROCESSES ABOVE THESURFACEEvapotranspirat ion

Water lost from vegetation via both

evaporation and transpiration.

15

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

5/26/2018 Hydrology

16/48

16

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.

5/26/2018 Hydrology

17/48

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.

17

5/26/2018 Hydrology

18/48

18

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

5/26/2018 Hydrology

19/48

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.

19

5/26/2018 Hydrology

20/48

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

20

5/26/2018 Hydrology

21/48

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.

21

5/26/2018 Hydrology

22/48

Main Concepts

Spatial and Temporal Changes

22

THE STORM HYDROGRAPH AND WATERBALANCE

5/26/2018 Hydrology

23/48

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.

23

5/26/2018 Hydrology

24/48

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.

24

5/26/2018 Hydrology

25/48

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.

25

5/26/2018 Hydrology

26/48

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.

26

5/26/2018 Hydrology

27/48

27

5/26/2018 Hydrology

28/48

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

5/26/2018 Hydrology

29/48

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.

29

5/26/2018 Hydrology

30/48

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.

30

5/26/2018 Hydrology

31/48

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

5/26/2018 Hydrology

32/48

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.

32

5/26/2018 Hydrology

33/48

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

33

5/26/2018 Hydrology

34/48

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.

34

5/26/2018 Hydrology

35/48

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

35

5/26/2018 Hydrology

36/48

Climatic

Physical

Biotic (human and vegetation)

36

FACTORS AFFECTING DRAINAGE BASINCHARACTERISTICS

5/26/2018 Hydrology

37/48

CLIMATIC FACTORS

37

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

5/26/2018 Hydrology

38/48

PHYSICAL FACTORS

38

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

5/26/2018 Hydrology

39/48

BIOTIC FACTORS

39

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

5/26/2018 Hydrology

40/48

Characteristics

Main factors influencing drainage patterns

40

DRAINAGE PATTERNS

5/26/2018 Hydrology

41/48

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.

41

5/26/2018 Hydrology

42/48

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).

42

5/26/2018 Hydrology

43/48

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.

43

5/26/2018 Hydrology

44/48

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.

44

http://www.uwsp.edu/geo/faculty/ritter/glossary/s_u/syncline.htmlhttp://www.uwsp.edu/geo/faculty/ritter/glossary/s_u/syncline.html

5/26/2018 Hydrology

45/48

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.

45

5/26/2018 Hydrology

46/48

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.

46

5/26/2018 Hydrology

47/48

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.

47

5/26/2018 Hydrology

48/48

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.