2nd_lecture.pdf

Engineering Hydrology (CVNG 1011)

Groundwater

Lecture 2

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Components of hydrologic cycle

Precipitation

Infiltration

Evapotranspiration

Inter flow

Groundwater flow

Base flow

Stream flow

(Runoff)

Total quantity of water in the world is estimated as 1386 M km3

1337.5 M km3 of water is contained in oceans as saline water

The rest 48.5 M km3 is land water

13.8 M km3 is again saline

34.7 M km3 is fresh water

10.6 M km3 is both liquid and fresh

24.1 M km3 is a frozen ice and glaciers in the polar regions and

mountain tops

World Water Budget

World Water Availability

Supply of Water Resources

Water covers 71% of the earths surface R

eadily accessib

le freshw

ater

Estimate of the World Water Balance

Mean residence time = Volume of water [m3] in a sub-system divided by flux [m3 s-1]

For example, atmosphere (values from Dyck & Peschke 1995): 13000 km3 / 577000

km3/a = 8.2 days

Water Balance of Continents Area (M km^2)

30.3

8.7 9.8

20.717.8

45

0

10

20

30

40

50

Africa Asia Australia Europe N.America S.America

Precipitation (mm/yr)

686 736 734 670726

1648

0

500

1000

1500

2000


Water Balance . Precipitation (mm/yr)

686 736 734 670726

1648

0

500

1000

1500

2000


Evaporation (mm/yr)

547 510415 383

1065

433

0

200

400

600

800

1000

1200


Total Runoff (mm/yr)

139

226

319287293

583

0

100

200

300

400

500

600

700


Drop of water ..

Matter..

Water Balance of Oceans

10712

75167

780

240

1010

1210

1040

120

1380

1140

0

200

400

600

800

1000

1200

1400

1600

Atlantic Arctic Indian Pacific

Area M km^2

Precp (mm/yr)

Evap. (mm/yr)

Water flow in Ocean

200 230

70 60

350

-300

130

-60

-400

-200

0

200

400

Atlantic Arctic Indian Pacific

Continental Inflow (mm/yr)

water exch. with ocean(mm/yr)

The Meridional Overturning Circulation plays a crucial role for life in the oceans

If this ocean conveyor belt slows down or changes as a result of melting ice and

increasing ocean temperatures, the impacts on marine life may become severe

Coastal regions in the World where dense shelf water cascading flushing

has been observed

The map shows sites with known dense shelf water cascading phenomena,

which often may involve the flushing effect

Dense shelf water cascading is highly sensitive to increases in temperature,

and hence, climate change

Sales

Barbados (90%) Bahamas (~ 90%)

Jamaica (60%) Trinidad and Tobago (40%)

Availability of freshwater = estimated reserves of rivers, lakes and aquifers

WATER STRESS CHALLENGES

GROWTH AND SECURITY

Draw on tomorrow's water to meet today's needs

Un

ited

Nati

on

s E

nv

iro

nm

en

t P

rog

ram

me

Water stress refers to the ratio of water

demand to renewable water resources

Libya's thirst for 'fossil water'

Libyans like to call it "THE EIGHTH WONDER OF THE WORLD"

Libya - arid nation, mostly desert, freshwater perpetually scarce

Rainfall is meagre - only 5% of the nation receives more than 100 mm of

rain each year

Groundwater reserves - coastal groundwater aquifers

- become brackish with an influx of seawater

In the mid-1950s - Oil exploration

Nations vast southern desert revealed

a precious resource and a potential

solutionfossil water

Deep under the Sahara sands, ancient

aquifers have been storing 40,000-year-old

reserves of pure drinking water

http://drinking-water.org/html/en/glossary.html#gloss60http://drinking-water.org/html/en/glossary.html#gloss64http://drinking-water.org/html/en/glossary.html#gloss64http://drinking-water.org/html/en/glossary.html#gloss59

Indus Valley - Pakistan

Preventing, controlling measures

Prevention of waterlogging and salinity

Traditional furrow and basin irrigation is being replaced by sprinklers,

drip and trickle micro-irrigation techniques.

By open ditches, tile drains or pumping from boreholes

There are many more.

USA Nuclear waste

Midwest USA Lowering of GW table

Saline aquifer problem in Nile Valley and Australia

In Caribbean -

Sea water Intrusion

Climate change

Sea Level Rise

Reduction in recharge

Pollution, Landfills, Fertilizers, Spills --- like other countries

Integrated water resources management (IWRM) in the Caribbean:

The table above shows that climate change is likely to increase the exposure of

small islands to water shortages for various reasons

Ratio of water withdrawal to water availability in Latin America and

Caribbean countries

* World Meteorological Organization and Inter.-American Development Bank

(WMO/IDB) (1996), Water resources assessment and management strategies in

Latin America and the Caribbean. Proceedings of the WMO/IDB Conference, San

Jose, Costa Rica.

....providing knowledge for good decisions in water

management

Extensive flooding, water scarcity, water quality deterioration, ecosystem

decline and effects of global changes initiated or facilitated through

hydrological processes

Mitigation strategy needs to address whole catchments in a holistic way

to support life, civilization and

sustainable development

Water Availability is Decreasing

Water availability is decreasing for:

Climate change (need to be very careful);

Overexploitation;

Pollution

The Future?

By the year 2025 nearly 2 billion people will live in

regions or countries with absolute water scarcity, even

allowing for high levels of irrigation efficiency

Year

World

Population

(billions)

2010 6.8

2020 7.6

2030 8.2

2040 8.7

25 The Lake Aral disaster

Soviet government decided in the 1960s to

divert those rivers so that they could irrigate the

desert region surrounding the Sea in order to

favour agriculture rather than supply the Aral

Sea basin

Those consequences range from unexpected climate feedbacks to public health issues,

affecting the lives of millions of people in and out of the region

Aral Sea gets approximately one fifth of its water

supply through rainfall, while the rest is delivered

to it by the Amu Darya and Syr Darya rivers

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It deprived the Aral Sea of its two main sources of water income, which almost

immediately led to less water arriving to the sea

Majority of water was being soaked up by the desert and blatantly wasted (between 25%

and 75% of it, depending on the time period)

- Major fault in conceptualization of the Irrigation Canal project

Evaporation causes the water level to decrease by the same amount that flows into

the Sea

Over the last 4 decades

2014 Level of salinity

rose from

approximately

10g/l to often

more than

100g/l in the

remaining

Southern Aral

28 What is the role of hydrology for water resources management ?

Estimation of water resources availability

Estimation and reduction of hydrological risks

Development of hydrological scenarios

Ensure proper information to decision makers

Zone of aeration/ vadose zone: A zone that contains both water and air

Saturated zone: Where all the interconnected openings between rock particles are

filled with water

Soil moisture: Water in the upper layers of zone of aeration

Groundwater: Called the water in the zone of saturation

Capillary fringe/ tension-saturated zone: The lower subdivision of the zone of

aeration that overlies the zone of saturation and in which the pressure of water in

the interstices is lower than atmospheric

The water table is the surface where hydraulic head or

water pressure head is equal to the atmospheric

pressure

Water Table

Hydraulic head is a specific measurement of water pressure which helps the water to

rise above a datum

Atmospheric pressure is the force per unit area exerted on a surface by the weight of

air above that surface in the atmosphere of Earth

Surface that divide saturated zone and unsaturated zone

Most accurate definition is:

Aquifer

Aquifer is an underground layer of water-bearing permeable rock or

unconsolidated materials that can produce significant quantities of water to

springs and wells

Aquiclude is an impermeable layer of rock or soil which therefore acts to stop the flow

of groundwater

Aq

uif

er Confined Aquifer

Unconfined Aquifer

Perched Aquifer

Alluvial

Leaky aquifer Mainly in

Fractured rocks

When a permeable stratum in overlain or underlain by a semipervoius or semi-

confining layer

Pumping from a well in a leaky aquifer water removes in horizontal flow as well

as vertical movement

Does a confined aquifer have a water table ?

Confined aquifer does not have a water table.

It is completely filled with groundwater.

The water levels in wells drilled into a confined aquifer correspond

instead to the potentiometric surface of the aquifer, also

known as pressure head or confined head.

If the pressure head falls below the top of the aquifer, the aquifer is no

longer confined.

WATER POTENTIAL IN POROUS MEDIA

Groundwater flow through porous media is a mechanical process

The forces driving the fluid (water) forward must overcome the frictional forces set

up between the moving fluid and the grains of the porous medium

From different types of mechanical energy, for practical purposes one takes into

consideration the gravitational potential energy, the energy of fluid pressure and the

kinetic energy

Water potential in saturated media

Arbitrary datum (z=0)

Ground surface

Consider a mass m of fluid contained in a

volume V, centered around a point P situated

at the elevation z above an arbitrary datum

Fluid pressure in the point P is p

Fluid velocity v = 0 at the datum

Fluid velocity v = V in point P

The total energy of the mass m of the fluid corresponding to the point P has three

components

=

1) Gravitational energy (): representing the work to lift the mass from the elevation 0 of the datum to the elevation z of the point P :

2) Pressure energy (): representing the work to increase the fluid pressure from a reference energy p0 to the energy p:

3) Velocity energy (): representing the work to accelerate the fluid from the velocity 0 to the velocity v :

The total energy of the mass m is thus composed by the potential energy

(gravitational energy and pressure energy) plus the kinetic energy:

Due to the low velocities in porous media, the kinetic

energy is very small, being neglected

Thus only the gravitational energy and the energy of fluid pressure are considered

The flow is directed from regions where the mechanical energy of the fluid is higher

to the regions where it is lower

This means that the water transfer is due to the difference of the potential energy

between two locations

The relative potential energy can be normalized, being expressed per unit mass, per

unit volume or per unit weight; this specific energy represents the water potential

(Mermoud, 1998)

If one considers m = 1 in the relation of the total energy, one obtains the FLUID

POTENTIAL =

For flows through porous media, velocities are extremely low, so the last term may

be neglected in practical cases

Integrate this equation

For incompressible fluids, density is constant, so after integration the previous

equation becomes:

= As water transfer is due to the difference of the potential energy between two locations

Reference energy one usually uses the energy of water under atmospheric pressure,

considered null (P0 =0)

=

This sum called Hydraulic Head

H =

after Mermoud, 1998

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