Hydrology and Hydrogeology

Dr. Shimelis G Setegn, Ph.D.

Program Executive Officer- GLOWS

Research Assistant Professor - RSCPHSW

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HYDROLOGY

Hydrology is the science that deals with the occurrence, circulation and distribution of water upon, over and beneath the earth surface.

It is the science concerned with the transportation of water vapours through the air, the precipitation occurring on the ground as rainfall and the flow of water over the ground surface and through the underground strata of the earth.

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HYDROLOGY

It also deals with the evaporation from water surface and soil surface, the infiltration through the ground surface and the transpiration from the plants and various other allied processes occurring in nature.

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Importance of Hydrology

A basic knowledge of hydrology is essential for the irrigation engineer engaged in the development, utilization and management of water resources

It helps assessing the quantity of water available for irrigation, hydropower, municipal and industrial water supply and other purposes.

It is required for the estimation of the maximum discharge for the design of spillways, aqueducts, bridges, and sewers.

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Hydrological studies are also necessary for flood control, erosion control, pollution control, etc.

Hydrology is basically an applied science. the subject is sometimes classified asScientific Hydrology: The study which is

concerned mainly with academic aspects.Engineering or applied Hydrology: a study

concerned with engineering applications.

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In general, engineering hydrology deals with

• Estimation of water resources.• The study of processes such as precipitation,

runoff, evapotranspiration and their Interaction

• The study of problems such as floods and droughts and strategies to combat them.

which enable a quantitative evaluation of the hydrologic processes that are of importance to a civil, agricultural or soil & water eng.

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Sources of data Depending upon the problem at hand, a

hydrologist would require data relating to the various relevant phases. The data normally required are:

1. Weather records:- temperature, humidity, and wind velocity,

2. Precipitation data,3. Stream-flow records.4. Infiltration and transpiration data,5. Evaporation characteristics of the area,6. Ground water characteristics7. Physical and geological characteristics of

the area under consideration

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Sources:Meteorological data are collected from

Meteorological service agency.Stream flow data of various rivers and

streams can be found from Ministry of water resources.

Data on Evaporation, transpiration, infiltration will be available in ministry of agriculture, or water resources or any other concerned departments.

The physical data of the area can be obtained from topographic map of the area available with mapping agencies.

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The Hydrologic cycle:

- The main components of the hydrological cycle are rainfall (precipitation), evaporation, Transpiration, Infiltration, runoff and ground water.

beginning with the evaporation of water from the ocean.

The resulting vapour is transported by moving air masses.

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The Hydrologic cycle:

Under the proper condition, the vapour is condensed to form clouds, which in turn may result in precipitation.

The precipitation which falls up on land is dispersed in several ways.

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The Hydrologic cycle: continued The greater part is temporarily retained in the soil and is ultimately returned to the atmosphere by evaporation and transpiration

a portion of the water finds its way over and through the surface soil to stream channels, while other water penetrates farther into the ground to become part of the ground water.

Phases of hydrologic cycle simulated by SWAT

Land phase

Water phase

Courtesy: SWAT Manual

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Under the influence of gravity, both surface stream flow and ground water move toward lower elevations and may eventually discharge into the ocean.

How ever, substantial quantities of surface and under ground water are returned to the atmosphere by evaporation and transpiration before reaching the ocean.

Groundwater Hydrology

Ground water Ground water is the most important source of fresh

water today, not surface waters (lakes and rivers). Supplies 34 out of 100 largest U.S. cities because

 

1)    Precipitation varies dramatically, particularly in arid areas where little or no surface water exists.

2)    Surface water often polluted.

3)    Often it is naturally filtered

4)    Largest available source of fresh, liquid water.

What is groundwater?

Terms

Porosity - Ratio of Void Volume to Total Volume of Soil

Terms cont.

Permeability - A measure of the resistance to flow through a porous media.

What is saturation?

In the saturated zone all of the pore space is occupied by water

In the unsaturated or vadose zone, both air and water are found in the pore space

What are aquifers?

GW cont.-Aquifer Large volumes of ground water require an aquifer that

holds and transmits the water. Material has a high porosity and permeability. Usually a well-sorted sediment rock or sedimentary

rock. Flow rates of 1-100 meters per day for sand and

sandstone and 100-500 per day for gravel or conglomerate are typical.

  Aquiclude: impermeable to water flow. Acts as a barrier

(i.e. shale). Aquitard: intermediate condition

Aquifer Types Unconfined: no confining aquiclude on top of aquifer.

Water is not under any pressure. Aquifer is open to surface waters (and pollution) throughout its entire area. Usually is regional in extent (100s of square miles or more).

 Perched water table: localized (10s of square miles or less) unconfined aquifer usually at a shallower depth than the regional aquifer. Caused by underlying aquiclude of limited extent. Cheaper to exploit, but can be quickly depleted and is more sensitive to local precipitation and pollution.

Confined of artesian: overlain by an aquitard or aquiclude. Water movement restricted to the sandwiched aquifer. Water usually under pressure due to its own weight (hydrostatic head). If drilled, water will rise to the potentiometric surface.

Aquifers types cont.Unconfined Aquifers: When saturated

conditions are found with impermeable material between the aquifer and ground surface, we term this an unconfined aquifer, water table aquifer, or a phreatic aquifer.

If a well is drilled into this aquifer, the water level in the well defines the water table, phreatic surface or the piezometric surface

Unconfined AquifersThe piezometric surface move up and down

depending on the amount of water in the aquiferThe addition of water to the aquifer is termed

rechargeThe elevation (above sea level) of the surface of

the aquifer is termed the headThe change in head, or head loss, with distance

is termed the hydraulic gradientGroundwater flows down the hydraulic gradient

Hydraulic gradient in unconfined aquifers

Hydraulic Gradient

where h1 = head at location 1

h2 = head at location 2

L = distance between locations 1 and 2

L

hh

L

h 12

gradienthydraulic

Confined aquifer Aquifer which is between 2

impermeable layers (aquicludes) Can be "leaky" in which case the aqucludes are replaced by aquitards.

Equation for flow per unit width (Q') in a confined aquifer;

K = Permeability D = Thickness of Aquifer L = Distance between Head (H)

Measurements R = Distance from Well to H

measurement

Unconfined aquifer Flowrate per unit width of

the aquifer K = Permeability H1 and H2 = Head above

confining layer L = Distance between H

measurements R = Distance from Well to H

measurement

Groundwater Flow Darcy’s Law

where v = groundwater “Darcy” velocity (m/d) K = hydraulic conductivity (m/d)

where Q = flow rate (m3/d)

L

hKv

AL

hKvAQ

Hydraulic Conductivity

Example

A medium of sand aquifer 20.0 m thick has two monitoring wells spaced 500 m thick along the direction of flow. The groundwater level in the first well is 239.0 m above sea level, and 237.0 m in the second well. Estimate the rate of flow per meter of width (distance perpendicular to the flow).

Example

004.0500

0.5370.53912

L

hh

L

h004.0

500

0.2370.23912

L

hh

L

h

d

m

d

s86,400

s

m 1.12104.1 4

sandmediumK

width

2

m

m

d

m20004.01.12A

L

hKQ

widthmeter per d

m

md

m 3

width

3

97.0968.0

Q

Average Linear Velocity Darcy velocity is

the flow per unit cross- sectional area of the aquifer

Much of the cross-sectional area is “blocked” by particles

Average Linear Velocity

Actual velocity of the water that moves through the pores is greater than the Darcy velocity

where v’water = average linear velocity (m/d)

v = Darcy velocity

η = poristy

v

vwater '

Example

In the previous example, what is the average linear velocity of the water?

d

m

d

m0484.0004.01.12

L

hKv

d

mdm

13.037.0

0484.0'

sandmediumwater

vv

Hydraulic conductivity (K)

Hydraulic conductivity (K) of soil or rock depends on physical factors and is an indication of an aquifer’s ability to transmit water

Has a unit of L/TTransmissivity (T)- a term applied to confined

aquifers, T = K.b– B = aquifer thickness– T has the unit of L2/T

Intrinsic permeability (k) It is the property of the medium only, independent

of fluid properties. It is related to hydraulic conductivity

Where m = dynamic viscosity, r = fluid density and g = gravitational acceleration

K has units of m2

Intrinsic permeability (k) is used in petroleum industry and K is used in groundwater hydrology

g

Kk

Determination of Hydraulic conductivity (K)

Lab- – constant and – falling head permeameters

Field-– pump test, – slug test and – tracer tests

Determination of Hydraulic conductivity (K)

Permeameter is used to measure K by maintaining flow through a small column of material and measuring flow rate and head loss

For a constant head permeameter, Darcy’s Law can be directly applied to find K, where V is volume flowing in time t through a sample of area A, length L and with constant head, h

Ath

VLK

Determination of Hydraulic conductivity (K) cont.

The falling head permeameter, test consists of measuring the rate of fall of the water level in an attached tube or column and nothing that

Darcy law can be written for the sample as

Where r, rc are radii of the tube and sample respectively and t is the time interval for the water to fall from h1 to h2

After equating and integrating

dt

dhrQ 2

dl

dhKrQ c

2

2

12

2

lnh

h

tr

LrQ

c

Determination of Hydraulic conductivity (K) cont.

In the field, slug tests, pump tests and tracer tests are preferable for determination of K

They provide better estimate of field conditions The slug test for shallow wells operates based on a

measurement of decline or recovery of the water level, in the well through time.

The well can be pumped to lower the water level and allowed to recover in time or

Water level can be increased and allowed to drain out in time

Hydraulic K is then determined by evaluating the rate of change in the water level with time.

Determination of Hydraulic conductivity (K) cont.

The pump test involves the constant removal of water from a single well and observations of water level declines at several adjacent wells

Field test give usually yield different values of K than lab since they are accurate and indicate the field condition

Anisotropic aquifers K can be different in the vertical and horizontal directions

for most geologic systems Mainly due to alluvial depositions For two layered aquifer of different K in each layer and

different thickness, we can apply Darcy’s law to horizontal flow to show

Or in general Where Ki = K in layer i and Zi = thickness of layer i

21

2211

zz

zKzKK x

i

iix z

zkK

Anisotropic aquifers cont For vertical flow

Or in general

Where Ki = K in layer i and Zi = thickness of layer i

2211

21

// KzKz

zzK z

ii

iz Kz

zK

/

Urban watersheds and Hydrology

Volume ?

Qp ?Tp ?

Time

Flow

•Volume

•Peak

•Time to peak

Hydrologic Response vs. Land Surface Parameters and Fluxes

Surface energy fluxes

Fractional vegetation

cover

ImperviousSurface

Surface Temperature

Topography

HYDROLOGIC RESPONSE

Land-cover

SnowmeltPrecipitation/Soil moisture

Original (natural)

Partiallydeveloped

Fullydevelope

d

(a) (b) (c)

Q

time

(From: Hydrology and Floodplain Analysis, 2nd ed. P.B. Bedient and W.C. Huber, Addison-Wesley Pub. © 1992)

Watersheds

Land-use change effect on hydrology

Afforestation– Annual flow

Increased interception in wet periods Increased transpiration in dry periods through increased

water availability to deep root systems– Seasonal flow

Increase interception and transpiration will increase soil moisture deficit and reduce seasonal flow

Drainage activities associated with planting may increase dry season flow through initial dewatering

Cloud water (mist or fog) deposition will augment dry season flows

Land-use change effect on hydrology cont.

Afforestation – floods

interception reduces floods Cultivation, drainage and road construction increase

floods water availability to deep root systems– Water quality

Leaching of nutrients is less from forests through reduced surface runoff and reduced fertilizer applications

Deposition of atmospheric pollutants is higher

Land-use change effect on hydrology cont.

Afforestation – erosion

High infiltration rates in natural, mixed forests reduce surface runoff and erosion

Slope stability is enhanced by reduced soil pore water pressure and binding of forest roots

Wind throw of trees and weight of tree crop reduce slope stability

Soil erosion, through splash detachment, is increased from forests without an understory of shrubs or grass

Cultivation, drainage, felling and road construction increase erosion

Land-use change effect on hydrology cont.

Afforestation – Climate

Increased evaporation and reduced sensible heat fluxes from forests affect climate

Land-use change effect on hydrology cont.

Agricultural intensification – Water quantity

Alteration of transpiration rates affects runoff Timing of storm runoff altered through land drainage

– Water quality Application of organic fertilizers Application of persistent pesticides poses health risks to

humans and animal life Farm wastes pollutes surface and groundwater bodies

– Erosion Cultivation without proper soil conservation measures

and uncontrolled grazing on steep slopes increase erosion

Land-use change effect on hydrology cont.

Draining wetlands– Seasonal flow

Upland peat bogs, groundwater fens have little effect in maintaining dry season flows

– Wetlands loose their desired function (flood control, water quality improvement, recharge etc)

Reducing urban runoff: Parking Lots

Reducing runoff– porous

pavement– Gravel parking lots

Porous or punctured asphalt

– Concrete vaults and cisterns beneath parking lots in high value areas

– Vegetated ponding areas around parking lots

Delaying runoff Grassy strips on parking

lots Grassed waterways

draining parking lot Ponding and detention

measures for impervious areas– Rippled pavement– Depressions– basins

Reducing urban runoff: Residential

Reducing runoff– cistern for

individual or group of homes

– Gravel driveways (porous)

– Contoured landscape– Groundwater

recharge Perforated pipe Gravel Trench Dry wells

– Vegetated depressions

Delaying runoff Reservoir or detention basin Planting grass with high

roughness value Gravel driveways Grassy channels increased length of travel of

runoff by means of gutters and diversions

Guidelines for Planning in an Urban Drainage Basin

Maximize the distance of storm water travel from the site to a collection area or stream.

Maximize the concentration time by slowing the rate of storm water runoff.

Minimize the volume of overland flow per unit area of developed land.

Utilize buffers such as forests and wetlands to protect collection areas and streams from urban impacts.

Divert storm water away from critical features such as steep slopes, unstable soils, or valued habitats.

1992 2001

19841974

FIS Area - Grand Forks

FIS

1.0 0.5

FVCFIS 1

FIS Area – Fargo/Morehead

1992

1974 1984

2001FIS

1.0 0.5