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WATER ON AND UNDER

GROUND

Objectives

• Define and describe the hydrologic cycle.

• Identify the basic characteristics of streams.

• Define drainage basin.

• Describe how floods occur and what factors may

make them worse.

• Define recurrence interval and show how it is used to

predict floods.

• Define water table.

• Identify two types of aquifers.

The Hydrologic Cycle

• Hydrology

– The scientific study of water

• Evaporation

– Water changes from a liquid into water

• Transpiration

– Water taken up by plants passes into the atmosphere

• Condensation

– Water changes from a vapor into a liquid or a solid

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The Hydrologic Cycle

• Water in the Earth system

– Precipitation

• Water that has condensed in the atmosphere falls back to the surface

as rain, snow, or hail

– Surface runoff

• Precipitation that drains over the land or stream channels

– Infiltration

• The process by which water works its way into the ground though

small openings in the soil

– Evapotranspiration

The Hydrologic Cycle

The Hydrologic Cycle

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The Hydrologic Cycle

How Water Affects Land

• Streams and stream flow

– Stream• A body of water that flows

downslope along a clearly defined natural passageway

How Water Affects Land

• Streams and stream flow

– Channel• The clearly defined

passageway through which a stream flows

– Straight

– Meandering

– Braided

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Qu’Appelle Valley, Manitoba

Meandering Stream

Braided stream

How Water Affects Land

• Streams and stream flows

– Gradient

• The steepness of a stream

channel

– Discharge

• The mount of water passing by

a point on a channel’s bank

during a unit of time

– Load

• The suspended and dissolved

sediment carried by a stream

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Discharge is the amount of water that flows through a channel. Discharge is

calculated by first finding the cross-sectional area of a stream and then

multiplying this times the velocity. The result will always be in a measure of

volume per unit time. Rather than trying to measure streams everywhere all the

time, stream gages (measure points) are used to estimate flow.

Where Does Sediment Come From?

Stream sediment is

called load. One way

streams procure

sediment is by erosion

itself. Particles loosened

by weathering are

picked up in surface

runoff and transported

to the stream channel.

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Mass movement

events may move

loose material

downslope into the

stream channel, the

second way streams

procure sediment.

The third way

streams procure

sediment: the

stream itself may

also erode the

banks of its

channel and that

sediment too

becomes part of

the stream’s load.

Here we can

visualize the various

motions of bedload

particles and see

how small eddy

currents keep

suspended load

entrained.

Sediment Load

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• http://media.pearsoncmg.com/bc/bc_0media_geo/activ

e_art/hdew_2e.html?OxbowLake

Floodplains

Meander Abandonment

Why a sinuous stream meanders

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How water Affects Land• Stream deposits

– Alluvium

• Stream sediment

• Recently deposited

(geologically speaking)

– Floodplain

– Natural Levee

– Alluvial fan (right)

– Delta

Suspended-load deposits build up floodplains.

Delta

• Stream deposit

– Delta: deposition of stream

sediment in a standing

body of water like a lake

sea, or ocean.

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•Erosion of the

Mississippi Delta

Building of artificial levees,

dams, etc. starve a delta of

sediment. Erosion by waves then

begins to erode the delta.

How Water Affects Land

• Drainage basin

– The total area from which

water flows into a stream

• Divide

– A topographic high that

separates adjacent drainage

basins

• Lakes

– Standing bodies of water that

have open surfaces, in direct

contact with the atmosphere

The Drainage Basin: all the area that supplies

water to a stream and its tributaries

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Boundaries of a

watershed are called

divides; indicated by

the black dashed lines.

Lakes: collect water from rain, runoff, and

groundwater. Usually have stream outlet

Water as a Hazard and a

Resource• Floods

– An event in which a

body of water

overflows its banks

• Flood prediction and

prevention

– Flood frequency curve

– Recurrence interval

– Channelization

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Floods (Flows greater than bankfull capacity)

Due to high intensity rainfall, usually seasonal.

Intense rain cannot be absorbed quickly enough, so it runs off into streams.

Flash floods associated with canyons are generally the deepest and often most damaging (below right).

These events are rapid (flash) and difficult to predict.

Flash floods

Water as a Hazard and a

Resource

• Hydrographs of stream discharge

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Water as a Hazard and a

Resource

Water as a Hazard and a Resource

• Flood Frequency Curve

Water as a Hazard and a

Resource• Surface water resources

– Reliable water supply critical to human survival and health, agriculture

and other economic activities

– Nearly 250 million people are designated as water-scarce

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Freshwater Underground

• Ground water

– Subsurface water

contained in pore

spaces in regolith

and bedrock

• Water table

– The top surface of

the saturated zone

Freshwater Underground

• How groundwater moves

– Porosity

• The percentage of the total volume of a

body of rock or regolith that consists of

open spaces (pores)

– Permeability

• Measure of how easily a solid allows

fluids to pass through it

30% 15%

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Freshwater Underground

• How groundwater

moves (continued)

– Percolation

• Groundwater seeps

downward

• Flows under the

influence of gravity

– Recharge

• Replenishment of

groundwater

Freshwater Underground

• How groundwater

moves (continued)

– Discharge

• Subsurface water

leaves the saturated

zone

• Becomes surface water

– Spring

• Occurs where the water

table intersects the land

surface

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Freshwater Underground

• Where groundwater is

stored

– Aquifer

• A body of rock or

regolith that is water

saturated, porous, and

permeable

– Aquiclude

• A layer of impermeable

rock

– Artesian well

Wells in confined aquifers

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Freshwater Underground

• When groundwater

dissolves rock

– Karst topography

– Sinkholes

– Cave

• Underground open space

• A cavern is a system of

connected caves

Major karst landforms

• Dolines (sinkholes)

Collapse dolines

Colorado Plateau

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Collapse dolines

Edwards Limestone, Texas: recharge area for Edwards

Aquifer

Solution or subsidence doline

Mitchell Plain, Indiana

Caves and Speleothems

Stalactite

Stalagmite

Column