Ch 18. Hydrologic Cycle and streams

Tom Bean

Wednesday’s outline1. the hydrologic cycle

“reservoirs”cycling between them Evaporation and the atmosphere

2. Surface hydrology infiltration and soil moisturestreamflowETSnow/ice

3. water balanceWater balance equationcontrol volume a drainge basinwater balance exampleunits

Why do we care about the hydrologic Cycle and streams

Water resourcesFloods = hazards

Rivers shapeEarth’s surface

1. Hydrologic Cycle: continuous circulation of earth’s water,

between oceans, atmosphere and continents

Major “reservoirs” of water, in 1015 kg

Land16,000

Oceans1,400,000

Atmosphere13.5

Cryosphere(ice and snow)43,000

Lithosphere (LOTS)

97% of all water(salty)

29% of fresh water as GROUNDWATER

70% of all fresh water

hydrosphere

Moving water between reservoirs

Land16,000

Oceans1,400,000

Atmosphere13.5

Cryosphere43,000

Lithosphere

precipitation: rain and snowEvaporation

rivers

Evaporation and hydro cycle

Evaporationliquid to gas

Water in atmosphere1. Gas

(you cannot see water vapor)

CondensationGas to liquid

or ice

Ocean LAND

Water in atmosphere2. Clouds: liquid and ice

Surface hydrology: land or terrestrial branch of the hydrologic cycle

s

Soil

Sediment/Rock

Infiltration: movement of water from the soil surface into the soil

Overland flow to streams

s

Soil

Sediment/Rock

Evapotranspiration (ET) is the combination of:1. Evaporation of water from soil (liquid to gas)2. Transpiration: water evaporates inside leaves; moves

into atmosphere as gas

Evaporation

Transpiration

Streamflow (or runoff)movement of water downhill in a channel

Sources:1. From groundwater2. Overland flow3. Precipitation on channel

Evaporation

Snow and ice:water is temporarily stored as a solid

Water Balance Change in vol. of water equals inputs minus outputs

Similar to 1.Balancing a bank account2.Inventory at a store

1. Need a “control volume”

• A user defined compartment • Need not be “spatially connected”

– for example, the earth’s glaciers

• An area of the earth’s surface that contributes the water passing a point on a stream

• the boundary is called a “divide”

Drainage basin or watershed

6th largest river

~4000 km long

41% of U.S.

Mississippi River basin

Fig. 13.22

Drainage Basin of the Colorado River

Change in volume of water = input minus outputCatchment water budget

retp in volume change

Mississippi River basin

Inputs?Precipitation (P)

Outputs?Evapotranspiration (ET)Runoff (R)

Catchment water budgetretp in volume change

retp 0pret

Precipitation that falls on a basin, leaves the basin via the sum of ET and runoff

Catchment water balance example:In a year, how much ET from Boulder creek watershed

What percent of precipitation leaves basin via ET?A) 20%B) 40%C) 60%D) 80%

rpet 1. P measured as a “depth/time” mm/year

2. R measured as a “volume/time” m3/second

Raingaugedepth per time (length per time)

Streamgauge; measure dischargevolume per time

What is stream discharge?

2 m

2 m

Area = height * width

DISCHARGE = area x AVERAGE velocityDischarge = 4 m2 x 3 m/s = 12 m3/ s

3 m/s

Volume = area * depthkm3 [=] km2 * km

Map of Boulder Creek drainage basin

Area

Does it rain more in Rhode Island or New Mexico?

By volume: more rain falls in New Mexico.

By depth: more rain falls in Rhode island

volume = area * depth

depth = volume / area

1. Boulder Creek example

P = 1 m/yearBasin area = 500 km2 =500,000,000 m2

What is volume of precipitation in Boulder creek basin?

Either: 500,000,000 m3/year or 0.5 km3

2. Boulder creek example

yrxxm

yrm

1sec1015.3

sec

733

Runoff = 3.2 m3/s

What is volume of runoff (per year)

yrkm

yrm

yrmx

yrsxxR

3338

71.0000,000,1001001.1

11015.3 2.3

Catchment water balance example:In a year, how much ET from Boulder creek watershed

P = 0.5 km3

R = 0.1 km3

Answer: ET = 0.4 km3 or 80% of p

rpet

What percent of precipitation leaves basin via ET?A) 20%B) 40%C) 60%D) 80%

Catchment water balance example:In a year, how much ET from Boulder creek watershed

P = 1.0 m / yr or 1000 mm/yr

R = 0.2 m / yr or 200 mm/yr

ET = 0.8 m/yr or 800 mm/yr

20% of precipitation ends up as runoff80% of returns to atmosphere via ET

rpet

Water balance by continent(units = mm yr-1)

Continent P (mm) R (mm) ET (mm) % runoff

Africa 690 140 550 20

Asia 720 290 430 40

Australia 740 230 510 31

Europe 730 320 410 44

N. America 670 290 380 43

S. America 1650 590 1060 36

Antarctica 170 30 140 18

All land 800 310 490 39

All oceans 1066 -10 1176 -1

Globally, only 40% of precipitation is runoff

Majority (60%) goes to ET!

In southwestern US:Nearly all P ET

172,000

108,000(62.7%)

CA

48,000

46,000(95.8%)

NV

68,000

67,000(98.5%)

AZ

75,000

73,000(97.3%)

NM

Precipitation

Evapotranspiration

Water Balance Component

Data in Million Gallon/Day. Source: USGS Water Use Report 1990

Water flowing in streams

Tom Bean

What is a “stream”: flow of water within any natural channel

bed: floor of the channel.floodplain: the flat region

adjoining the channel occupied in times of flood

How does water get into a stream?

Sources:1. From groundwater2. Overland flow3. Precipitation on channel

Streams flow downhill gravity

Continual conversion of:Potential energy kinetic

energy

1. Where does energy come from?

2. How come water doesn’t keep going faster?

The sun is the energy source for the hydrologic cycle

Streams are turbulent flows

Turbulence

Even a quiet, gentle stream is a turbulent flow, not laminar

Energy loss to friction in turbulent flowsso, water doesn’t keep moving faster

Drainage basins and “SUB-BASINS”

Political boundaries rarely follow basin boundaries

A basin can be divided into a series of “sub-basins”

Boulder Creek

Every basin has a “stream network” Pattern of stream channels in a basin

Some drainage basins are VERY VERY BIG

Some drainage basins are VERY VERY small

Water flowing in stream channelsWhat controls how water flows down a channel?

Channel controls on flow1. cross sectional AREA

2 m

2 m

Area = height * width

DISCHARGE = area x AVERAGE velocityDischarge = 4 m2 x 3 m/s = 12 m3/ s

3 m/s

Velocity varies throughout a channel cross-section

Channel controls on flow: 2. Gradient or slope

2 m

2 m

Low gradient High gradient

“rise”“run”

Gradient or slope = rise/run

Channel controls on flow: 3. material on bed

Boulders

Concrete channelsSand

All else equal: Smooth bed = high velocity

Fig. 13.11

Meandering River

Point Bar

Peter Kresan

Fig. 13.12

Braided River

Tom Bean

Rivers and Geology

• Erosion• Transportation• Deposition

EROSION: Waterfall Retreating Upriver stream is eroding bedrock

Fig. 13.7Donald Nausbaum

EROSION Pebbles Caught in Eddies Form Potholes

Carr Clifton/Minden Pictures

Transport: Streams move material in three forms

• Dissolved load• Suspended load• Bed load

Fig. 13.3

Bedload -> Saltation

Fig. 13.26

Deposition 1: Mississippi Delta

Landsat 2 image annotated by Moore, 1979

Delta: sediments deposited where river meets the ocean or a lake

Fig. 13.28

Shifting Mississippi River Delta Over the Past 6000 Years

Deposition 2. Formation of Natural Levees

Levees built up further for flood control

FLOODING

Xie Jiahua/China Features/Sygma

Discharge varies through time

Boulder Creek, near Orodell

1986 1987 1988 1989 1990 1991 1992 1993 1994 1995Date

Mea

n da

ily s

tream

flow

, ft3 /

sec

Flooding

• Discharge exceeds capacity of channel • Interval between floods depends on climate, channel type,

basin size and stream network• Flooding is natural, but human activities affect flood frequency

and magnitude

Recurrence interval- Average time between occurrences of a given

event

The “100-year” flood: a flood of this size is expected every 100 years

The “500-year” flood: a flood of this size is expected every 500 years

ZONING and INSURANCE

The floodplain in Boulder

Fig. 13.1

Annual Flood Frequency Curve