© 2012 Pearson Education, Inc. Water. © 2012 Pearson Education, Inc. Hydrologic Cycle The hydrologic cycle is a model that describes the movement of water

© 2012 Pearson Education, Inc.

Water


Hydrologic Cycle• The hydrologic cycle is a model that

describes the movement of water through the reservoirs of the Earth

• Processes involved in the hydrologic cycle

• Precipitation• Evaporation• Infiltration• Runoff• Transpiration


Hydrologic CycleEvaporation: Water

changes from liquid to vapor

Transpiration: Water taken up by plants passes into the atmosphere

Condensation: Water changes from vapor to liquid

Precipitation: Water returns to the surface of the earth as rain or snow.


Hydrologic Cycle

Surface Runoff: precipitation that drains over the land or in streams

Infiltration: Water works its way into the ground through small openings in the soil.


The Hydrologic Cycle


Sources of Earth’s water


Streamflow

• Surface Runoff begins as overland or sheet flow as precipitation moves downhill

• Sheet flow eventually becomes a stream - water that flows downslope along a clearly-defined natural pathway.


Streamflow Characteristics• Stream gradient - steepness of the slope. The gradient is highest near the stream’s headwaters (source) and decreases as it moves downslope toward the ocean.

• Stream discharge - the amount of water passing any point during a unit of time (example: ft3/sec).

• Stream load – suspended and dissolved sediment carried by the stream.


Types of Stream Channels - Straight

Straight Channel

– Usually occur only in short stretches

– Generally have a high gradient (near headwaters)

– Classic V-shaped valley (as opposed to glacial U-shaped valley)


Types of Stream Channels- Braided

Braided Channel

– Low gradient

– Large, variable sediment load that varies seasonally (e.g. snowmelt, storm activity)


Types of Stream Channels- Meandering

Meandering Channel

– Usually occur downstream, near the mouth

– Low gradient

– Cut bank erosion and point bar deposition

– May eventually form an oxbow lake


Oxbow Lake– Form when the meanders get too close together.

– River cuts a new, direct channel between the meanders. Meandering Channel

– Oxbow lake will eventually become a swamp and dry up.


Stream Deposits

Floodplain – Flat valley floor adjacent to the stream, which is inundated when the stream overflows its banks

Alluvium – unconsolidated sediment deposited by the stream


Stream Deposits – Alluvial Fans• Rapid change from

high gradient to low gradient environment causes stream to slow down and drop its sediment load.

• Fan-shaped deposit as water escapes its channel.

• Often found in arid to semiarid environments with infrequent rainfall/flash flood conditions.


Stream Deposits – Delta• Similar to an alluvial

fan, but found at the mouth of the stream

• Fan-shaped deposit as stream flows into ocean and slows down, dropping its sediment load.

• Distinctive fan-shape looks like the Greek letter delta: Δ


Drainage Basins• Drainage Basin: The

total land area that contributes water to a stream. Also called a watershed, or catchment basin.

• The drainage pattern consists of the interconnected network of streams in an area

• A drainage basin of one stream is separated from the drainage basin of another by an imaginary line called a divide.


Successive drainage basins: The Yellowstone, Missouri, and Mississippi


Mississippi River Drainage Basin


Surface water resources

• Fresh water supply– Essential for

• Direct human consumption• Crop and livestock• Industry

– Scarcity of water resources

• Many populations around the world lack access to clean drinking water

• Cause of socio-political conflicts


Water-related conflicts in California• Colorado River

• Colorado River Compact among 7 states• Dispute between Imperial County and San Diego.

• Effect of Los Angeles Aqueduct on Mono Lake, Owens Lake, Owens River Gorge


Groundwater - Subsurface water contained in pore spaces in sediment and rock

• Zone of aeration (vadose zone): pore space mainly filled with air

• Saturated zone (phreatic zone): pore space filled with water

• Water Table: Top surface of the saturated zone


Groundwater• Groundwater moves

through the interconnected pore spaces of rock and sediment from areas of higher pressure to areas of lower pressure

• Rock porosity– Relative amount of space in underground rock

• Rock permeability– Ease of flow through rock


Groundwater

• Water table tends to mimic topography

• The water table comes up to the surface at streams, springs, and lakes


Discharge and Recharge

• Water percolation - Process of water seepage through rocks and sediment

• Recharge - Replenishment of groundwater• Discharge – process of water moving from the

subsurface to the surface.


Aquifers and Aquicludes

• Aquifer– Saturated rock or sediment that is porous and permeable, so able to be accessed by well.

• Aquiclude– Layer of impermeable rock or sediment from which water cannot be drawn


Types of Aquifers• Unconfined aquifer

– Water table not protected from surface recharge

– Vulnerable to surface and near surface contamination

– Well water rises to water table and must be pumped to surface

• Confined aquifer– Protected from surface by aquiclude,

– Recharge area is often distant

– Water under pressure and rises above water table in well


Artesian Well– Well which accesses water from a confined aquifer

– Water rises in artesian wells without pumps due to release of confining pressure

– Less likely to be contaminated by surface runoff, but otherwise the same H2O.

Documents

© 2012 Pearson Education, Inc. Water. © 2012 Pearson Education, Inc. Hydrologic Cycle The hydrologic cycle is a model that describes the movement of water