Hydrologic Cycle

The Hydrologic Cycle

More than 97% of all Earth’s water is in the oceans.

Only 1 % of Earth’s water is available to us as water vapor, groundwater, and freshwater.

Earth’s waters are constantly circulating.

The driving forces are:• Heat from the Sun • Force of gravity

The Hydrologic Cycle

The hydrologic cycle is the set of processes that controls the circulation of water on Earth.

Processes involved in the hydrologic cycle:•Evaporation •Precipitation• Infiltration•Runoff

The Hydrologic Cycle

The Hydrologic CycleWater that goes from the ocean

back to the ocean makes a complete loop in the hydrologic cycle.

The journey is not always direct.

•Water can flow as streams, rivers, and groundwater

•Water can also be frozen in ice caps and glaciers

GroundwaterWater beneath the ground exists

as groundwater and soil moisture.

Groundwater occurs in the saturated zone—water has filled all pore spaces.

Soil moisture is above the saturated zone in the unsaturated zone—pores filled with water and air.

The water table is the boundary between these two zones.

GroundwaterThe depth of the water table varies with

precipitation and climate.• Zero in marshes and swamps, hundreds of

meters in some deserts.• At perennial lakes and streams, the water table is

above the land surface.• The water table tends to rise and fall with the

surface topography.

Groundwater

Factors that influence storage and movement of groundwater:• Porosity: ratio of open space in soil, sediment,

or rock to total volume of solids plus voids—the amount of open space underground.

•Greater porosity equals more potential to store greater amounts of groundwater.

• Particle size, shape, and sorting influence porosity.

— Soil with rounded particles of similar size has higher porosity than soil with various sizes.

GroundwaterPermeability

• Degree to which groundwater can flow through a porous material—higher permeability, greater potential for fluid flow.

• Sediment packing and connectedness of pores influences permeability.

• Hydraulic conductivity—a measure of permeability—tells us the degree to which the material can transmit water.

GroundwaterAquifers are reservoirs of groundwater. Aquifers generally have high porosity and high

permeability.Aquifers underlie the land surface in many

areas; they are a vital source of fresh water.It is important to keep this vital source of fresh

water clean and contaminant free.

Groundwater

A perched water table occurs when discontinuous, low-permeability layers in an unconfined aquifer intercept percolating water above the water table.

Groundwater

Geologists can often use springs to locate faults, because a spring can indicate that there are cracks or breaks in the rock.

Groundwater

The elevation of a water table above a particular location—usually sea level—is called the hydraulic head.

Groundwater

Darcy’s law:

Groundwater flow rate =

hydraulic conductivity

cross-sectional area hydraulic gradient

The Work of Groundwater

Flowing groundwater can alter and change features at the surface:

•Land subsidence

•Caves and caverns

•Sinkholes

The Work of Groundwater

Land Subsidence:• Extreme groundwater withdrawal

by pumping from wells can result in lowering of the land—land subsidence.

• Land subsidence is especially prevalent in areas underlain by aquifers made of sandy sediments and interbedded clays. The clays leak water to the sand, then when water is pumped out, the clays shrink and compact, causing subsidence.

The Work of Groundwater

Caverns and caves• The dissolving action of groundwater “eats

away” at rock—limestone in particular.

•Rainwater chemically reacts with CO2 in the air and soil, producing carbonic acid. The acidified water seeps into rock (especially limestone), partially dissolving it.

The Work of Groundwater

Groundwater has carved out magnificent caves and caverns (a cavern is a large cave).

The Work of Groundwater

Karst regions are characterized by soft rolling hills or sharp, rugged surfaces.

Karst regions are areas where sinkholes, caves, and caverns define the land surface.

The Work of Groundwater

Sinkholes are funnel-shaped cavities in the ground that are open to the sky; they are formed in a manner similar to caves. They can also be formed from conditions of drought and the over- withdrawal of groundwater.

Surface Water and Drainage Systems

Surface water includes streams, rivers, lakes, and reservoirs.

Infiltration of water is controlled by:• Intensity and duration of precipitation• Prior wetness condition of the soil• Soil type• Slope of the land•Nature of the vegetative cover

The Work of Surface Water

Flowing surface water sculpts and shapes Earth’s surface:

• Erosion—erosive sculpting action carves the landscape

• Deposition—shapes the land as sediment is deposited

The Work of Surface Water

Running water shapes Earth’s surface in two opposing ways: fast water transports sediment, slow water deposits sediment.

Streamflow—two types of flow; stream speed•Laminar flow—slow and gentle•Turbulent flow—fast and rapid

Factors that determine velocity:• Gradient, or slope• Channel characteristics (shape and size)• Discharge—volume of water moving past a given

point in a certain amount of time

The Work of Surface Water

Average stream speed =

discharge / cross-sectional area

Stream speed is usually not constant along the length of a stream. As the stream moves downslope, the gradient decreases and the channel widens. Discharge usually increases as tributaries add water.

The Work of Surface Water

Stream erosion:• Loosely consolidated particles are lifted by

abrasion and dissolution.

Stronger currents lift particles more effectively:•Stronger currents have “higher” energy•Lift and transport more and bigger particles•Turbulent versus laminar flow

The Work of Surface Water

The land area that contributes water to a stream is called the drainage basin.

Drainage basins are separated by drainage divides.

The largest drainage divides are continental divides.

The Work of Surface Water

Stream water carries substances that chemically weather and erode rock.

Abrasion occurs when sediments and particles scour a channel.Hydraulic action erodes and moves great quantities of sediment and rock.

The Work of Surface Water

Streams transport great amounts of sediment from one location to another.

Laminar flows can lift and carry only the very smallest and lightest particles.

A turbulent flow can move and carry a range of particle sizes—it moves particles downstream mainly by lifting them into the flow or by rolling and sliding them along the channel bottom. The smaller, finer particles remain suspended to make the water murky.

The Work of Surface Water

Stream channels in high mountain areas cut into underlying rock.

Fast-moving rapids and beautiful waterfalls are characteristic of V-shaped mountain stream valleys.

The Work of Surface Water

Stream speed plays a role in erosion and deposition.

Water speed varies within a channel. It is slower along the stream bed and greater near the surface. Maximum flow speed occurs mid-channel.

The Work of Surface Water

Meandering streams create a wide belt of almost flat land: a floodplain.

When a flood occurs, sediment is deposited in the floodplain. Large, coarse sediment creates natural levees.

The Work of Surface Water

A delta is where a flowing stream meets a standing body of water. The flow slows down and the stream dumps sediment.

The result is a fan-shaped deposit of new land.

Glaciers and Glaciation

Glaciers are powerful agents of erosion. A glacier is like a plow as it scrapes and plucks up rock and sediment.

Glaciers are also powerful agents of deposition. A glacier is like a sled as it carries its heavy load to distant places.

Glaciers and GlaciationA glacier is an accumulation of snow and ice

thick enough to move under its own weight.• Two types of glaciers:

— Alpine — Continental

Glaciers and Glaciation

Alpine glaciers develop in mountainous areas, generally confined to individual valleys.

—Cascades, Rockies, Andes, Himalayas—Erosional landforms: cirque, arête, horn, hanging

valley, U-shaped valley

The Work of Glaciers

When a glacier’s ice mass becomes thick enough—about 50 meters—the pressure of the overlying material causes the base of the ice to move plastically—the entire mass shifts.

Also, meltwater at the base of the glacier creates basal sliding.

The Work of Glaciers

When glacial ice melts, it drops a poorly sorted, heterogeneous load of boulders, pebbles, sand, and clay.

A wide range of particle sizes is the hallmark that differentiates glacial sediment from the much-better-sorted material deposited by streams and winds.

The Work of Glaciers

The mass of a glacier changes over time. As snow falls, accumulation makes the glacier grow. As ice melts, sublimates, or breaks off, ablation occurs.

The Work of Air

Wind blows everywhere, but its impact on sculpting the land is minor.

Impact is greatest where:• Strong winds blow frequently• Vegetation is sparse or absent

— Plant roots keep particles together— Plants deflect wind and shelter particles

• Surface particles are small — Small particles are more easily lifted and

transported

Runoff

• Water that can`t be absorbed and it is moving in the surface

• Runoff is helping to get more water in the aquifers, but also helps to the erosion of the land