EASC 1000 - Topic 8: Hydrologic Cycle and Hydrosphere

Essential vocabulary terms and definitions covering the hydrologic cycle, stream mechanics, drainage networks, groundwater properties, karst features, glaciers, and isostasy.

Cycle

A series of interrelated steps that occur in succession and can be repeated.

Earth cycle

Refers to the continuous movement of water within the hydrosphere, including processes such as evaporation, condensation, precipitation, and runoff.

Pools

The various storage locations of water within the hydrologic cycle, including oceans, rivers, glaciers, and underground aquifers.

Reservoirs

Specific locations where water is stored within the hydrologic cycle, such as lakes and artificial water bodies. These reservoirs can hold water temporarily or long-term.

Sources

Refers to the origins of water within the hydrologic cycle, including precipitation, surface runoff, and groundwater recharge. These sources supply water to various pools and reservoirs.

Sink

A location where water is removed from the hydrologic cycle, such as evaporation into the atmosphere or water flow into oceans. Sinks play a crucial role in regulating water distribution and balance.

Steady State

The condition in a mass-transfer cycle where the rate of material entering a reservoir from sources equals the rate exiting into sinks ($\text{Sources} = \text{Sinks}$).

Solar energy

is the primary driving force behind the hydrologic cycle, affecting processes like evaporation and influencing weather patterns.

Evaporation

The process by which water is transformed from a liquid state to a vapour state, primarily due to solar energy, and is a key component of the hydrologic cycle.

Precipitation

The process where water vapor condenses and falls back to the Earth's surface in various forms, such as rain, snow, sleet, or hail, playing a crucial role in the hydrologic cycle.

Ice caps and glaciers

Large masses of ice that stores 1.8% of the 2.5% of freshwater on the planet (the largest reservoir of fresh water) and influence sea levels, playing a pivotal role in the earth's climate system and hydrologic cycle.

Residence Time

The average period of time that a substance or molecule remains within a specific reservoir before being transferred to another. Depends on mass and flux.

Flux

The mass or volume of material flowing into or out of a reservoir per unit of time.

Surface runoff

The movement of water, from rain or melting snow, across the land surface and into rivers, lakes, and oceans. It is a key component of the hydrologic cycle, significantly influencing water distribution.

Drainage Basin

A geographic area, also called a catchment or watershed, from which a drainage network collects water.

Drainage Divide

A topographic ridge or highland that separates one drainage basin from an adjacent basin.

Local topographical divide

Occur at a change in direction of a slope (e.g., a small peak with streams flowing in opposite direction from the peak). A local high point in the landscape that directs water flow into different drainage basins.

Regional divide

Separates two large drainage basins (e.g., the Appalachians separating the Atlantic Ocean and Gulf of Mexico drainage basins). A large topographical feature that separates multiple drainage basins and directs water runoff into different watersheds.

Continental Divide

A major drainage divide that separates drainage systems discharging into different oceans, such as the North American Cordillera separating flow to the Pacific from flow to the Gulf of Mexico and Atlantic.

Interior drainage basins

Drainage basins that have no outlet to the sea. A hydrological area where water does not flow out to external bodies of water, instead collecting in lakes or evaporating.

Ephemeral stream

A stream that is temporary and only flows during certain periods, typically after rainfall, and may not have a continuous flow throughout the year.

Permanent stream

A stream that flows continuously throughout the year, receiving water from rainfall, melting snow, or groundwater, and maintaining a constant channel.

Dry Wash

An empty stream channel in an arid or semi-arid region that carries water only during or immediately following storms.

Ultimate Base Level

The lowest elevation to which a trunk stream can downcut its bed, which corresponds to sea level.

Sediment load

The total amount of sediment carried by a stream, including particles suspended in the water and those transported along the bed.

Dissolved Load

Ions derived from sediment, rock, or groundwater dissolution that are transported in solution within river water.

Suspended Load

Fine-grained solid particles, such as silt- or clay-sized material, carried swirling in a stream without settling to the streambed.

Bed Load

Coarse clastic sediment, including sand, pebbles, and cobbles, that moves along the bottom of a channel by rolling, sliding, or bouncing.

Saltation

A transport process in which clasts bounce along the streambed and dislodge other clasts into the flow.

Stream Competence

The maximum particle size that a stream can carry, determined primarily by water velocity.

Stream Capacity

The total quantity or mass of sediment that a stream is able to transport, governed by water volume and velocity.

Alluvial Fan

A broad, gently-sloping wedge of sediment deposited where a fast-flowing ephemeral stream abruptly emerges from a steep mountain canyon onto an open plain.

Braided Stream

A stream consisting of an interwoven network of channels separated by sediment bars, formed where high sediment load chokes channels during drops from flood stage to normal flow.

Meandering Stream

A stream exhibiting continuous sweeping curves (meanders) that develops across gentle gradients, eroding the outer channel wall and depositing sediment on point bars along the inside bend.

Groundwater

Water residing in underground sediment and rock pores beneath the surface, holding $60$ times more water than surface reservoirs combined and providing two-thirds of human freshwater resources.

Porosity

The percentage of the total volume of a rock or sediment consisting of open pore spaces or cracks, expressed as a %.

Primary Porosity

The original pore space formed within a rock during its genesis, such as un-cemented spaces in sedimentary rock or vesicles in extrusive igneous rock.

Secondary Porosity

New voids and open spaces produced in a rock after its initial formation, created through fracturing or dissolution by circulating fluids.

Permeability

The capacity of a material to transmit fluids through interconnected open pore spaces and channels. Depends on the number of available conduits, the size of the conduits, and the straightness of conduits through which a fluid can pass.

Aquifer

A permeable, porous layer of sediment or rock that can store and transmit significant volumes of groundwater.

Aquitard

A low-permeability rock or sediment layer that impedes the transmission and flow of groundwater.

Unconfined Aquifer

An aquifer whose water table is open to the surface, allowing water to infiltrate directly downward from the ground.

Confined Aquifer

An aquifer that is isolated from the land surface by an overlying low-permeability aquitard.

Water Table

The subsurface boundary separating the unsaturated (vadose) zone above from the fully saturated (phreatic) zone below.

Saturated (Phreatic) Zone

The subsurface region below the water table where all available pore spaces and cracks are completely filled with water.

Unsaturated (Vadose) Zone

The subsurface zone above the water table where pore spaces contain both air pockets and surface moisture films.

Spring

A natural surface emergence of water occurring where the water table intersects the ground surface.

Cone of Depression

A downward-curving, funnel-shaped depression of the water table produced around an actively pumped well.

Karst Landscape

A topography characterized by caves, subterranean drainage, and sinkholes, formed by the dissolution of limestone via the reaction: $CaCO_3(s) + H_2O + CO_2 \rightarrow Ca^{2+}(aq) + 2HCO_3^-(aq)$.

Sinkhole

A depression or collapse structure on the land surface formed when the roof of a subterranean limestone cave collapses.

Ice

Classified as a mineral, is solid H20 with a defined crystal structure with a hexagonal form.

Firn

Granular, compacted snow containing approximately $25\%$ air, representing an intermediate stage in the transition of loose snow into glacial ice.

Alpine Glacier

A glacier that originates at high elevations in mountainous topography and flows downward under gravity, including cirque, valley, and piedmont glaciers.

Continental Glacier

A vast, thick ice sheet spreading over thousands of square kilometers of continental crust, presently found in Greenland and Antarctica.

Ice ages

Periods on Earth when the climate was significantly cooler and glaciers were able to grow far larger than they are today. Within ice ages there is also times with shorter-term glacial advances and retreats. Ice ages can be seen at certain times in the geological record (e.g., tillites) that suggest 4-5 major cases: Pleistocene, Permian, Late Proterozoic, Early Proterozoic, and possibly in the Archean.

Pleistocene Ice Age

Glaciers reached their maximum extent 11,700 years ago during the last ice age before receding. Ice sheets extended from different areas with topographical highs and merged in several places (such as the Laurentide ice sheet merged from the Labrador and Keewatin ice sheets).

Glacial Subsidence

The downward bending and depression of the lithosphere into the asthenosphere caused by the heavy weight of an overlying ice sheet.

Glacial Rebound

The slow, upward isostatic uplift of the Earth's crust following the melting and retreat of an ice sheet as asthenospheric material flows back.

Isostasy

The condition of gravitational equilibrium between the Earth's lithospheric crust and the underlying flowing asthenosphere.