Hydrologic Cycle

what is the hydrologic cycle?

The hydrologic (water) cycle is the continuous circulation of water between:

• the atmosphere

• the Earth’s surface

• the subsurface

• the biosphere

Stream

• a body of running water that flows into a channel.

• moves from upstream to downstream.

• heatwaters = starting point of stream

• mouth = the outlet of a stream where it discharges into another body of water

what is the hydrologic cycle driven by?

Solar energy: drives evaporation (warms liquid water into water vapor that rises into atmosphere) and transpiration (release of water vapor from plants)

Gravity: causes that water to fall as precipitation (rain, snow) and flow as rivers and groundwater back to oceans (runoff) and groundwater flow

why does water demand roughly equal water supply in natural environments?

because ecosystems evolve in response to climate

what human activities disrupts natural balance between water supply and water demand?

• irrigation

• damming rivers

• groundwater pumping

• urbanization (impermeable surfaces)

definition of percipitation

The process by which water falls from the atmosphere to Earth’s surface in solid or liquid form (rain, snow, sleet, hail).

why does precipitation occur?

• Warm air holds more water vapor than cold air.

• When moist air rises, it:

  1. expands

  2. cools

  3. reaches saturation

  4. condenses into cloud droplets

  5. produces precipitation

definition of infiltration

the downward movement of water from the land surface into the soil.

what are three things that can happen once water is infiltrated?

• some evaporates

• some is taken up by plants (transpiration)

• some percolates downward to become groundwater

factors controlling infiltration

• soil texture - permeability (sand has fast infiltration rate because it has large and well-connected pores vs. clay has low permeability so it acts like a sponge and retains a lot of water)

• vegetation cover (higher vegetation cover increases infiltration rate - roots create channels and breaks up compacted soil)

• soil saturation (higher soil saturation with water decreases infiltration and forces water to run off)

• land use (urban areas with concrete and other impervious surfaces blocks water from entering soil, reducing infiltration vs. natural areas have natural pores in soil that allow water to go through)

Runoff definition

Runoff occurs when precipitation flows over the land surface rather than infiltrating. If the rain is falling faster than it can infiltrate into the ground, then water will remain on top of the soil and flow downhill

• very fast process

• delivers water to streams and lakes within minutes to hours

• dominant in urban environments

Causes of runoff

• rainfall intensity exceeds infiltration capacity

• saturated soils

• impermeable surfaces (e.g. concrete or asphalt roads in urban areas)

evaporation

the process of turning from liquid into water vapor.

Driven by solar energy

Occurs from oceans, lakes, rivers, soil

Transpiration

Plants absorb water from soil through their roots. Transpiration is the process where plants release water vapor into the air, primarily through tiny pores (stomata) on their leaves.

evapotranspiration

The sum of evaporation from bodies of water and the ground surface and transpiration from plants.

Sublimation

Solid ice or snow turns directly into water vapor

Common in cold, dry environments

What. is relative humidity (RH)? provide formula

Relative humidity is the ratio of actual water vapor in the air to the maximum amount the air can hold at that temperature.

RH = (actual water vapor) / (maximum possible water vapor)

describe the relationship between temperature and relative humidity

Warm air can hold more water vapor than cold air, so when air cools its relative humidity increases; once relative humidity reaches 100%, the air becomes saturated, causing water vapor to condense into droplets that eventually fall as precipitation.

• Cooling air → RH increases (maximum amount the air can hold decreases)

• Warm Air → Higher Capacity for Water Vapor before becoming saturated

• RH reaching 100% → condensation → precipitation

summary of global rainfall patterns

• High precipitation where air rises

  • equatorial regions

  • mid-latitude storm belts

• Low precipitation where air descends

  • subtropical deserts

steps of the Rain Shadow Effect (orographic effect)

1. Prevailing winds carry warm air over oceans, where it gathers moisture as water vapor. Moist air moves inland from ocean

2. Air rises over mountain range

3. Rising air cools → condensation into clouds → precipitation as rain or snow

4. Windward slope becomes wet

5. Drier air descends on the leeward side of the mountain

6. Descending air warms → relative humidity drops

7. Leeward side becomes a dry area with little to no parcipitation (aka rain shadow)

what happens to the level of runoff and soil saturation during wet seasons?

• high runoff

• saturated soils

• groundwater recharge

hydrology during dry seasons

• low runoff

• streams sustained by groundwater discharge

why is snowmelt so critical for recharging hydrological systems during dry seasons?

• Snowmelt acts as a natural reservoir

• Snowfall that gets stored on high mountains melts during the spring

• melt runoff into river systems and recharge aquifers, groundwater systems, and agriculture

meteroic water

Groundwater derived from precipitation that infiltrates the ground.

definition of porosity

Porosity is the percentage of total rock or sediment volume that consists of open space (pores).

Primary porosity

Formed during deposition or rock formation (Ex: spaces between sand grains, vesicles in basalt)

Porosity decreases with burial compaction and cementation

Crystalline rocks have very little primary porosity

Secondary porosity

new pore space created after the rock was first formed. (ex: fractures, fault breccia, solution cavities)

Intragranular Porosity

pore spaces within grains

fracture porosity

type of secondary porosity formed by cracks/fractures in rocks

factors affecting porosity

• grain size

  • coarse-grained rocks tend to have higher porosity than fine-grained rocks

• Grain shape

  • rounder → higher porosity

  • More angular → tighter packing → less pore space → lower porosity

• how loosely or tightly grains are packed

• cementation

  • When spaces are filled by parcipitation of minerals, it decreases porosity

definition of permeability

Permeability is the ability of a material to allow fluids to flow through connected pore spaces.

*depends on the interconnectedness pores

does high porosity always mean high permeability?

No!

ex) Clay = high porosity, low permeability

Aquifer

A rock or sediment unit that:

• has high porosity

• has high permeability

• can store and transmit groundwater

Aquitard

a lower-permeability and lower porosity rock that hinders water flow

Unconfined Aquifer

• directly connected to the surface, with its top boundary being the water table. Directly exposed to atmospheric pressure

• Easily recharged by rain and water level fluctuates depending percipitaton ion levels

• easily contaminated

Confined Aquifer

• lies in between impermeable layers (aquitards)

• Lower risk of pollution/contamination due to protective layers

Vadose Zone

• Unsaturated zone

• Contains air + water

Phreatic Zone

• Saturated zone

• All pores filled with water

Water Table

 the top of the groundwater reservoir in the subsurface. Boundary between the the unsaturated (vadose) zone and the saturated (phreatic) zone. The capillary fringe sits at the top of the boundary.

How does the depth of the water table vary based on climate/season?

• Humid regions: water table lies close to the surface, ponds and streams are connected to water table and remain filled

• Dry regions: water table sinks deep below the surface. Ponds and streams are disconnected from the water table and dry up. 

what direction does groundwater flow?

Groundwater flows from areas of high hydraulic head to areas of low hydraulic head, moving in the direction of the hydraulic gradient

Recharge

groundwater recharge occurs where surface water (rain, rivers, lakes, irrigation) infiltrates through permeable soil and rock into aquifers, replenishing groundwater

(typically high elevations)

Discharge

• Water exits groundwater system typically at low elevations

• Occurs at:

  • springs

  • streams

  • lakes

  • wetlands

Darcy’s Law

• equation that predicts the volume of water passing through an area of an aquifer per time. Driven by the slope of the water table (hydraulic gradient) and the material’s permeability.

• Q=A⋅K((ha​−hb​)​/L)

• groundwater discharge (Q) is directly proportional to hydraulic conductivity (K) and cross-sectional area (A), and directly proportional to the hydraulic gradient

hydraulic head (h)

• level to which water rises in a well / drill hole

• For a given aquifer & distance of travel, the rate at which water flows from one point to another is directly proportional to the vertical drop in elevation between the two points. (As vertical drop increases, rate of flow increases)

• For a given aquifer and vertical drop, the rate of flow is inversely proportional to the distance the water travels. (As distance increases, rate of flow decreases)

Hydraulic gradient (H)

• represents the slope of the water table (change in hydraulic head (water pressure + elevation) per unit of distance along a flow path

• Steeper gradient → faster flow

• Longer distance → slower flow

(h1-h2) / L

Well

• Artificial access to groundwater. Holes are excavated or drilled to obtain water

• Pumping wells lowers water table locally

Cone of depression

• a funnel-shaped area where the groundwater level drops around a well because water is being pumped out faster than it can be naturally replenished

• Occurs when rate of pumping > rate of recharge

Springs

• Natural groundwater discharge points

• Occur where water table intersects surface

Artesian Well

• A well that taps a confined, pressurized aquifer, causing water to rise to the potentiometric surface (the pressure level of the aquifer)

• If the well opening is below the potentiometric surface, water is naturally pressurized and can flow upward without mechanical pumping

Hot springs

groundwater that is heated either by shallow bodies of magma or by circulation through faults to hot rock deep in the Earth's crust (driven by geothermal gradient)

gyser

a type of hot spring that experiences cyclic eruptions of steam and hot water. These eruptions are caused by the interaction of groundwater with magma, leading to pressure-controlled boiling erupting through a narrow channel

Identify 4 issues related to groundwater

1. depletion

2. reversing flow direction

3. saline intrusion

4. contamination plumes

Groundwater depletion

Long-term groundwater withdrawal lowers the water table, reducing groundwater discharge to streams, wetlands, and swamps, causing them to dry up.

How does groundwater depletion lead to aquifer collapse and land subsidence?

Excessive groundwater extraction causes sediment grains to compact, reducing pore space and permeability, which lowers aquifer volume and causes the land surface to sink.

how does recharge diversion lead to groundwater depletion?

Diverting water from natural recharge areas (streams, canals, wetlands) reduces groundwater replenishment, leading to aquifer depletion and drying of surface waters.

Reversal of Groundwater Flow Direction

Heavy pumping creates a cone of depression that reverses the hydraulic gradient, drawing groundwater toward wells

Saltwater intrusion

In coastal aquifers, freshwater floats on denser saltwater; pumping lowers freshwater pressure, causing the saltwater boundary to rise and contaminate wells.

contaminant plume

A zone of polluted groundwater formed as contaminants are transported down-gradient, with highest concentrations near the source and decreasing with distance.

groundwater contamination clean-up via pumps

Contaminated groundwater can be treated by pump-and-treat systems (putting a lot of wells into the area to extract the contaminants and prevent plume from flowing into adjacent regions)

*very expensive and slow process

Bioremediation

A remediation method that uses microorganisms or bacteria to break down or neutralize contaminants