Water Systems and the Hydrological Cycle

Water Systems

4.1 Water

Introduction to Water Systems

  • Water systems are vital for supporting life on Earth.
  • They interact with other systems, including the carbon cycle.

Movements of Water in the Hydrosphere

  • Solar Radiation: Drives evaporation and evapotranspiration.
  • Condensation: Releases heat.
  • Precipitation: Moves water from the atmosphere to the Earth's surface due to gravity.
  • Air Movements: Redistribute water from place to place.

Phase Changes in the Hydrological Cycle

  • Freezing: Liquid to solid.
  • Melting: Solid to liquid.
  • Evaporation: Liquid to gas.
  • Condensation: Gas to liquid.
  • Sublimation: Solid to gas.
  • Deposition: Gas to solid.

Global Hydrological Cycle Components

  • Condensation: Water vapor turning into liquid.
  • Advection: Horizontal movement of water, especially in the atmosphere.
  • Sublimation: Conversion of snow or ice directly to vapor.
  • Snow & Glaciers: Solid water storage.
  • Melt: Conversion of solid water to liquid.
  • Runoff: Surface water flowing into streams or rivers.
  • Evapotranspiration: Combined evaporation and transpiration from vegetation.
  • Soil Moisture: Water content in the soil.
  • Groundwater: Water beneath the Earth's surface.
  • Infiltration: Water seeping into the soil.
  • Overland Flow: Water flowing over the land surface.
  • Percolation: Downward movement of water through soil and rock.
  • Groundwater Flow: Movement of groundwater.
  • Lake: Storage of surface water.
  • Streamflow: Water flowing in a stream.
  • Stream: A channel carrying flowing water.
  • Ocean: Major water reservoir.

Detailed Hydrological Cycle Processes

  • Precipitation: Water falling from the atmosphere.
  • Advection: Wind-blown movement affecting water distribution.
  • Freezing: Phase transition from liquid to solid water.
  • Melting: Phase transition from solid to liquid water.
  • Sublimation: Direct phase transition from solid to gas.
  • Evaporation: Phase transition from liquid to gas (water vapor).
  • Surface Flow (Overland/Surface Run-off): Water flowing over the land surface.
  • Water Table: Upper level of saturated soil.
  • Throughflow: Lateral movement of water through the soil.
  • Reservoir: Storage of water.
  • Infiltration (Percolation): Downward movement of water into the soil and bedrock.
  • Groundwater Flow: Movement of water beneath the earth’s surface.
  • Condensation: Phase transition from gas/vapor to liquid.
  • Evapotranspiration: Combined processes of evaporation and transpiration.
  • Transpiration: Release of water vapor from plants.
  • Evaporation: Phase transition of liquid to gas/vapor.
  • Channel Floor (River): Bottom of a river channel subjected to flooding.
  • Interception: Precipitation that is captured by vegetation.
  • Ocean: Large body of saltwater.
  • Stream Flow: Flow of water in a defined channel.
  • Interception Storage: Water temporarily stored on plant surfaces.
  • Stemflow and Drip: Water flowing down plant stems and dripping from leaves.
  • Surface Storage: Water stored on the land surface.
  • Soil Moisture Storage: Water stored in the soil.
  • Seepage: Water leaking out of the ground.
  • Aeration Zone Storage: Unsaturated zone in the soil.
  • Groundwater Recharge: Replenishment of groundwater.
  • Groundwater Storage: Water stored underground.
  • Through Flow (Interflow): Lateral water movement in the soil.
  • Groundwater Flow (Baseflow): Contribution of groundwater to streamflow.
  • Channel Storage: Water stored within a river or stream channel.
  • Run-off: Total water that flows off the land surface.

Global Water Storage Distribution

  • Total Global Water:
    • Saltwater: 96.5\%
    • Freshwater: 3.5\%
  • Freshwater Distribution:
    • Glaciers and Ice Caps: 1.7\%
    • Groundwater: 1.7\%
    • Surface Freshwater: 0.02\%
    • Atmosphere: 0.001\%
    • Organisms: 0.0001\%

Hydrological Stores

  • Ice: Covers approximately 10\% of the Earth’s terrestrial surface.
  • Groundwater: Much lies deep (around 600m), but shallower groundwater is accessible to people and plants, some being tens of thousands of years old.
  • Rivers and Lakes: Most accessible freshwater sources but often polluted.

Flow in the Hydrological Cycle

  • The cycle includes evaporation, water vapor transport (advection), condensation, precipitation, runoff, groundwater flow, and evapotranspiration.
  • Global Precipitation: Annually averages 860 mm.
    • 77\% falls over the oceans.
    • 23\% falls on land.
  • Evaporation:
    • 84\% from the oceans.
    • Net horizontal transfer of 7\% from land to sea.
  • Precipitation over Land:
    • 16\% evaporates or transpires.
    • 7\% runs off to the oceans.
  • 96.5\% of all free water is stored in the oceans.

Annual Water Exchange (Thousands of km³)

  • Evaporation: 496.0
    • From Oceans: 425.0
    • From Land: 71.0
  • Precipitation: 496.0
    • From Oceans: 385.0
    • From Land: 111.0
  • Run-off to Oceans: 41.5
    • From Rivers: 27.0
    • From Groundwater: 12.0
    • From Glacial Meltwater: 2.5

Key Terms

  • Precipitation, Interception, Evaporation, Transpiration, Infiltration, Condensation, Sublimation, Deposition, Advection, Percolation, Groundwater flow, Stream flow, Aquifers

Systems Diagram of the Hydrological Cycle

Flows

  • Transfer and transformation of water.

Stores

  • Atmosphere, Ice + glaciers, Surface Water (lakes+rivers), Oceans, Groundwater, Plants, Animals

Processes

  • Sublimation, respiration, evaporation, condensation, melting, freezing, precipitation, advection, infiltration, percolation, absorption, evapotranspiration, surface runoff, streamflow, flooding, currents

Rainfall and Evapotranspiration

Rainfall Pathways

  • Interception loss from intercepted rainfall on canopy.
  • Throughfall from trees.
  • Stem flow from wet stems.
  • Drip from canopy.
  • Net rainfall from litter.
  • Rain onto open water and bare soil.

Water Movement

  • Overland flow.
  • Subsurface flow.
  • Infiltration into soil water store.
  • Water table reaching groundwater.
  • Groundwater recharge and storage.
  • Groundwater discharge into streams.
  • Soil water uptake leading to streamflow.

Infiltration Variations by Soil Texture

  • Soil types (Sand, Silt, Clay) influence infiltration rates.

Influences of Deforestation

  • Deforestation impacts runoff, groundwater, and sediment transport.
  • Trees and plants filter water, reducing pollutants.

Groundwater Flow

  • Soil filtration removes pollutants.

Human Activities Altering Hydrological Flows

Natural vs. Urban Water Cycle

  • Natural landscapes have high infiltration and groundwater flow.
  • Urban landscapes have less infiltration, more runoff due to impermeable surfaces (roofs, roads).
  • Leads to increased stream flooding and reduced groundwater recharge.

Agriculture's Influence

  • Arable Farming: Removes vegetation, increasing erosion vulnerability.
  • Pastoral Farming: Reduces interception and infiltration.

Interception in Agriculture

  • Depends on vegetation type and density.
  • Cereals intercept less than broadleaf crops.
  • Row crops leave soil exposed.
  • Forests promote greater infiltration than grasslands.

Land-Use and Infiltration

  • Grazing compacts soil, reducing infiltration.
  • Poor drainage leads to waterlogging and salinization.
  • Infiltration rates vary by ground cover type.

Deforestation Impacts

  • Reduces interception and increases overland flow.
  • Leads to more severe flooding.
  • Increases flood runoff and decreases channel capacity due to increased sediment deposition.

Deforestation Effects

  • Causes increased sediment transport.
  • Increases overland flow, leading to erosion.
  • Reduces interception, infiltration, and evapotranspiration.

Runoff and Erosion Following Deforestation

  • Rainfall and slope relationships significantly impact runoff and erosion.
  • Data examples illustrate these relationships across different localities.

Urbanization Effects

  • Increases impermeable surfaces, reducing infiltration.
  • Increases drainage via sewers and drains.

Impact of Impervious Surfaces

  • Natural ground cover vs. 10-20\%, 35-50\%, and 75-100\% impervious surfaces show differences in water infiltration.

Urbanization Effects on Drainage Basins

  • Greater impact on lower parts of drainage basins.
  • Increases erosion by increasing river water volume and flow speed.
  • Can lead to less erosion with riverbank protection schemes.

Urbanizing Influence

  • Vegetation removal decreases evapotranspiration and interception.
  • Construction increases stream sedimentation.
  • Development decreases porosity, increasing runoff and peak discharges.
  • Storm drains increase runoff volume and flood potential.

Water Harvesting and Storage

  • Improves yields of rainfed crops and provides water for domestic use.
  • Farmers globally use it for erosion reduction and increased crop yields.
  • Rainwater harvesting collects runoff from roofs or ground surfaces.
  • Floodwater harvesting collects discharges from watercourses.
  • Storage includes soil moisture, groundwater, and surface storage.

Steady State of Water Bodies

  • Flow diagrams illustrate inputs (precipitation, transfers, groundwater flow) and outputs (evaporation, transpiration, discharge).
  • Used to calculate sustainable water-harvesting rates.

Water Balance for the Amazon Rainforest

  • Flow diagrams represent long-term water stores.
  • Soil and bedrock moisture may not be included.

Impact of Water Use on Aquifers

  • The volume of water used must be less than precipitation minus evapotranspiration for sustainability.
  • Groundwater use must not exceed annual recharge.
  • Ogallala Aquifer under the High Plains irrigates over 20\% of the USA's cropland but is depleting in parts.

U.S. Freshwater Withdrawals (2010)

  • Thermoelectric Power: 45\%
  • Irrigation: 32\%
  • Public Supply: 12\%
  • Self-Supplied Industrial: 5\%
  • Aquaculture: 3\%
  • Mining: 2\%
  • Self-Supplied Domestic: 1\%-2\%
  • Livestock is less than 1\%.

Global Water Withdrawals

  • Trends in Total Water Withdrawals over time.
  • Breakdown by sector (Agriculture, Domestic, Industry, Power generation, etc.).
  • Regional comparisons (Latin America & Caribbean, Asia, North America, Europe, Africa).

Domestic Water Use

  • Total domestic water use (km³) by region.
  • Per capita domestic water use (L/day) by region.

Exercises

  1. Identify the main inputs and outputs from the basin hydrological cycle.
  2. State the approximate amount of the world’s water that is freshwater.
  3. Identify the largest store of the world’s freshwater. Comment on its availability to people.
  4. Outline the main impacts of urbanization on the hydrological cycle.