4.1A Hydrological cycle Notes

Movements of Water in the Hydrosphere

1. Water is constantly circulating within the hydrosphere, which includes all water found on, under, and above the Earth’s surface.
2. These continuous movements are driven by solar energy and gravity, two fundamental forces maintaining the global water cycle.

Solar Radiation as a Driving Force

1. Solar energy provides the heat that drives evaporation and transpiration.
2. During evaporation, water molecules gain energy and transform from a liquid to a vapour state, entering the atmosphere.
3. Transpiration occurs when plants lose water vapour through small openings (stomata) in their leaves.
4. Together, these two processes are known as evapotranspiration, responsible for most of the water vapour in the atmosphere.

Gravity: The Force of Downward Flow

1. Gravity causes precipitation to fall to the Earth’s surface and directs surface runoff, infiltration, percolation, and streamflow.
2. Water at high altitudes (e.g., mountains) possesses gravitational potential energy that converts to kinetic energy as it flows downhill.
3. This gravitational movement returns water from land back to the oceans, completing the global water cycle.

The Global Hydrological Cycle as a System

1. The global hydrological cycle operates as a closed system, meaning that water is neither added to nor removed from Earth.
2. It only moves and changes state within the system.
3. However, it remains open in terms of energy because solar radiation and gravity drive these processes.
4. It is composed of stores (reservoirs of water) and flows (movements between them).
5. Flows are the transfers (movement without state change) and transformations (state change, e.g. evaporation) that move water between stores.

Main Components of the Global Water Cycle

1. Inputs: Solar radiation (energy source).
2. Stores: Oceans, glaciers, groundwater, rivers, lakes, soil moisture, and the atmosphere.
3. Flows (Transfers): Precipitation, infiltration, runoff, and groundwater flow.
4. Transformations: Evaporation, condensation, and freezing or melting.

Interactions Within the System

1. The hydrological cycle connects the atmosphere, biosphere, hydrosphere, and lithosphere.
2. It transfers energy through processes such as evaporation (absorbs heat) and condensation (releases heat).
3. It transfers matter (water and dissolved nutrients) through precipitation, runoff, and groundwater movement.
4. These interactions influence climate patterns, weather events, and ecosystem productivity.

Major Stores in the Water Cycle

1. Earth’s water exists in six main reservoirs: oceans, ice and snow, groundwater, surface water, the atmosphere, and living organisms.
2. Each store differs in volume, state (solid, liquid, vapour), and residence time (the average duration water remains there).

1. Oceans

1. Oceans contain approximately 96.5% of Earth’s total water, making them the largest water reservoir.
2. This water is saline and unsuitable for direct human consumption.
3. Energy from the Sun causes evaporation, transferring vast amounts of water vapour to the atmosphere.
4. Ocean currents also play a major role in redistributing heat energy globally.

2. Glaciers and Ice Caps

1. Glaciers and polar ice caps store around 1.7% of Earth’s water, nearly all of which is freshwater in solid form.
2. Found mainly in Antarctica, Greenland, and high mountain ranges, these frozen stores represent long-term water reservoirs.
3. When melting occurs, freshwater enters oceans, influencing sea levels and climate regulation.

3. Groundwater

1. Groundwater resides in aquifers, which are permeable rock layers capable of storing and transmitting water.
2. It makes up around 1.7% of total water but represents the largest accessible source of liquid freshwater.
3. Groundwater is replenished through infiltration and percolation from precipitation, though recharge rates vary with soil type and climate.
4. Over-extraction for agriculture and urban use can lead to aquifer depletion and land subsidence.

4. Surface Water

1. Includes rivers, lakes, wetlands, and streams, accounting for only 0.02% of Earth’s total water.
2. Although small in volume, it is the most accessible freshwater source for human use.
3. Surface water responds quickly to climatic events, such as precipitation or drought, making it vital for ecosystems and agriculture.

5. Atmospheric Water

1. The atmosphere holds a small yet crucial amount of water (≈0.001% of global total) in the form of vapour and clouds.
2. Water vapour absorbs and retains heat, making it an important greenhouse gas that regulates Earth’s temperature.
3. Average residence time is only about 8–10 days, reflecting rapid turnover through precipitation and evaporation cycles.

6. Water in Living Organisms

1. A minuscule fraction of total water (0.0001%) exists within plants, animals, and microbes.
2. Despite its small amount, it is essential for biochemical processes such as photosynthesis, digestion, and nutrient transport.
3. Organisms continually exchange water with their surroundings through respiration, transpiration, and excretion.

Flows in the Hydrological Cycle: Transfers and Transformations

1. The hydrological cycle consists of flows that transfer water between different stores through physical and atmospheric processes.
2. These flows include phase changes (transformations) and movement of water across Earth's surface and subsurface (transfers).

Transfers in the Hydrological Cycle

1. Precipitation

1. The process by which condensed water vapour in the atmosphere falls to Earth as rain, snow, sleet, or hail.
2. Returns atmospheric water to the lithosphere and hydrosphere.
3. Precipitation distribution varies greatly with latitude, altitude, and wind patterns.
4. Plays a key role in recharging groundwater and maintaining stream flow.

2. Advection

1. The horizontal movement of water vapour, clouds, or precipitation through the atmosphere due to wind.
2. Responsible for redistributing moisture across the globe.
3. Without advection, regional climates would become highly unbalanced.

2. Surface Runoff and Streamflow:

1. Surface runoff occurs when rainfall exceeds the soil’s infiltration capacity, causing excess water to flow overland into rivers and lakes.
2. Streamflow is the movement of water within river channels, ultimately transporting it to the oceans.
3. Both processes are driven by gravity and are key components in shaping landscapes.

3. Infiltration

1. The downward movement of water through soil pores.
2. Influenced by soil texture, vegetation cover, and land use.
3. Allows water to recharge aquifers and sustain base flow in rivers during dry periods.

4. Percolation

1. The deeper movement of infiltrated water through rock layers to the groundwater table.
2. Slow process, creating long-term water storage.

5. Throughflow and Groundwater Flow

1. Throughflow moves laterally through soil, contributing to stream discharge.
2. Groundwater flow represents the slowest transfer, occurring deep below the surface, often taking years or centuries to reach oceans.

Transformations in the Hydrological Cycle

1. Evaporation

1. Conversion of liquid water into water vapour due to solar energy.
2. Major energy transfer process linking the hydrosphere and atmosphere.
3. Influenced by temperature, wind speed, humidity, and surface area.

2. Condensation

1. Cooling of water vapour into liquid droplets, forming clouds and dew.
2. Releases latent heat energy, which drives atmospheric circulation and storms.

3. Freezing and Melting

1. Changes between liquid and solid states store or release latent heat energy.
2. Important in regulating polar climates and seasonal water availability.

4. Sublimation and Deposition

1. Sublimation: Direct change from solid ice to water vapour (common in polar regions).
2. Deposition: Reverse process, forming ice crystals without passing through a liquid state.

5. Transpiration

1. Water released from plant leaves through stomata.
2. Converts liquid water in plants to water vapour, contributing to atmospheric humidity.
3. Vital for regulating plant temperature and global water cycling.

Systems Diagram of the Hydrological Cycle

To visualize these processes, a systems diagram can be created, where:

1. Stores (e.g., oceans, glaciers, groundwater) are represented as boxes.
2. Flows (e.g., evaporation, infiltration, runoff) are represented as arrows connecting the stores.
3. Transformations (e.g., condensation, sublimation) show phase changes of water.

How Human Activities Alter the Water Cycle

1. Human activities are altering the rate, distribution, and quality of water flows and stores worldwide.
2. These changes can disrupt the natural equilibrium of the hydrological cycle, leading to floods, droughts, and long-term ecosystem degradation.

1. Agriculture and Irrigation

1. Irrigation withdraws large amounts of groundwater and surface water, leading to aquifer depletion.
2. Fertilizer use can contaminate runoff, affecting downstream water quality.
3. Drainage systems in agricultural lands lower soil moisture levels, reducing natural storage capacity.

2. Deforestation

1. Removal of vegetation reduces interception and root absorption, increasing surface runoff.
2. Decreased transpiration lowers local humidity and cloud formation, potentially reducing rainfall.
3. Soil erosion intensifies, decreasing infiltration and leading to flash flooding.
4. Over time, reduced water retention contributes to desertification in dry regions.

3. Urbanization

1. Urban surfaces like concrete and asphalt create impermeable layers, drastically reducing infiltration.
2. Leads to rapid surface runoff, flash flooding, and reduced groundwater recharge.
3. Urban heat islands increase local evaporation rates, altering microclimates.

4. Dams and Water Diversion Projects

1. Dams alter the natural flow regime, trapping sediments and changing downstream ecosystems.
2. They create artificial reservoirs that increase evaporation losses and displace communities.
3. Reduced river flow impacts delta formation and coastal ecosystems.
4. Conversely, they can provide flood control and hydroelectric energy, showing the trade-offs between human benefit and ecosystem cost.

5. Pollution and Climate Change

1. Air pollution affects cloud formation by altering aerosol concentrations, influencing rainfall patterns.
2. Climate change accelerates the hydrological cycle by increasing evaporation and precipitation extremes.
3. Melting glaciers and polar ice add freshwater to oceans, contributing to sea-level rise.
4. Warmer air holds more water vapour, intensifying storms and flooding.

Consequences of Altered Water Flows

1. Reduced Evapotranspiration

1. Deforestation and Urbanization: These activities decrease evapotranspiration, reducing atmospheric moisture and disrupting rainfall patterns.
2. Agriculture: Irrigation increases evaporation but often at the expense of natural water cycles, leading to imbalances.

2. Increased Run-off

1. Flash Floods: Rapid run-off overwhelms rivers and drainage systems, causing floods.
2. Soil Erosion: Without vegetation, water erodes soil, reducing its fertility and increasing sedimentation in rivers.
3. Water Pollution: Run-off carries pollutants like fertilizers, pesticides, and urban waste into water bodies, degrading ecosystems.

Steady-State Equilibrium and Dynamic Balance

1. The global hydrological cycle maintains a steady-state equilibrium where long-term water inputs and outputs remain balanced, even though fluctuations occur locally and seasonally.
2. However, natural and human-induced changes can temporarily or permanently shift this equilibrium.

Steady-State Equilibrium

1. The total volume of water on Earth remains constant over geological time.
2. Local or short-term fluctuations (floods, droughts) are balanced by compensatory processes elsewhere in the system.
3. The system self-regulates through feedback mechanisms, maintaining stability.

Inputs and Outputs of a Water Body

1. Inputs may include:
   1. Precipitation falling directly on the surface.
   2. Surface inflow from rivers or streams.
   3. Groundwater inflow from aquifers.
   4. Surface runoff from surrounding land.
2. Outputs may include:
   1. Evaporation and transpiration from the surface.
   2. River outflow to other water bodies.
   3. Groundwater outflow into deeper aquifers.
   4. Human extraction for agriculture, domestic, or industrial use

Flow Diagrams of Inputs and Outputs

1. Flow diagrams are visual tools used to illustrate and quantify the movement of water into and out of a system.
2. Arrows indicate direction and magnitude of water movement, helping to determine if the system is in equilibrium.
3. These diagrams form the basis for calculating sustainable harvesting limits.

Sustainable Water Harvesting

1. Sustainable harvesting means withdrawing water at a rate that does not exceed natural replenishment.
2. If extraction surpasses recharge, the water body enters a deficit, lowering water levels over time.
3. Long-term imbalance leads to aquifer depletion, reduced river flow, and ecosystem collapse.

Consequences of Unsustainable Harvesting

1. Lowered water tables, leading to land subsidence.
2. Saltwater intrusion in coastal aquifers.
3. Loss of wetland habitats and biodiversity.
4. Reduced agricultural productivity due to water scarcity.

Dynamic Equilibrium and Feedback Mechanisms

1. The system is dynamic, responding to natural and anthropogenic changes.
2. Positive feedbacks amplify changes (e.g., ice melt reducing albedo, leading to further warming).
3. Negative feedbacks restore balance (e.g., increased evaporation leading to more rainfall that replenishes stores).
4. Such mechanisms ensure that while local cycles fluctuate, the global hydrological system remains resilient.

Calculating Sustainable Rates of Water Harvesting

1. The steady-state concept is essential for determining sustainable extraction rates of freshwater resources.
2. Harvesting (e.g., pumping from aquifers or withdrawing from lakes) must not exceed the net rate of recharge.
3. Exceeding this rate leads to water level decline, ecosystem degradation, and loss of resilience.
4. To maintain equilibrium:
   $$\text{Sustainable yield} = \text{Total input} - \text{Total output (excluding harvest)}$$

Disruptions to Equilibrium

Natural Causes:

1. Droughts reduce precipitation and soil moisture, decreasing streamflow.
2. Volcanic eruptions can block solar radiation, lowering evaporation rates.
3. Glacial melt events increase water flow temporarily, altering long-term stability.

Human Causes:

1. Urbanization accelerates runoff and reduces groundwater recharge.
2. Deforestation reduces transpiration and increases flood frequency.
3. Overextraction and climate change shift precipitation patterns, leading to new hydrological regimes.