Water Cycle and Mass Balance Practice Flashcards
# Fundamental Components of the Water Cycle: Inputs, Outputs, Stores, and Flows
Conceptual Overview: * The water cycle is a system comprising inputs, outputs, stores, and flows that operate at various scales from the local hillslope to the global level. Understanding these components is essential for calculating the mass balance of any hydrological system.
Inputs: Water entering a system from an external source. * Precipitation: The primary input, including rain, snow, and hail. * Condensation from the Atmosphere: The process where water vapor changes into liquid water, contributing to the system's moisture.
Outputs: Water leaving a system to enter another store or the atmosphere. * Evaporation: Conversion of liquid water from surfaces into water vapor. * Transpiration: The release of water vapor from vegetation into the atmosphere. * River Discharge to Oceans: The movement of surface water out of a drainage basin and into oceanic stores. * Sublimation: The direct phase transition from ice to water vapor, bypassing the liquid phase.
Stores: Locations where water is held for varying durations. * Oceans: The largest global reservoir. * Atmosphere: Water held as vapor or in clouds. * Cryosphere: Water frozen in ice sheets, glaciers, and permafrost. * Lakes and Rivers: Significant surface water stores. * Soil Moisture: Water held within the soil profile. * Groundwater/Aquifers: Water stored in underground rock layers. * Vegetation/Interception: Water held on the leaves and branches of plants.
Flows / Transfers: The movement of water between different stores. * Key Processes: * Infiltration: Downward movement of water from the surface into the soil. * Percolation: Downward movement of water through soil into underlying rock layers. * Surface Runoff: Water flowing over the land surface. * Throughflow: Lateral movement of water through the soil towards a river or stream. * Groundwater Flow: Very slow movement of water through aquifers. * Steamflow: Water flowing down the stems/trunks of vegetation to the ground. * Interception: The trapping of precipitation by vegetation before it reaches the soil surface. * Operational Variables: It is necessary to explain how fast these processes operate and to distinguish between the dynamics of a drainage basin versus the global water cycle.
The Dynamics of Mass Balance and Storage Change
Mass Balance Equation: Understanding the relationship between inputs and outputs is critical for assessing the health of a hydrological system. * Equation:
Positive Balance: * Occurs when . * Result: Stores increase in size. * Examples: Flooding in a drainage basin or growth in a glacier's mass.
Negative Balance: * Occurs when . * Result: Stores shrink or deplete. * Examples: Drought conditions in a region or the melting of continental ice sheets.
Applications of Mass Balance: * Drainage Basins: Used to predict floods or water scarcity. * Glaciers: Used to monitor glacial health and response to climate change. * Global Sea Level: Used to track the net transfer of water from land-based stores to the ocean.
Characteristics and Distribution of Global Water Stores
Oceans: * Relative Size: The largest store, containing approximately of all water on Earth. * Characteristics: Saline (saltwater). * Residence Time: Very long, estimated at approximately . * Distribution: The Pacific Ocean is the largest individual ocean; evaporation rates are unevenly distributed across the global ocean surface. * Importance: Acts as the primary source for global evaporation.
Cryosphere: * Components: Includes ice sheets, glaciers, sea ice, and permafrost. * Relative Size: The largest store of freshwater on Earth. * Location: Primarily concentrated in Antarctica and Greenland. * Characteristics: Frozen water with significant seasonal variation; sensitive and highly responsive to climate change. * Movement: Characterized as a slow-moving store.
Atmosphere: * Relative Size: A very tiny store, representing only about of Earth's water. * Characteristics: Water is held as vapor or in clouds. * Transfer Speed: Features very rapid transfer rates. * Residence Time: Approximately to . * Importance: It is the primary control for global precipitation patterns.
Lakes and Rivers: * Characteristics: Small stores relative to the ocean/cryosphere but feature rapid movement and are vital for ecosystems and human civilization. * Dynamics: Subject to seasonal lake level changes and variations in river discharge.
Soil Moisture: * Characteristics: A temporary store. * Influencing Factors: Soil type, density of vegetation, amount of precipitation, and ambient temperature.
Groundwater: * Key Concepts: Aquifers (underground water-bearing rock), the water table (top of the saturated zone), and permeable rock types. * Characteristics: A large store of freshwater with slow transfer rates; critically important in dry climates. * Management Concepts: * Recharge: Water entering the groundwater store. * Abstraction: Human extraction of water from aquifers. * Groundwater Depletion: The result of abstraction exceeding recharge.
Vegetation Store: * Components: Water held within biological mass (biomass) and through interception. * Controls: Climate, vegetation type, and seasonality. * Significance: Particularly important in tropical rainforest ecosystems.
Spatiotemporal Changes in Store Sizes and Sea-Level Dynamics
Rationale for Change: Stores change in size due to shifts in climate and human activity, and the speed of these changes varies across different spatial scales.
Causes of Rising Sea Level: * Thermal Expansion: As oceans warm, the water molecules expand, increasing the overall volume. * Melting Land Ice: Addition of water from the Greenland ice sheet, the Antarctic ice sheet, and mountain glaciers. * Water Transfer from Land: Primarily driven by groundwater extraction, where water is pumped from the ground and eventually enters the ocean.
Impacts of Sea-Level Change: * Coastal flooding. * Increased rates of coastal erosion. * Saltwater intrusion into freshwater aquifers.
Cryospheric Processes (Glacier Dynamics): * Accumulation: The gain of ice and snow mass. Examples include snowfall and the refreezing of meltwater. * Ablation: The loss of ice mass. Examples include melting, sublimation, and calving (ice breaking off into the sea). * Glacier Mass Balance Formula: * Positive Mass Balance: The glacier grows as accumulation exceeds ablation. * Negative Mass Balance: The glacier retreats as ablation exceeds accumulation. * Clinical Observations: Climate warming increases ablation; there are significant seasonal differences in balance; these changes directly impact global sea levels.
Mechanisms of Transfer and Spatial/Temporal Scopes
Atmospheric Processes: Controlled by temperature, wind, and humidity. * Transfers: Evaporation, condensation, advection (horizontal movement of moisture in the air), and precipitation.
Hillslope Processes: Controlled by rock permeability, soil saturation levels, vegetation cover, and the angle of the slope. * Transfers: Overland flow, throughflow, infiltration, and percolation.
Ocean-Atmosphere Transfers: * Evaporation from ocean surfaces. * Moisture transfer driven by atmospheric winds. * The influence of ocean currents on moisture distribution.
Comparison of Time Scales: * Minutes to Hours: Rainfall events and immediate surface runoff. * Days to Months: Changes in soil moisture levels. * Seasons: Accumulation and melting of snow packs. * Years to Decades: Groundwater recharge processes. * Centuries to Millennia: Changes in major ice sheets and global sea-level shifts.
Comparison of Spatial Scales: * Hillslope Scale: Example process is throughflow. * Drainage Basin Scale: Example process is river discharge. * Regional Scale: Example process is monsoon systems. * Global Scale: Example process is the Thermohaline Circulation.