Hydrologic Cycle Study Guide

Fundamentals of the Hydrologic Cycle

• Water Cycle Definition: The continuous, dynamic movement of water between the atmosphere, land, living things, surface water, and underground environments.

• Law of Conservation of Mass: Water is neither created nor destroyed during the hydrologic cycle; rather, it moves continuously between various pools and undergoes phase changes between solid, liquid, and gaseous states.

• Primary Energy Drivers:

  • The Sun: Supplies the thermal energy necessary to drive evaporation and phase transformations into vapor.

  • Gravity: Pulls precipitation downward toward the Earth's surface and drives the downhill movement of water across landscapes and downward through subsurface layers.

• System Components:

  • Pools: Reservoirs or locations where water is stored within the cycle. Examples include:

  • Oceans

  • Lakes

  • Glaciers

  • Soil

  • Groundwater

  • Plants (living biomass)

  • The atmosphere

  • Fluxes: Dynamic processes that transfer water from one pool to another. Examples include:

  • Evaporation

  • Precipitation

  • Runoff

  • Infiltration

  • Groundwater flow

Above-Ground Hydrologic Processes and Features

• Evaporation: The physical process in which liquid water absorbs heat energy from the Sun and changes into water vapor, rising from open water bodies or moist land surfaces into the atmosphere.

• Transpiration: The biological process in which plants release water vapor into the atmosphere directly through their leaves.

• Evapotranspiration: The collective, integrated movement of water vapor into the atmosphere resulting from both evaporation (from land and open water) and transpiration (from vegetative cover).

• Sublimation: The direct phase change wherein snow or ice transitions into water vapor without first melting into liquid water.

• Condensation: The process where water vapor cools as it rises and transitions into liquid water droplets, facilitating the formation of clouds.

• Precipitation: Water returning to the Earth's surface from clouds in various forms, including:

  • Rain

  • Snow

  • Sleet

  • Freezing rain

  • Hail

• Snowmelt: Water produced by the melting of accumulated snow cover. Snowmelt either infiltrates the underlying soil or flows over the land surface as runoff.

• Runoff: Water originating from precipitation or melting snow that travels across the land surface into surface water bodies such as streams, rivers, lakes, or oceans. Runoff acts as an agent of physical erosion and transports pollutants across watersheds.

• Surface Water Storage and Utilization:

  • Reservoir: An artificial, human-made lake created by constructing a dam across a river, allowing water to collect and be stored behind the structure.

  • Wetland: An ecosystem characterized by soil that is flooded, ponded, or completely saturated with water for all or part of the year.

  • Types of Wetlands: Marshes, swamps, bogs, and mangroves.

  • Ecological and Hydrological Functions: Wetlands act as natural water storage basins, attenuate flooding, and naturally filter out certain contaminants and pollutants.

  • Domestic Water Use: Water utilized within households for everyday residential purposes, specifically including:

  • Drinking

  • Cooking

  • Bathing

  • Cleaning

  • Flushing toilets

  • Watering residential plants

Underground Hydrologic Processes and Hydrogeology

• Subsurface Hydrologic Movement:

  • Infiltration: The initial entry and downward movement of water from the surface of the land into the upper soil layer.

  • Percolation: The continuous, deeper downward movement of infiltrated water through the void spaces, cracks, and pores of deeper soil strata and rock formations.

  • Groundwater Recharge: The process by which percolating water successfully reaches the saturated zone, replenishing and adding volume to an aquifer.

• Subsurface Zones and Boundaries:

  • Unsaturated Zone: The subsurface zone situated immediately above the water table where pore spaces and fractures contain a mixture of both air and water.

  • Saturated Zone: The deep underground region where every available crack, fracture, and pore space is completely filled with water.

  • Water Table: The upper boundary or surface of the saturated zone. For a water extraction well to function successfully, it must be drilled deep enough to reach below the water table.

  • Impermeable Layer: A geologic formation—such as dense clay or solid, unfractured rock—that prevents or severely obstructs the flow and movement of water.

• Subsurface Reservoirs and Geologic Hazards:

  • Groundwater: Water held and stored underground within fractures, pore spaces, and voids in soil, unconsolidated sediment, and bedrock.

  • Aquifer: An underground geologic layer composed of permeable rock or sediment that can both store and transmit usable quantities of groundwater.

  • Subsidence: The gradual settling or sudden sinking of the land surface resulting from excessive groundwater extraction. As pore water pressure is removed, underground void spaces collapse under the weight of overlying sediment, causing structural compaction that is often permanent and irreversible.

Material Characteristics: Porosity and Permeability

• Core Definitions:

  • Porosity: The volumetric proportion or total amount of empty void space within a geologic material that is available to hold or store water.

  • Permeability: A measure of the ease with which water can flow and transmit through interconnected pore networks and fractures within a material.

• Material Behaviors:

  • Sand and Gravel: Typically exhibit high permeability because their large, well-connected pore spaces allow water to move through rapidly and with minimal resistance.

  • Clay: Frequently displays high porosity due to an abundance of tiny internal void spaces; however, it possesses exceptionally low permeability because these microscopic pores are poorly connected, greatly restricting fluid flow.

  • Fractured Rock: Can exhibit high permeability despite a dense rock matrix if water is capable of traveling readily through connected fracture networks and structural joints.

• The Porosity-Permeability Disconnect: A material can possess high porosity while simultaneously having very low permeability. The total capacity to store water within a material does not imply or guarantee that water can freely transmit through it.

Comparative Analysis of Key Hydrologic Concepts

• Evaporation vs. Transpiration:

  • Evaporation: Originates directly from open water surfaces, damp soil, or non-living terrestrial landforms.

  • Transpiration: Originates exclusively from the biological tissues of living plants via their foliage.

• Infiltration vs. Percolation:

  • Infiltration: Specifically describes the physical entry of water crossing the boundary from the atmosphere/ground surface into the topsoil.

  • Percolation: Specifically describes the ongoing, downward migration of water through subsurface soil profiles and bedrock layers.

• Porosity vs. Permeability:

  • Porosity: Quantifies how much total fluid volume a geologic medium can store.

  • Permeability: Quantifies how easily and rapidly fluids can transmit through the interconnected void channels of that medium.

• Aquifer vs. Groundwater:

  • Groundwater: Represents the actual liquid water located underground.

  • Aquifer: Represents the geologic rock or sediment body that holds, contains, and conveys that liquid water.

• Water Table vs. Aquifer:

  • Water Table: Defines the distinct upper horizontal boundary of the saturated zone.

  • Aquifer: Encompasses the entire three-dimensional underground geological unit capable of yielding usable volumes of water.

• Runoff vs. Groundwater Recharge:

  • Runoff: Represents surface flux that carries water overland across the landscape.

  • Groundwater Recharge: Represents subsurface flux that directs downward-percolating water into the saturated zone to refill aquifers.

• Reservoir vs. Aquifer:

  • Reservoir: An artificial, open surface water containment basin held back by engineering structures (dams).

  • Aquifer: A naturally occurring, concealed subsurface reservoir of water embedded within rock and sediment matrices.

Fundamental Hydrologic Sequences and Pathways

• Pathway 1: Water Entering the Atmosphere from Surface Bodies:

  • Liquid water on the surface absorbs thermal solar energy.

  • Evaporation occurs as water undergoes a liquid-to-gas phase transition.

  • Water vapor ascends and mixes into the atmosphere.

• Pathway 2: Water Entering the Atmosphere via Vegetation:

  • Plant roots absorb moisture from the surrounding soil.

  • Transpiration releases water vapor through the leaf stomata.

  • Water vapor enters the atmospheric pool.

• Pathway 3: Water Returning from the Atmosphere to Earth:

  • Water vapor cools at higher atmospheric altitudes.

  • Condensation converts vapor back into liquid droplets.

  • Liquid droplets coalesce to form clouds.

  • Precipitation falls to the surface as rain, snow, sleet, freezing rain, or hail.

• Pathway 4: Subsurface Migration into an Aquifer:

  • Precipitation reaches the land surface.

  • Infiltration occurs as water penetrates the topsoil.

  • Percolation draws water deeper downward through soil pores and rock fractures.

  • Groundwater recharge delivers the percolating fluid past the unsaturated zone.

  • Water enters the saturated zone of the aquifer.

• Pathway 5: Overland Flow across Watersheds:

  • Precipitation falls or snowmelt generates liquid water on the ground surface.

  • Runoff travels overland across impervious or saturated surfaces.

  • Flowing runoff feeds into a local stream or river channel.

  • The river system drains downstream into a lake or ocean pool.

Anthropogenic Impacts on the Hydrologic Cycle

• Surface Modifications and Runoff Dynamics:

  • Pavement and Impervious Surfaces: Replacing natural soils with paved surfaces prevents natural water infiltration, sharply accelerating the volume and velocity of surface runoff.

  • Vegetation and Soil Preservation: Maintaining healthy vegetative cover and undisturbed soils preserves high infiltration rates and markedly reduces erosive surface runoff.

• Overexploitation of Groundwater Resources:

  • Pumping groundwater out of an aquifer at a rate exceeding the natural recharge rate draws down the local water table.

  • If the regional water table drops below the intake depth of an extraction well, the well runs dry and ceases to supply water.

  • Protracted overdraft and pore space collapse trigger irreversible land subsidence.

• Water Quality Degradation via Nonpoint Source Runoff:

  • Overland runoff collects and sweeps environmental contaminants directly into aquatic ecosystems, including:

  • Agricultural and residential fertilizers

  • Petroleum oils from roadways

  • Deicing road salts

  • Displaced sediments from bare ground erosion

  • Miscellaneous urban and industrial pollutants

• Hydrological Engineering and Wetland Drainage:

  • Dams and Reservoirs: Alter natural stream discharge patterns, manipulate flow velocities, and reshape regional surface water distribution.

  • Wetland Destruction: Eliminating natural wetland areas magnifies downstream flood damage and dismantles critical natural bio-filtration systems that purify water supplies.

Case Study: Municipal Water Supply of Ann Arbor

• Municipal Supply Distribution:

  • Surface Water Contribution: Approximately $85\%$ of Ann Arbor's municipal drinking water is drawn directly from the surface waters of the Huron River.

  • Groundwater Contribution: Approximately $15\%$ of Ann Arbor's municipal drinking water is extracted from municipal groundwater supply wells.