🌊Hydrology Unit 1 – Hydrology: Intro to the Water Cycle

Key Concepts and Definitions

• Hydrology studies the movement, distribution, and management of water resources on Earth
• Hydrosphere encompasses all water on Earth's surface, including oceans, lakes, rivers, and groundwater
• Water cycle (hydrologic cycle) describes the continuous movement of water through evaporation, condensation, precipitation, and runoff
• Watershed (drainage basin) represents an area of land where all water drains to a common outlet
• Hydrologic processes involve the exchange of water between the atmosphere, land surface, and subsurface
• Water balance equation quantifies the inputs, outputs, and storage changes within a hydrologic system
• Hydrograph graphically represents the rate of flow versus time past a specific point in a river, channel, or conduit
• Infiltration refers to the process by which water on the ground surface enters the soil

Components of the Water Cycle

• Evaporation converts liquid water into water vapor, primarily from oceans, lakes, and soil moisture
  • Driven by solar radiation and influenced by temperature, humidity, and wind speed
• Transpiration releases water vapor from plants through stomata during photosynthesis
• Condensation forms clouds and fog when water vapor cools and becomes liquid water droplets
• Precipitation occurs when water falls from the atmosphere as rain, snow, sleet, or hail
  • Influenced by atmospheric conditions, topography, and seasonality
• Surface runoff moves water over land towards streams, rivers, and oceans
• Groundwater storage occurs in aquifers, which are permeable rock or sediment layers that hold water
• Streamflow represents the movement of water within rivers and streams, fed by surface runoff and groundwater discharge

Hydrologic Processes

• Infiltration allows water to move from the surface into the soil, controlled by soil properties and antecedent moisture conditions
• Percolation is the downward movement of water through soil layers due to gravity and capillary forces
• Groundwater recharge replenishes aquifers through infiltration and percolation
• Groundwater discharge releases water from aquifers into streams, springs, or the ocean
• Overland flow occurs when precipitation exceeds infiltration capacity, causing water to flow over the land surface
• Interception captures precipitation by vegetation canopy, reducing the amount reaching the ground
• Evapotranspiration combines evaporation and transpiration, transferring water from land and plants to the atmosphere
• Baseflow sustains streamflow during dry periods, supplied by groundwater discharge

Water Balance Equation

• Quantifies the inputs, outputs, and changes in water storage within a hydrologic system over a specified time period
• Expressed as: $P = Q + E + ΔS$, where $P$ is precipitation, $Q$ is runoff, $E$ is evapotranspiration, and $ΔS$ is change in storage
• Precipitation represents the primary input of water to a hydrologic system
• Runoff includes surface runoff and groundwater discharge, representing water leaving the system
• Evapotranspiration accounts for water losses to the atmosphere through evaporation and transpiration
• Change in storage reflects the difference in water stored within the system (soil moisture, groundwater, lakes) over the time period
  • Positive $ΔS$ indicates an increase in storage, while negative $ΔS$ indicates a decrease

Measuring and Monitoring Techniques

• Rain gauges measure precipitation depth at a specific location, using manual or automated methods
• Streamflow gauges record water level (stage) and convert it to discharge using a rating curve
  • Rating curves are developed through periodic field measurements of stream velocity and cross-sectional area
• Soil moisture sensors (tensiometers, time-domain reflectometry) measure the volumetric water content in the soil
• Piezometers measure groundwater pressure head and can be used to determine groundwater level and flow direction
• Remote sensing techniques (satellite imagery, radar) estimate precipitation, evapotranspiration, and soil moisture over large areas
• Tracer studies use chemical or isotopic tracers to track water movement and residence times in hydrologic systems
• Eddy covariance systems measure evapotranspiration by quantifying vertical turbulent fluxes of water vapor and heat in the atmosphere

Human Impacts on the Water Cycle

• Land use changes (deforestation, urbanization) alter infiltration, runoff, and evapotranspiration rates
  • Impervious surfaces in urban areas increase surface runoff and reduce groundwater recharge
• Water withdrawals for irrigation, industrial, and municipal use can deplete surface and groundwater resources
• Dams and reservoirs regulate streamflow, alter sediment transport, and modify downstream ecosystems
• Groundwater pumping can lead to aquifer depletion, subsidence, and saltwater intrusion in coastal areas
• Climate change affects precipitation patterns, sea level rise, and the frequency and intensity of extreme hydrologic events (floods, droughts)
• Water pollution from point sources (wastewater discharge) and nonpoint sources (agricultural runoff) degrades water quality and ecosystem health
• Interbasin water transfers divert water from one watershed to another, altering natural hydrologic regimes

Applications in Environmental Science

• Water resource management involves the allocation and conservation of water for various uses (agriculture, industry, municipal)
• Flood forecasting and management aim to predict and mitigate the impacts of extreme hydrologic events
  • Flood control measures include levees, dams, and natural flood retention areas
• Drought monitoring and planning help communities prepare for and respond to water scarcity
• Groundwater remediation addresses contamination and restores aquifer water quality
• Ecohydrology studies the interactions between hydrologic processes and ecosystems, informing conservation and restoration efforts
• Watershed management takes a holistic approach to protect and enhance water resources within a drainage basin
• Water quality modeling predicts the fate and transport of pollutants in surface and groundwater systems

Case Studies and Real-World Examples

• The Colorado River Basin, USA, exemplifies the challenges of managing water resources in an arid region with competing demands (agriculture, urban growth, environmental flows)
• The Ogallala Aquifer, USA, is a critical groundwater resource for irrigation in the Great Plains, facing depletion due to overexploitation
• The Nile River Basin, Africa, demonstrates the complexities of transboundary water management, involving multiple countries with varying water needs and development goals
• The Aral Sea, Central Asia, illustrates the devastating environmental and socioeconomic consequences of unsustainable water diversion for irrigation
• The Mississippi River Basin, USA, showcases the impacts of land use change, nutrient pollution, and flood management on a large river system
• The Mekong River Basin, Southeast Asia, highlights the trade-offs between hydropower development, fisheries, and ecosystem services in a transboundary context
• The Murray-Darling Basin, Australia, provides an example of adaptive water management in response to climate variability and competing water demands