🌊Hydrology Unit 6 – Surface Water Hydrology

Key Concepts and Definitions

• Hydrology studies the movement, distribution, and management of water resources on Earth
• Watershed refers to an area of land that drains water to a common outlet point (river, lake, ocean)
• Hydrologic cycle describes the continuous movement of water through evaporation, condensation, precipitation, and runoff
  • Driven by solar energy and gravity
• Surface water includes water found in rivers, lakes, wetlands, and oceans
  • Accounts for a small percentage of Earth's total water supply
• Groundwater is water stored beneath the Earth's surface in soil pores and rock fractures
• Infiltration process by which water on the ground surface enters the soil
• Evapotranspiration combination of evaporation from land and water surfaces and transpiration from plants
• Runoff portion of precipitation that flows over land surfaces towards streams, rivers, or other water bodies

The Hydrologic Cycle

• Continuous movement of water on, above, and below the surface of the Earth
• Driven by solar energy, which causes evaporation and transpiration
• Evaporated water condenses in the atmosphere to form clouds
• Precipitation occurs when water vapor condenses and falls back to Earth as rain, snow, or hail
• Infiltration allows water to enter the soil and recharge groundwater aquifers
• Surface runoff flows over land towards streams, rivers, and other water bodies
  • Influenced by factors such as land use, soil type, and topography
• Groundwater flow moves through soil pores and rock fractures, eventually discharging into surface water or the ocean
• Human activities (water withdrawals, land use changes) can impact the hydrologic cycle

Surface Water Components

• Rivers and streams primary conduits for surface water flow
  • Fed by precipitation, snowmelt, and groundwater discharge
• Lakes and reservoirs natural or man-made water bodies that store surface water
  • Provide water supply, flood control, and recreational opportunities
• Wetlands areas where water covers the soil or is present near the surface for a significant portion of the year
  • Play crucial roles in water purification, flood attenuation, and habitat provision
• Oceans largest component of the Earth's surface water, covering approximately 71% of the planet
• Glaciers and ice caps store a significant amount of freshwater in solid form
  • Melting of these ice masses contributes to sea level rise and changes in surface water dynamics
• Estuaries transitional zones where rivers meet the ocean, characterized by a mix of fresh and saltwater
• Floodplains low-lying areas adjacent to rivers that are subject to periodic inundation during high flow events

Precipitation and Runoff Processes

• Precipitation primary input of water to the Earth's surface
  • Can occur as rain, snow, hail, or sleet
• Interception process by which precipitation is caught and stored by vegetation or other surfaces before reaching the ground
• Infiltration movement of water from the surface into the soil
  • Influenced by soil properties, land cover, and antecedent moisture conditions
• Surface runoff flow of water over the land surface towards streams or other water bodies
  • Occurs when precipitation exceeds the rate of infiltration
• Baseflow sustained flow in a stream or river derived from groundwater discharge
• Overland flow movement of water over the land surface as a thin, continuous film
• Channel flow movement of water within a defined stream channel
  • Influenced by channel geometry, roughness, and gradient
• Hydrograph graphical representation of stream discharge over time, showing the response of a watershed to a precipitation event

Stream Flow Measurement and Analysis

• Stream gauging measurement of water level (stage) and velocity to determine discharge
  • Discharge volume of water passing a specific point per unit time, typically expressed in cubic meters per second (m³/s)
• Rating curve relationship between water level (stage) and discharge for a specific stream cross-section
  • Used to estimate discharge based on measured water levels
• Hydrograph analysis study of the shape, timing, and magnitude of stream flow response to precipitation events
  • Rising limb steep increase in discharge following the onset of a precipitation event
  • Peak flow maximum discharge during a hydrograph
  • Recession limb gradual decrease in discharge following the peak flow
• Baseflow separation techniques used to estimate the contribution of groundwater to stream flow
• Flood routing methods used to predict the timing and magnitude of flood waves as they move downstream
• Flow duration curve graphical representation of the percentage of time that specific discharge values are equaled or exceeded
  • Useful for characterizing the flow regime of a stream and assessing water resource availability

Flood Frequency and Risk Assessment

• Flood frequency analysis statistical assessment of the likelihood of a flood event of a given magnitude occurring
  • Typically expressed as a return period (e.g., 100-year flood)
• Probability of exceedance likelihood that a flood of a given magnitude will be equaled or exceeded in any given year
• Annual exceedance probability (AEP) probability that a flood of a given magnitude will be equaled or exceeded in a given year
  • Inverse of the return period (e.g., 100-year flood has an AEP of 1%)
• Flood risk assessment evaluation of the potential consequences of flooding, considering factors such as population, infrastructure, and economic activity
• Flood hazard maps delineate areas at risk of flooding based on historical data and hydrologic modeling
  • Used for land use planning, insurance, and emergency management
• Flood mitigation strategies measures taken to reduce the impact of flooding (levees, floodwalls, detention basins)
• Non-structural measures policies and practices that reduce flood risk without physical infrastructure (building codes, land use regulations, early warning systems)

Water Quality in Surface Systems

• Physical parameters characteristics of water that can be observed or measured (temperature, turbidity, dissolved oxygen)
  • Temperature influences chemical and biological processes in water bodies
  • Turbidity measure of water clarity, influenced by suspended sediment and organic matter
• Chemical parameters concentrations of various substances in water (nutrients, metals, pesticides)
  • Nutrients (nitrogen and phosphorus) can lead to eutrophication and algal blooms when in excess
  • Metals (lead, mercury) can be toxic to aquatic life and human health
• Biological parameters indicators of the health of aquatic ecosystems (bacteria, algae, macroinvertebrates)
  • Fecal coliform bacteria indicate the presence of human or animal waste and potential pathogens
  • Benthic macroinvertebrates sensitive to water quality changes and used as bioindicators
• Point source pollution pollution originating from a single, identifiable source (wastewater treatment plants, industrial discharges)
• Non-point source pollution pollution originating from diffuse sources over a wide area (agricultural runoff, urban stormwater)
• Total Maximum Daily Load (TMDL) regulatory framework for identifying and restoring impaired water bodies by setting pollutant load limits
• Best Management Practices (BMPs) strategies and techniques used to minimize water quality impacts from land use activities (buffer strips, retention ponds)

Practical Applications and Case Studies

• Watershed management integrated approach to managing water resources within a watershed context
  • Involves stakeholder collaboration, land use planning, and water quality monitoring
• Stormwater management strategies for managing runoff from urban areas to reduce flooding and water quality impacts
  • Green infrastructure (permeable pavement, rain gardens) mimics natural hydrologic processes
• Flood forecasting and warning systems use hydrologic models and real-time data to predict and communicate flood risks
  • Early warning allows for evacuation and emergency response
• Reservoir operations management of water levels and releases from dams to balance competing objectives (water supply, flood control, hydropower)
• Instream flow requirements minimum water flows needed to maintain the ecological health of rivers and streams
  • Often established through legal or regulatory frameworks
• Wetland restoration and construction projects that aim to restore or create wetland habitats for water quality, flood attenuation, and biodiversity benefits
• Agricultural water management practices and technologies used to optimize irrigation efficiency and reduce water quality impacts from agricultural runoff (precision irrigation, cover crops)
• Urban stream restoration projects that aim to restore the hydrologic and ecological function of degraded urban streams (channel reconfiguration, riparian buffers)