🌡️Climatology Unit 5 – Hydrologic Cycle and Precipitation

Key Concepts and Definitions

• Hydrologic cycle the continuous movement of water on, above, and below the surface of the Earth
• Precipitation any product of the condensation of atmospheric water vapor that falls under gravity (rain, snow, sleet, or hail)
• Evaporation the process by which water changes from a liquid to a gas or vapor
• Transpiration the process by which water is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere
• Infiltration the process by which water on the ground surface enters the soil
• Runoff the variety of ways by which water moves across the land (includes both surface runoff and channel runoff)
• Groundwater the water found underground in the cracks and spaces in soil, sand, and rock
• Water budget an accounting of the inflow to, outflow from, and storage in, a hydrologic unit (drainage basin, aquifer, soil zone, lake, reservoir, or irrigation project)

Components of the Hydrologic Cycle

• Evaporation occurs when water changes from a liquid to a gas or vapor
  • Driven by solar energy and influenced by factors such as temperature, humidity, and wind speed
• Transpiration the process by which water is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere
  • Accounts for a significant portion of water vapor in the atmosphere (up to 10% globally)
• Condensation the process by which water vapor in the air is changed into liquid water
  • Occurs when the air becomes saturated and cannot hold any more water vapor (reaches dew point temperature)
• Precipitation occurs when the condensed water vapor falls to the Earth's surface under gravity
  • Forms include rain, snow, sleet, and hail
• Infiltration the process by which water on the ground surface enters the soil
  • Influenced by factors such as soil type, soil moisture content, and surface conditions (vegetation, slope)
• Surface runoff the flow of water that occurs when excess water from rain, snowmelt, or other sources flows over the Earth's surface
  • Occurs when the rate of precipitation exceeds the rate of infiltration
• Groundwater flow the movement of water through the subsurface soil and rock layers
  • Recharges streams, lakes, and oceans, and is a major source of water for human use (wells, springs)

Precipitation Formation Processes

• Condensation occurs when water vapor in the air cools and changes into liquid water
  • Requires the presence of condensation nuclei (tiny particles such as dust, salt, or smoke) around which water vapor can condense
• Collision-coalescence process the growth of precipitation droplets by the collision and merging of smaller cloud droplets
  • Efficient process for producing precipitation in warm clouds (above freezing temperatures)
• Bergeron process the formation of precipitation in cold clouds (below freezing temperatures) through the growth of ice crystals
  • Occurs when ice crystals grow at the expense of surrounding supercooled water droplets (remain liquid below freezing temperatures)
• Riming the rapid freezing of supercooled water droplets as they collide with an ice crystal or other frozen particle
  • Produces graupel (small, soft ice pellets) and hail (larger, layered ice particles)
• Melting the process by which frozen precipitation (snow, sleet, hail) changes into liquid form (rain) as it falls through a layer of warmer air
  • Can result in a variety of precipitation types reaching the surface depending on the temperature profile of the atmosphere

Types of Precipitation

• Rain liquid water in the form of droplets that have condensed from atmospheric water vapor and then become heavy enough to fall under gravity
• Snow solid precipitation in the form of ice crystals or aggregates of ice crystals that fall from a cloud
  • Requires below-freezing temperatures throughout the atmosphere from the cloud to the surface
• Sleet a type of precipitation consisting of partially melted snow or ice pellets
  • Forms when snowflakes partially melt as they fall through a layer of warmer air before refreezing in a layer of colder air near the surface
• Freezing rain rain that freezes on contact with the ground or other surfaces, forming a coating of ice
  • Occurs when snowflakes completely melt in a layer of warmer air before falling through a shallow layer of cold air near the surface
• Hail solid precipitation in the form of balls or irregular lumps of ice
  • Forms by the repeated freezing of liquid water onto an initial ice crystal or frozen droplet as it cycles through strong updrafts and downdrafts within a thunderstorm cloud

Measurement and Distribution of Precipitation

• Rain gauges instruments used to collect and measure the amount of liquid precipitation over a set period of time
  • Standard rain gauge consists of a funnel that collects the rain into a small container and a measuring tube that allows for precise measurements
• Weather radar a remote sensing method used to locate precipitation, calculate its motion, and estimate its type (rain, snow, hail) and intensity (light, moderate, heavy)
  • Sends out electromagnetic waves that bounce off precipitation particles and return to the radar, providing information about the particles' location, size, and shape
• Satellite observations provide a global view of precipitation patterns and can fill in gaps in ground-based observations
  • Techniques include visible and infrared imagery (cloud top temperatures), passive microwave sensing (emission and scattering by precipitation particles), and active radar sensing (precipitation profiles)
• Geographic distribution of precipitation highly variable across the Earth's surface
  • Influenced by factors such as latitude, altitude, proximity to moisture sources, and atmospheric circulation patterns
• Temporal distribution of precipitation varies on daily, seasonal, and annual timescales
  • Influenced by factors such as diurnal heating and cooling, seasonal changes in solar radiation and atmospheric circulation, and large-scale climate patterns (El Niño/La Niña, monsoons)

Factors Affecting Precipitation Patterns

• Latitude a major control on the global distribution of precipitation
  • Equatorial regions (low latitudes) receive high amounts of precipitation due to strong solar heating, convergence of trade winds, and frequent convective activity
  • Polar regions (high latitudes) receive low amounts of precipitation due to cold temperatures, low moisture content of the air, and stable atmospheric conditions
• Altitude precipitation generally increases with increasing elevation up to a certain point (orographic enhancement)
  • Rising air cools adiabatically, leading to condensation and precipitation on the windward side of mountain ranges
  • Leeward side of mountain ranges often experiences a rain shadow effect, with descending air warming adiabatically and suppressing precipitation
• Proximity to moisture sources precipitation amounts tend to be higher in coastal areas and regions with access to large bodies of water (oceans, lakes)
  • Warm, moist air from these sources can fuel precipitation processes and contribute to the formation of heavy precipitation events
• Atmospheric circulation patterns the large-scale movement of air and moisture in the atmosphere, driven by differences in pressure and temperature
  • Hadley cell circulation near the equator (rising motion, frequent precipitation)
  • Ferrel cell circulation in the mid-latitudes (variable precipitation depending on location and season)
  • Polar cell circulation near the poles (descending motion, low precipitation)
• Seasonal variations changes in precipitation patterns throughout the year, influenced by factors such as the position of the sun, the movement of the Intertropical Convergence Zone (ITCZ), and the development of monsoon systems
  • Monsoon a seasonal reversal of wind patterns that brings heavy rainfall to certain regions (South Asia, West Africa, Australia) during the summer months

Climate Change and the Hydrologic Cycle

• Warming temperatures climate change is causing an increase in global average temperatures, which can lead to changes in the hydrologic cycle
  • Higher temperatures increase the water-holding capacity of the atmosphere (Clausius-Clapeyron relationship), leading to more evaporation and the potential for more intense precipitation events
• Changes in precipitation patterns climate change is expected to alter the geographic and temporal distribution of precipitation
  • Some regions may experience increases in precipitation (high latitudes, wet tropics), while others may experience decreases (dry subtropics, Mediterranean climate regions)
  • More frequent and intense extreme precipitation events (heavy downpours, floods) are likely in many areas
• Impacts on water resources changes in the hydrologic cycle can have significant impacts on the availability and management of water resources
  • Reduced snowpack and earlier spring melting can affect the timing and amount of runoff in snow-dominated regions
  • Increased evaporation and changes in precipitation patterns can lead to more frequent and severe droughts in some areas
• Sea level rise warming temperatures are causing thermal expansion of the oceans and melting of land-based ice sheets and glaciers, leading to rising sea levels
  • Higher sea levels can increase the risk of coastal flooding, saltwater intrusion into freshwater aquifers, and erosion of coastlines
• Ecosystem impacts changes in the hydrologic cycle can have wide-ranging impacts on natural ecosystems and the services they provide
  • Shifts in the timing and amount of precipitation can affect plant growth, species distributions, and ecosystem productivity
  • Droughts and floods can stress or damage ecosystems, leading to changes in biodiversity and ecosystem function

Real-World Applications and Case Studies

• Agricultural water management understanding the hydrologic cycle is crucial for managing water resources in agricultural systems
  • Techniques such as irrigation scheduling, soil moisture monitoring, and drought-resistant crop varieties can help optimize water use and maintain crop yields in the face of variable precipitation patterns
• Urban stormwater management cities and towns must manage stormwater runoff to prevent flooding, protect water quality, and maintain infrastructure
  • Green infrastructure approaches (permeable pavements, rain gardens, green roofs) can help mimic natural hydrologic processes and reduce the impacts of urbanization on the hydrologic cycle
• Flood forecasting and warning systems knowledge of precipitation patterns and hydrologic processes is essential for predicting and mitigating the impacts of floods
  • Flood forecasting models use real-time data on precipitation, streamflow, and soil moisture to provide early warning of potential flood events
  • Effective communication and dissemination of flood warnings can help communities take action to protect lives and property
• Drought planning and response understanding the factors that influence precipitation patterns and the impacts of drought on water resources is important for developing effective drought management strategies
  • Drought monitoring systems use a combination of ground-based observations, remote sensing data, and hydrologic models to track the onset, severity, and extent of drought conditions
  • Drought response plans outline the actions that communities and water managers can take to reduce the impacts of drought, such as implementing water conservation measures, securing alternative water supplies, and providing assistance to affected sectors (agriculture, industry, public health)
• Ecosystem restoration and management restoring and managing ecosystems requires an understanding of the hydrologic processes that support them
  • Restoration projects may aim to restore natural hydrologic regimes (flow patterns, groundwater levels) to support the recovery of aquatic and riparian habitats
  • Adaptive management approaches can help ecosystems and the communities that depend on them to be more resilient to changes in the hydrologic cycle, such as those associated with climate change