12.1 The hydrologic cycle and drainage systems

Water is constantly moving and transforming on Earth. The hydrologic cycle describes how water circulates through various reservoirs, driven by processes like evaporation and precipitation. Understanding this cycle is crucial for grasping Earth's water distribution and availability.

Drainage systems are the pathways water takes as it flows across land. These systems form distinct patterns influenced by geology and topography. Recognizing drainage patterns helps us understand how landscapes evolve and how water shapes the Earth's surface over time.

The Hydrologic Cycle

Components of hydrologic cycle

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• Water reservoirs store and circulate water throughout Earth's systems
  • Oceans hold 97% of Earth's water as vast saline bodies
  • Atmosphere contains water vapor crucial for weather patterns
  • Land surface includes rivers, lakes, and ice caps (Antarctica)
  • Subsurface encompasses groundwater aquifers and soil moisture
• Processes drive water movement between reservoirs
  • Evaporation transforms liquid water to vapor from oceans, lakes, and land surfaces driven by solar energy
  • Transpiration releases water vapor from plant leaves through tiny pores (stomata)
  • Condensation forms clouds and fog as water vapor cools and collects on particles
  • Precipitation returns water to Earth's surface as rain, snow, sleet, and hail
  • Infiltration allows water to percolate into soil and rock, replenishing groundwater
  • Surface runoff carries water across land, shaping landscapes (Grand Canyon)
  • Groundwater flow moves water through subsurface rock and soil layers, feeding springs and wells

Factors in hydrologic balance

• Climate dictates water distribution and movement patterns
  • Temperature affects evaporation rates higher temps increase evaporation
  • Precipitation patterns influence water availability wet vs dry regions
• Vegetation impacts water retention and release
  • Plant cover increases transpiration and enhances infiltration
  • Root systems create pathways for water to enter soil
• Topography shapes water flow across landscapes
  • Steep slopes accelerate runoff and reduce infiltration
  • Flat areas promote ponding and slower drainage
• Soil properties determine water absorption and retention
  • Porosity affects water storage capacity sandy soils drain quickly
  • Permeability influences water movement clay soils slow infiltration
• Human activities alter natural water cycles
  • Urbanization increases impervious surfaces leading to more runoff and flooding
  • Deforestation reduces transpiration and infiltration, causing soil erosion
• Geological factors control water storage and movement
  • Rock type influences groundwater aquifer formation (limestone karst systems)
  • Fault lines can create pathways for groundwater flow

Drainage Systems

Types of drainage patterns

• Dendritic pattern resembles tree-like branching structure
  • Develops in areas with uniform rock resistance (sedimentary plains)
  • Main stream acts as trunk with smaller tributaries as branches
• Trellis pattern forms parallel main streams with right-angle tributaries
  • Occurs in areas with alternating resistant and weak rock layers (Appalachian Mountains)
  • Streams follow paths of least resistance between rock layers
• Rectangular pattern creates right-angle bends in streams
  • Forms in areas with intersecting joints and faults (New England)
  • Streams abruptly change direction along fracture lines
• Radial pattern shows streams flowing outward from a central high point
  • Typical of volcanic cones or domes (Mount Rainier)
  • Streams radiate like spokes on a wheel
• Centripetal pattern displays streams flowing inward toward a central basin
  • Found in crater lakes or enclosed basins (Great Basin)
  • Water converges toward lowest point in depression
• Parallel pattern exhibits streams flowing side by side
  • Develops on steep, uniform slopes (coastal plains)
  • Streams maintain consistent direction due to topography

Drainage systems and geology

• Rock type influence shapes landscape and stream paths
  • Resistant rocks form ridges and influence stream direction (granite outcrops)
  • Softer rocks erode faster, creating valleys and lowlands (shale formations)
• Structural controls guide water flow paths
  • Faults and fractures provide preferential pathways for streams
  • Folded rock layers create alternating ridges and valleys affecting drainage
• Topographic influence determines initial water flow
  • Slope direction guides initial drainage formation
  • Elevation differences control stream gradients and erosion rates
• Bedrock permeability affects surface and subsurface flow
  • Permeable rocks promote groundwater flow reducing surface streams
  • Impermeable rocks increase surface runoff and stream density
• Geological history shapes current drainage networks
  • Past tectonic events create mountain ranges and basins altering drainage
  • Glaciation carves valleys and leaves behind features affecting water flow (U-shaped valleys)
• Karst topography creates unique drainage features in soluble rocks
  • Dissolution forms sinkholes, caves, and underground streams (Mammoth Cave system)
  • Surface streams may disappear into sinkholes and reappear as springs