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• Hydrologic cycle


  The continuous movement of water from the oceans to the atmosphere, from the atmosphere to the land, and from the land back to the sea
• Earth is the only planet in the solar system with a global ocean and a hydrologic cycle
• Reservoirs of water on Earth

  • Oceans
  • Glaciers
  • Rivers
  • Lakes
  • Air
  • Soil
  • Living tissue
• The hydrosphere contains about 1.36 billion cubic kilometers of water
• About 97% of Earth's water is stored in the global ocean
• Ice sheets and glaciers account for slightly more than 2% of Earth's water
• Less than 1% of Earth's water is divided among lakes, streams, subsurface water, and the atmosphere
• Ecclesiastes 1:7: 'All the rivers run into the sea; yet the sea is not full; unto the place from whence the rivers come, thither they return again'
• Hydrologic cycle


  1. Water evaporates from the ocean, plants, and soil

  2. Moves through the atmosphere

  3. Falls as precipitation
• Infiltration


  Water soaking into the ground
• Runoff


  Water flowing over the surface
• Transpiration


  Water absorbed by plants and released into the atmosphere
• Evapotranspiration


  The combined process of evaporation and transpiration
• More water falls on land as precipitation than is lost by evapotranspiration
• The excess water is carried back to the ocean mainly by streams, with less than 1% returning as groundwater
• Groundwater provides storage that sustains stream flow between storms and during droughts
• If present-day glaciers melted, sea level would rise by several tens of meters
• Earth's hydrologic cycle is balanced, with 320,000 cubic kilometers of water evaporated from the oceans and 284,000 cubic kilometers returning as precipitation
• The remaining 36,000 cubic kilometers of water erode the land during the journey back to the oceans
• Runoff is the dominant process when surface material is highly impermeable or saturated
• Runoff is high in urban areas due to impermeable surfaces
• Runoff


  1. Sheet flow

  2. Rills

  3. Gullies

  4. Streams, brooks, creeks

  5. Rivers
• Drainage basin


  The area of land drained by a river and its tributaries
• Drainage divide


  The imaginary line that bounds a drainage basin
• The Mississippi River has the largest drainage basin in North America, collecting 40% of the flow in the United States
• Three zones of a river system

  • Sediment production (erosion)
  • Sediment transport
  • Sediment deposition
• Sediment is being eroded, transported, and deposited along the entire length of a stream
• The zone of sediment production is located in the headwater region of the river system
• Trunk streams rework their channels over time but are not a source of sediment nor do they accumulate or store it
• When a river reaches the ocean, most of the sediments accumulate at the mouth to form a delta
• Laminar flow


  Water moving in roughly straight-line paths parallel to the stream channel
• Turbulent flow


  Water moving in an erratic fashion characterized by a series of horizontal and vertical swirling motions
• Erosion


  The process of wearing away of the earth's surface by water, wind, ice, or other natural agents
• Sediment transport


  The movement of sediment, such as sand, silt, and clay, by water, wind, or ice
• Sediment deposition


  The process by which sediment is laid down or accumulated in a new location
• Streamflow


  • Water in river channels travels downslope under the influence of gravity
  • In very slow flowing streams, water moves in roughly straight-line paths parallel to the stream channel (laminar flow)
  • Streams typically exhibit turbulent flow, where the water moves in an erratic fashion characterized by a series of horizontal and vertical swirling motions
• Turbulence


  • Contributes to the stream's ability to erode its channel because it acts to lift sediment from the streambed
• Flow velocities can vary significantly from place to place along a stream channel, as well as over time, in response to variations in the amount and intensity of precipitation</b>
• Velocity increases as you move into deeper parts of the channel due to frictional resistance, which is greatest near the banks and beds of stream channels
• Some sluggish streams have flow velocities of less than 1 kilometer per hour, whereas stretches of some fast-flowing rivers may exceed 30 kilometers per hour