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Watersheds

Learning Objectives

  1. Explain the watershed concept and why it is the main spatial unit used in hydrology
  2. List the five factors that determine watershed characteristics
  3. Explain at least eight specific watershed parameters
  4. Infer how watershed characteristics (soil type, mean slope, etc.) may affect different response factors
  5. Sketch the global and terrestrial hydrologic cycles with their major sotrages and processes
  6. Understand the major components of the hydrologic cycle
  7. Write and understand the basic water balance equation
  8. Define with your own words, and put into context, all the terms in the glossary.

Watershed concept

Definition

  • The area that appears on the basis of topography to contribute all the water that passes through a given cross section of a stream
  • An area of land with a topograpic divide that collects rain and snow, discharging most of this water to a stream, river or other water body.
  • Synonyms: basin, drainage basin, catchment

Why is the Fraser river water dirtier than the Thompson river?

Silt / sediment that is produced from the flow throughout mountains.

Factors that influence watershed characteristics

A number of factors affect the way water and sediment move from upland areas to the stream and from there to its terminus.

  • Climate
  • Topography
  • Geology
  • Vegetation
  • Human intervention

Basic parameters

  • Area (ha or km^2)
  • Shape, topography, slope
  • Elevation
  • Land use
  • Stream discharge
  • Soil characteristics
  • Road network

Stream orders

Horton, Strahler, Shreve

Strahler: Starts at 1, when two 1's join together, it becomes 2. When two 2's come together, it becomes 3, etc.

Stream types:

  • Perennial: Flowing all the time regardless of season

  • Intermittent: Always full in the rainy season, always dry in the summer

  • Ephemeral: Goes dry when it doesn't rain

  • Influent: Losing water into the ground

  • Effluent: Gaining water from the ground

  • Bedrock controlled: Rocky

  • alluvial: more wide

  • Headwater / middle-order / lowland

Response factors affected by watershed characteristics

  • Discharge
  • Response speed
  • Water velocity
  • Flood risk
  • Sediment load
  • Water availability
  • Ecosystems and habitats

Two types of watersheds:

  • Boring
  • Crazy

Hydrologic cycle

Precipitation (496 km3/y) == evaporation (496 km3/y) ## Processes * Precipitation * Evaporation * Evapotranspiration * Melt runoff * Submlimation * Percolation * Groundwater flow * Streamflow * Infiltration * Overland flow * Condensation ## Storage * Groundwater

  • Snow & Glaciers * Lake * Vegetation * Stream * Ocean * Atmospheric Water

Water balance equation

P + G_(in) - (Q + ET + G_(out)) = ΔS Q = P - ET ± ΔS (simplified) Q = P - ET (Ultra-simplified)

P = precipitation G_(in) = groundwater Q = stream outflow ET = evapotranspiration G_(out) = groundwater out S = storage

**All in depth* [e.g., mm] or volume [e.g. m³]*