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Runoff

Runoff Processes

Runoff within the water & energy balances

  • D -- Direct precipitation
  • O -- Overland Flow
  • Ss -- Subsurface flow

 

Water balance:

  • Falls under Q; is an intermediary process

Energy Balance:

  • Kinetic energy (E_(k))
  • Potential energy (gravitational potential, pressure potential, matric potential, osmotic potential)

 

Fundamentals of runoff

  • Importance:
    • Directly related to erosion
    • System responses to precipitation are key to establish flooding risk
    • Initiator of landslides
    • Directly related to water quality

In humid areas, annual precipitation is disposed of approximately as shown by the percentages listed.

Surface stormflow leaves the small basin within minutes, whereas some baseflow may be retained within the soil mantle for years.

Q = P - ET

  • P = 100%
  • ET = 67%
  • Q = 33%
    • 23% baseflow
    • 8% subsurface flow
    • 2% surface stormflow

 

Subsurface Flow

  • "A runoff generating mechanism operating in most upland terrains"
  • Occurs when water moves laterally down a hillslope through soil layers or permeable bedrock to contribute to the storm hydrograph
  • In humid environments and steep terrain with conductive soils, subsurface flow may be the main mechanism of storm runoff generation
  • Subsurface flow rate is highly dependent on hydraulic conductivity

micropore (matrix) flow

  • Upward and downwards flow

macropore flow

  • Downward flow only

Overland flow

 

Infiltration-excess overland flow

  • Most likely in arid and semiarid landscapes where vegetation densities and infiltration rates are low, or in disturbed areas
    • Hortonion overland flow

 

Saturation-excess overland flow

  • Develops where soil becomes saturation by perennial groundwater table rising to the surface or by lateral or vertical percolation above an impeding horizon
  • Tends to dominate quickflow in thickly vegetated landscapes with thin soils, high water tables, and long gentle concave hillslopes
  • Subsurface flow and saturation overland flow might both be simultaneously important

Variable source area

  • Because of temporal and spatial variation of zones generating subsurface stormflow, the terms variable source area concept and dynamic watershed concept were created.
    • The flow of water in a stream at any given moment is under the influence of a dynamic, expanding or shrinking source area
  • The area actively involved in producing quickly varies from less than 1% to more than 50% in extreme storms.
    • Stormflow & source area will increase at the beginning & decrease at the end of a storm / snowmelt event
      • Caused by subsurface flow & channel expansion / shrinkage

Overview of runoff processes

 

Variable Source Area Hydrology

Variable Source Area Hydrology is the concept that the majority of runoff exiting a watershed is driven by a relatively small portion of the watershed. The abundance of these runoff generating sources may increase and decrease depending on temporal and spatial variability.

 

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Factors that affect runoff

  • Surface and bedrock topography
  • Soil depth and compaction
  • Vegetation type & abundance
    • Higher vegetation = lower soil moisture
  • Antecedent soil moisture
  • Precipitation form, intensity, duration, distribution
  • Flow pathway: matrix vs. macropores
  • Spatial distribution and occurrence of geomorphic features (e.g., deep alluvial valley bottoms, degree of convergence, etc.
  • Watershed characteristics: circularity, drainage density, slope, etc.

 

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The hydrograph concept

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Runoff estimation

Direct measurement

I.e., Sprinkling experiment

Quickflow -- baseflow separation

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We'll use the constant slope method

 

 

Constant slope method by Hewlett & Hibbert (1967)

  • Authors felt that all other methods have the limitation of being completely arbitrary
  • Examined hydrograph events from ~200 water-years of record collected on fifteen small forested watersheds
  • Determined that a line projected from the beginning of any stream rise at a slope of 0.05 cubic feet per second per square mile per hour until it intersected the falling side of the hydrograph would be satisfactory for separating streamflow into quick and delayed flow

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"Old" and "new" water

  • 75% of quickflow is pre-event water on average worldwide
  • There's water that is standing still in the watershed, not moving, happy
    • Rain comes and makes it all connected and flow to the outlet
    • I.e., the rain pushes the water that was already there out of the watershed
  • Isotopic signature: ratio of stable or unstable isotopes of particular elements
    • Can be used to determine old and new water under the following assumptions:
      • Groundwater and baseflow have a single isotopic concentration
      • Rain or meltwater have a single isotopic concentration which is significantly different from that of groundwater and baseflow
      • Vadose water is either same isotopic composition as groundwater or makes a negligible contribution to streamflow

 

The seperation of streamflow into "old" and "new" components is based on the following mixing equation:

CtQt = CoQo + CnQn

Where:

  • C is the concentration of the isotope in the water
  • Q is the flow rate (volume per time unit)
  • The subscripts o, n, and t respectively refer to the old, new, and total components of flow.
    • Given that:
      • Qt = Qo + Qn

 

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Dominant runoff processes:

  • Mapping of dominant runoff processes:
    • Field mapping
    • Soil profiles
    • Vegetation, topography
  • Field experiments:
    • Infiltration
    • Tracer studies
    • Event observations
  • GIS methods