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Runoff and Stream flow. Ali Fares, PhD Watershed Hydrology, NREM662 UHM-CTAHR-NREM. Presentation Outline. Runoff Components of runoff Hortonian Overland Flow Hydrograph Definition Factors affecting hydrographs variable saturated area runoff

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runoff and stream flow

Runoff and Stream flow

Ali Fares, PhD

Watershed Hydrology, NREM662

UHM-CTAHR-NREM

NREM 662

presentation outline
Presentation Outline
  • Runoff
    • Components of runoff
    • Hortonian Overland Flow
    • Hydrograph
      • Definition
      • Factors affecting hydrographs
    • variable saturated area runoff
  • Human impacts on rainfall-runoff relationship:
    • Change land use:
      • Urbanization
      • Intensive agriculture

NREM 662

runoff processes
Runoff Processes
  • The processes that produce storm runoff are:
      • Horton overland flow
      • Subsurface stormflow,
      • Return flow
      • And direct precipitation onto saturated areas

NREM 662

hortonian overland flow hof
Hortonian Overland Flow (HOF)
  • If rainfall exceeds soil infiltration capacity:
    • Water fills surface depression then
    • Water spills over downslope as overland flow and
    • Eventually to the stream

NREM 662

horton was wrong
Horton was wrong...
    • Horton’s theory can be applied to arid areas, badlands, frozen ground or areas where soils have been compacted - e.g., some agricultural soils, road surfaces, hardpan near surface.
  • Hewlett was among the first to observe that infiltration is seldom a limiting factor in forested basins - 1950’s at Coweeta
    • widespread overland flow was not observed
    • Variable Source Area Concept
runoff in high infiltration soils
Runoff in High Infiltration Soils
  • Under most humid forest regions, infiltration capacities are high as a result of:
    • Vegetation protecting soil surface
    • Supply of humus & microfauna create an open soil structure
  • As a result, HOF does not occur on large areas of the landscape.
  • This was observed by Hewlett and Hebert in the Coweeta forested regions.
  • They proposed a dynamic response of catchment to storm rainfall: variable source area (VSA)

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variable source area
Variable Source Area

Ritter et al., 1995

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variable source area11
Variable Source Area

Ritter et al., 1995

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variable saturated area
Variable Saturated Area
  • RO from a steep well-drained hillside, VT, rain: 44 mm, in 2h

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vsa single 46 mm storm
VSA, single 46 mm storm
  • Expansion of saturated zone as a result of a single 46mm rain storm in a catchment with steep, well-drained hillsides and a narrow valley floor, Vermont.

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vsa seasonal dynamic
VSA: Seasonal Dynamic
  • Seasonal variation of pre-storm saturated area in a catchment with steep, well-drained hillsides and a narrow valley floor, Vermont.

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runoff processes their major control
Runoff processes & their major control

Thin Soils gentle concave slopes

Deep Soils Steep Hillslope

NREM 662

Arid, thin veg. & dist.

humid, dense veg. & <dist.

slide16
VSAC
  • only part of a watershed supplies runoff to stream channels - generally the riparian areas
  • during rainfall, subsurface flows from upslope exceed the capacity of riparian soils to transmit it - when this happens, water comes to the surface and the stream channel will grow
  • this embodies a concept known as translatory flow, where new water will displace old water from the groundwater system to the channel
more on vsac
More on VSAC
  • VSAC allows for interflow to occur as lateral flow through near surface soil layers, as a kind of perched water table that forms as a result of a discontinuity in hydraulic conductivity between upper and lower soil horizons
    • interflow comes to the surface and feeds the stream on the rising limb by saturation overland flow
    • subsurface interflow feeds the recession limb of the hydrograph
    • drainage of unsaturated soil can sustain baseflow in headwater catchments
interflow
Interflow
  • Towards the late 1960’s the concept of interflow being able to supply rapid subsurface to the stream channel began to gain wide acceptance
    • studies by Whipkey, Ragan, Dunne and Black involved intercepting these flows with a trench and measuring them
    • Whipkey first postulated the existence of “biological and structural channels” that carry such flow
theory of macropore flow
Theory of macropore flow
  • The idea of macropore flow came into being because of experimental evidence that showed throughflow appeared too soon in streams to have got there by Darcian flow through the soil matrix - thus translatory flow could not account for all subsurface runoff (Weyman, 1973)
    • Pilgrim et al. (1978) confirmed that throughflow was mostly new water not translatory flow
    • Mosley (1979) also confirmed this using dye tracers
old water vs new water
Old water vs. new water
  • Conservative naturally occurring tracers have been used to determine the relative contribution of “old” or “pre-event” water (water already in the watershed - groundwater, soil water etc.) and “new” or “event” water (water added to the system by rain or snowmelt
    • it is necessary that the old and new water have different tracer signatures
    • O and H isotopes, specific conductance, silica
isotopic analysis
Isotopic analysis
  • Studies involving ratios of naturally occurring isotopes of oxygen and hydrogen
    • e.g., ratios of 18O/16O
    • ocean water has a fixed Oxygen isotope ratio
    • once away from the ocean, 16O evaporates preferentially to 18O because it is lighter
    • as a result, old water becomes enriched in 18O relative to new water
hydrograph analysis
Hydrograph analysis
  • Hydrograph separation methods
    • baseflow separation
    • groundwater components
  • Unit hydrograph
    • a simple method to model streamflow due to rainfall
baseflow separation methods
Baseflow separation methods
  • Many older baseflow separation methods suggested using arbitrary assumptions in separating baseflow
    • for example, one method suggested extending the antecedent baseflow recession to a point under the hydrograph peak and then joining that point to an arbitrary point on the hydrograph recession limb
    • a more meaningful approach involves matching sections of the hydrograph recession with curves based on a recession constant
      • each successive value is a constant fraction of the preceeding value
hydrograph separation

Hewlett & Hibbert, 1967

Hydrograph Separation
  • Aims to provide estimate of each component of hydrograph:
    • Overland – Baseflow -Interflow
  • To relate effective rainfall to runoff
  • Ultimately design safe structures for future events.
  • Various techniques used.

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baseflow separation
Baseflow separation

lag time

Baseflow recession = 0.9886/0.5 hr

recession = 0.997/0.5 hr

time to peak

what does it represents
What does it represents?
  • It represents the effects of:
    • climate,
    • hydrologic losses,
    • surface runoff,
    • interflow and
    • ground water flow (Bedient and Huber, 1988).

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annual hydrograph
Annual hydrograph
  • Useful for:
      • water budgeting
      • supply conditions and
      • potential flood periods.

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impacts on ro r relationship
Impacts on RO-R Relationship
  • Rain varies from year to year
  • Human impact:
    • Urbanization
    • Mining
    • Forest management: logging
    • Land use change: from forest to intensive agriculture:
      • Fertilizers, pesticides, plowing, etc..

NREM 662