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Runoff and Stream flow

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Runoff and Stream flow

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  1. Runoff and Stream flow Ali Fares, PhD Watershed Hydrology, NREM662 UHM-CTAHR-NREM NREM 662

  2. 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

  3. NREM 662

  4. Runoff Processes • The processes that produce storm runoff are: • Horton overland flow • Subsurface stormflow, • Return flow • And direct precipitation onto saturated areas NREM 662

  5. 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

  6. VSA: Dry Season NREM 662

  7. VSA: Wet Season NREM 662

  8. 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

  9. 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) NREM 662

  10. Variable Source Area Ritter et al., 1995 NREM 662

  11. Variable Source Area Ritter et al., 1995 NREM 662

  12. Variable Saturated Area • RO from a steep well-drained hillside, VT, rain: 44 mm, in 2h NREM 662

  13. 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. NREM 662

  14. VSA: Seasonal Dynamic • Seasonal variation of pre-storm saturated area in a catchment with steep, well-drained hillsides and a narrow valley floor, Vermont. NREM 662

  15. Runoff processes & their major control Thin Soils gentle concave slopes Deep Soils Steep Hillslope NREM 662 Arid, thin veg. & dist. humid, dense veg. & <dist.

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

  21. 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

  22. Tsihrintzis, 2002 What are the components of a Hydrograph? NREM 662

  23. Hydrograph analysis • Hydrograph separation methods • baseflow separation • groundwater components • Unit hydrograph • a simple method to model streamflow due to rainfall

  24. 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

  25. 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. NREM 662

  26. Baseflow separation lag time Baseflow recession = 0.9886/0.5 hr recession = 0.997/0.5 hr time to peak

  27. Stream flow separation conducted using the Sliding Interval Method NREM 662

  28. Expanding channel network NREM 662

  29. What does it represents? • It represents the effects of: • climate, • hydrologic losses, • surface runoff, • interflow and • ground water flow (Bedient and Huber, 1988). NREM 662

  30. Factors affecting Hydrograph NREM 662

  31. NREM 662

  32. Effect of Watershed Shape NREM 662

  33. Annual hydrograph • Useful for: • water budgeting • supply conditions and • potential flood periods. NREM 662

  34. 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

  35. Long-term R-RO relationship NREM 662

  36. Impacts on Runoff - Rainfall NREM 662

  37. Impact of Land use NREM 662

  38. Urbanization effects NREM 662