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LUMPED HYDROLOGIC MODELS

LUMPED HYDROLOGIC MODELS. BLACK BOX. Before GEO3280!. UNIT HYDROGRAPH THEORY. Sherman - 1932 Horton - 1933 Wisler & Brater - 1949 - “the hydrograph of surface runoff resulting from a relatively short, intense rain, called a unit storm.”

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LUMPED HYDROLOGIC MODELS

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  1. LUMPED HYDROLOGIC MODELS BLACK BOX Before GEO3280!

  2. UNIT HYDROGRAPH THEORY • Sherman - 1932 • Horton - 1933 • Wisler & Brater - 1949 - “the hydrograph of surface runoff resulting from a relatively short, intense rain, called a unit storm.” • The runoff hydrograph may be “made up” of runoff that is generated as flow through the soil (Black, 1990).

  3. WHAT IS THE THEORY BEHIND THE UNIT HYDROGRAPH ?

  4. Impervious Parking Lot

  5. ASSUMPTIONS: RESPONSE IS ALWAYS DIRECTLY PROPORTIONAL TO THE INPUT.

  6. NOTE: RESPONSE IS ALWAYS PROPORTIONATE TO THE INPUT. DOUBLE INPUT- DOUBLE RESPONSE.

  7. NOTE: TWO POINT TWO TIMES THE INPUT- TWO POINT TWO TIMES THE RESPONSE!

  8. ASSUMPTIONS: RESPONSE IS ALWAYS DIRECTLY PROPORTIONAL TO THE INPUT. TIME-BASE OF RESPONSE IS ALWAYS FIXED REGARDLESS OF THE INPUT

  9. NOTE: TWO CONSECUTIVE INPUTS OF 1mm/min PRODUCESTWO CONSECUTIVE, BUT SUPERIMPOSED RESPONSES, THE TIME BASES OF WHICH ARE EQUAL.

  10. NOTE: THE TIME BASE OF EACH RESPONSE REMAINS AT 7 MINUTES, REGARDLESS OF WHETHER THE IMPUT IS 0.2 2 OR 1 mm/minutes AS IN THIS MORE COMPLEX EVENT

  11. HOW CAN A UNIT HYDROGRAPH BE DERIVED IN REALITY?

  12. 1. Select fairly clearly defined single events from available records Avoid this sort of complex response where it is hard to separate one storm from the next

  13. 2. Extract observed hydrograph

  14. 3. Separate out the “Direct Runoff” response from the groundwater or baseflow. Direct Runoff Hydrograph GROUNDWATER?

  15. HOW TO COMPUTE MASTER RECESSION CURVE

  16. 4. Determine the quantity and temporal distribution of “Effective Rainfall” that gave rise to that Direct Runoff Hydrograph As is assignments and lectures, this involves consideration of water lost to interception, depression storage and infiltration.

  17. Storm Duration, d Average Storm Effective Precipitation, p

  18. Effective Hyetograph 5. Determine the Direct Runoff Hydrograph resulting from an Effective Precipitation of p mm over d hours/minutes in duration Direct Runoff Hydrograph

  19. HOW DO YOU PROCEED FROM THIS 37.6 mm 90 min UNIT HYDROGRAPH TO ONE OF MORE CONVENTIONAL DEPTH (e.g. 25mm) AND TIME (e.g. 1 hour) UNITS?

  20. 1. Use the Equal Time Base of Response assumption

  21. This S-curve showing the response to a very large number of sequential 37.6mmm in 90 events would be the same no matter what time we actually started the sequence, at time 0, or after 5 minutes, or 60 minutes. • Imagine that we set such a sequence running at time 0, and a second identical sequence running at time 60 minutes. • We have two s-curves offset by 60 minutes. • The difference in discharges between the two S-curves, measured at any time, represents the response of the first S-curve, that the second one hasn’t yet experienced because of the 60 minute shift. • This difference therefore represents the “missing” rainfall in that 60 minute period, or the 37.6mm for 60 minutes hydrograph.

  22. HOW DO WE CONVERT THIS 37.6mm for 1hr UNIT HYDROGRAPH INTO SOMETHING MORE STANDARD LIKE A 25mm (1”) for 1 hour UNIT HYDROGRAPH? Use the assumption of a proportional response.

  23. HERE IS OUR BASIC “BUILDING BLOCK”, DERIVED WITHOUT HAVING TO GO THROUGH THE EXERCISE OF ISOCHRONES ETC., BUT WHICH CAN BE USED TO FORECAST RESPONSES TO STORM DELIVERING VARYING QUANTITIES OF WATER IN DIVERSE PATTERNS OVER TIME.

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