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Estimating Catchment Runoff

Estimating Catchment Runoff. Direct runoff or Effective rainfall. Storm. Initial abstraction. Infiltration. Losses. Surface depression storage. Infiltration methods. Soil Conservation Service (SCS) Curve Number (CN) method Horton’s equation ‘moving curve’ method Green & Ampt model.

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Estimating Catchment Runoff

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  1. Estimating Catchment Runoff Direct runoff or Effective rainfall Storm Initial abstraction Infiltration Losses Surface depressionstorage

  2. Infiltration methods • Soil Conservation Service (SCS) Curve Number (CN) method • Horton’s equation ‘moving curve’ method • Green & Ampt model

  3. SCS Curve Number method CN depends on soil type and pre-wetting inches P(t) = depth of rainfall Q(t) = depth of runoff Ia = initial abstraction S = potential storage CN = curve number  100 mm

  4. Horton equation

  5. Green & Ampt model where M=moisture deficit S =suction head K =hydraulic conductivity

  6. Rainfall-Runoff models (1) Rainfall Effectiverainfall Infiltration Model Runoff Losses Catchment Model Losses subtracted from rainfall to get effective rainfall which is then applied to catchment.

  7. Rainfall-Runoff models (2) Losses and infiltration calculated along with runoff as part of Runoff Model Rainfall Runoff Catchment Model SurfaceDepressionStorage Losses andinfiltration

  8. Calculating the Runoff (1) Runoff from pervious and impervious fractions computed and added together Flow lengths can be:- (a) equal (b) proportional (c) user supplied

  9. Calculating the Runoff (2) Symmetrical catchmentArea = 2.2 ha Overland flow length can be estimated as area divided by length of stream bank available for inflow. 75m 96m 63m One-sided catchmentArea = 2.4 ha 192m

  10. Calculating the Runoff (3) Overland flow routing choices: • Combine effective rainfall with: • triangular response function • rectangular response function • single linear reservoir response function • Combine infiltration & other losses with outflow from idealized inclined plane.(Similar to SWMM RUNOFF method)

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