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DES 606 : Watershed Modeling with HEC-HMS

DES 606 : Watershed Modeling with HEC-HMS. Module 10: Average Rainfall Theodore G. Cleveland, Ph.D., P.E, M. ASCE, F. EWRI 26-28 August 2015. Module 10. Outline for Module 10. FHWA-NHI-02-001 Highway Hydrology Chapter 2, Section 2.1; Chapter 3

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DES 606 : Watershed Modeling with HEC-HMS

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  1. DES 606 : Watershed Modeling with HEC-HMS Module 10: Average Rainfall Theodore G. Cleveland, Ph.D., P.E, M. ASCE, F. EWRI 26-28 August 2015 Module 10

  2. Outline for Module 10 • FHWA-NHI-02-001 Highway Hydrology • Chapter 2, Section 2.1; Chapter 3 • Examine spatial distribution of rainfall and averaging techniques. • Examine how to put multiple gages into HMS and assign these gage depths to a particular watershed. Module 10

  3. Total and Incremental Rainfall • The total amount (measured as depth) of rainfall that occurs in a storm is the important input characteristic for describing the response of a watershed to rainfall. • The depth-time of rainfall (hyetograph) is equally important. Module 10

  4. Rainfall Properties • There are a number of time-related and space-related factors that are used in explaining rainfall input. • The four most important are: • Intensity (a rate: i.e. inches/hour) • Duration (a time: 15 minutes) • Frequency (a probability: 1%) • Average Depth (a length: inches) • Actually better thought of as volume/area, but dimensionally it is a length. Module 10

  5. HEC-HMS Rainfall Input • HEC-HMS has precipitation input at “gages” that are assigned to basins. • The examples so far assume a single gage is assigned to a sub-basin. • HEC-HMS inputs are in depths, either incremental or cumulative • Intensity x Duration = Depth • These computations are typically external to HMS. • Here duration is simply used as a time interval, but the term really refers to an entire storm length and not some portion. Module 10

  6. Average Rainfall • Precipitation • Meterology, Climate • Runoff • Fraction of precipitation signal remaining after losses Spatial distribution – these precipitation arrows may not be identical. Unless we wish to route hydrographs, need some way to “average” the input. • Watershed • Losses • Transformation • Storage • Routing Module 10

  7. Average Rainfall • Averaging is used to generate uniform inputs for unit hydrograph applications • One implicit assumption of the UH is spatially uniform input time series. • Averaging avoids having to route hydrographs • Routing would probably be required on larger watersheds. • If the data justify distribution, then could route subdivided watersheds to capture storm patterns. Module 10

  8. Average Rainfall • The entire volume of rainfall applied to an entire basin is called the precipitation volume • If the basin area normalizes this volume the resulting value is called the equivalent uniform depth • Methods to compute equivalent depth • arithmetic mean • theissen polygon network • iso-heyetal method Module 10

  9. Arithmetic Mean • The mean value of all nearby gages is used • Not all gages actually on the watershed Module 10

  10. Polygon Weighting • A weighted mean based on polygon area is used • Not all gages actually on the watershed • Polygons formed using Theissen method • Can use a minimum distance algorithm to semi-automatically generate the weights Module 10

  11. Polygon Weighting • A weighted mean based on polygon area is used Subarea A Subarea B Subarea C Subarea D Area ratios are called Theissen weights Subarea E Module 10

  12. Isohyetal • A weighted mean based on iso-hyetal panel areas is used • Not all gages actually on the watershed • Areas formed by intersection of isohyetal contours and underlying drainage area Module 10

  13. Isohyetal • A weighted mean based on isohyetal panel areas is used Module 10

  14. Averaging Rainfall • Theissen polygons and arithmetic mean are probably the most common because the weights are constants with respect to geography. • Arithmetic mean is easiest to automate Module 10

  15. Example • Illustrate use of multiple gages on Ash Creek. • Known Theissen weights are • 0.12 and 0.88 • Simulate using these known weights. Module 10

  16. Summary • Multiple rain gage data can be used to estimate an equivalent uniform depth • Gage weights by a variety of methods • Arithmetic mean • Minimum distance (Theissen polygons) • Isoheyetal • Inverse distance methods Module 10

  17. Summary • HEC-HMS models multiple gages in the Meterological Model Manager • The example illustrates how to set-up multiple gages • Weights were supplied Module 10

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