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Precipitation Measurement

Precipitation Measurement. Area Average Precipitation. From Mays, 2011, Ground and Surface Water Hydrology. NOAA Hydrometeorological Design Studies Center Precipitation Frequency Data Server (PFDS). http://hdsc.nws.noaa.gov/hdsc/pfds/ The standard source for design storm data.

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Precipitation Measurement

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  1. Precipitation Measurement

  2. Area Average Precipitation From Mays, 2011, Ground and Surface Water Hydrology

  3. NOAA Hydrometeorological Design Studies CenterPrecipitation Frequency Data Server (PFDS) http://hdsc.nws.noaa.gov/hdsc/pfds/ The standard source for design storm data CEE 3430 – Spring 2011

  4. Terminology used in Precipitation Frequency Analysis D = i Td • Duration (Td). The interval over which precipitation occurs. • Intensity (i). The average precipitation rate over a specified duration • Depth (D). The total precipitation over a specified duration • Exceedanceprobability (P). The probability associated with exceeding a given amount in any given period (usually year) once or more than once. • Recurrence interval or Return period (T). The average interval between events of a set magnitude, evaluated as the inverse of the exceedenceprobability • A 50 year return period event has a 1/50 = 0.02 probability of being exceeded in any one year • Frequency. General term for specifying the average recurrence interval or annual exceedanceprobability. Rate i Time Td • Annual maximum series (AMS). Time series of the largest amounts in a continuous period (usually year). • Partial duration series (PDS). Time series that includes all amounts above a pre-defined threshold regardless of year; it can include more than one event in any particular year. Based on http://www.nws.noaa.gov/oh/hdsc/glossary.html

  5. Water Budget in a Watershed From Mays, 2011, Ground and Surface Water Hydrology

  6. Evaporation • Calculate Evaporation by • Energy balance (p304) • Aerodynamic method • (p307) • Combined method • (p309) • Priestley-Taylor method • (p309)

  7. Method Information Requirements Aerodynamic Energy Balance RN Combination

  8. + + RN - - - + Summary • Energy exchanges and energy balance • Turbulent diffusion into the atmosphere • Adjustment and balance Conditions adjust to varying inputs. Calculations can interpret measurement, but should not be used to predict the effect of changing one variable without considering the adjustments of connected variables

  9. Surface Runoff occurs when surface water input exceeds infiltration capacity. (a) Infiltration rate = rainfall rate which is less than infiltration capacity. (b) Runoff rate = Rainfall intensity – Infiltration capacity. (from Dunne and Leopold, 1978)

  10. Green-Ampt model idealization of wetting front penetration into a soil profile • for ho = 0 From Mays, 2011, Ground and Surface Water Hydrology

  11. Infiltration capacity – Depth Function

  12. Example Consider a watershed with XXX soil and Green-Ampt parameters given in Mays Table 7.4.1 (page 317). Consider a storm where X cm of rainfall occurs in X hours. Calculate the following using the Green-Ampt approach. a) Time to ponding b) Depth of infiltration excess runoff generated from this storm

  13. Unit hydrographs • The unit hydrograph reflects the unchanging characteristics of a watershed that relates excess precipitation to direct runoff. • U(t) is the response to 1 unit of precipitation over a watershed in duration D. • Direct application. Given U(t) and P(t) calculate Q(t) (8.3.1, 8.3.2) • Inverse application. Given P(t) and Q(t) deduce U(t) for use with different P(t) inputs (8.3.2, 8.3.1) • Deduce losses and excess precipitation – CN,  index (8.2.1, 8.7.2, 8.2.6, 8.7.1, 8.7.3, 8.7.5) • Synthetic unit hydrograph. Determine U(t) from watershed attributes (8.4.1, 8.8.1, 8.4.1, #6, 8.8.1, 8.8.2) • S-Hydrograph. Technique to change the duration D associated with a unit hydrograph (8.5.1, 8.3.6, 8.5.4) ExampleHW Problem

  14. Synthetic Unit Hydrographs • A unit hydrograph is intended to quantify the unchanging characteristics of the watershed • The synthetic unit hydrograph approach quantifies the unit hydrograph from watershed attributes 1/3 2/3 Follow the procedure of table 8.4.1 • Widths (4.05,570) (3.09,427.5) (5.97,427.5) W75 (7.41,285) W50 1/3 2/3 (14.1,0)

  15. Effective Precipitation from the SCS Curve Number Equation From Mays, 2011, Ground and Surface Water Hydrology

  16. Hydrologic Response from the unit Hydrograph Excess Precipitation Precipitation Infiltration Capacity Excess Precipitation Time

  17. Calculating a Hydrograph from a Unit Hydrograph and visa versa ... ... for n=1 ... N From Mays, 2011, Ground and Surface Water Hydrology

  18. Baseflow separation and hydrograph recession Direct Runoff Baseflow End of direct runoff and beginning of baseflow recession

  19. Determining the  index and Excess Rainfall Hyetograph

  20. Rainfall – Runoff Analysis From Mays, 2011, Ground and Surface Water Hydrology

  21. S Curve to change the duration associated with a unit hydrograph

  22. Practice Problem

  23. Practice 2

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