1. Finishing Precipitation
Learning Objectives
Be able to construct and interpret intensity-duration-frequency graphs (P37-38)
Be able to calculate area average precipitation (P39-43)
CEE 3430 – Spring 2011

2. Intensity-Duration-Frequency
IDF design curves
All major cities
Based on NWS data
Various return periods & durations
Used for drainage design of pipes & roads
Used for floodplain designs - watersheds
From Bedient

3. Design Rainfalls Design Storm from HCFCD and NWS
Based on Statistical Analysis of Data
5, 10, 25, 50, 100 Year Events
Various Durations of 6 to 24 hours
Six Hour Rainfall
From Bedient

4. NOAA Hydrometeorological Design Studies Center Precipitation Frequency Data Server (PFDS)
The standard source for design storm data
CEE 3430 – Spring 2011

5. Example – Design Rainfall for Logan
What amount of rain in Logan in a 6 hr period is a 100 yr storm
What amount of rain in Logan in a 1 hr period is a 100 year storm
Develop a 6 hour 100 year design storm using 1 hour time steps
CEE 3430 – Spring 2011

6. From NOAA Atlas 14 (for Utah)
CEE 3430 – Spring 2011

7. From NOAA Atlas 2 http://www. nws. noaa
CEE 3430 – Spring 2011

8. From NOAA Atlas 2 http://www. nws. noaa
CEE 3430 – Spring 2011

9. Rainfall Averaging Methods
From Bedient
Rainfall Averaging Methods

10. Thiessen Polygons - Areal Average Rainfall from Gages
Connect gages with lines
Form triangles as shown
Create perpendicular bisectors of the triangles
Each polygon is formed by lines and WS boundary
P = S (Ai*Pi) / AT
From Bedient

11. RADAR Rainfall Estimates
NEXRAD provides real-time data on a ~16 km2 (6 mi2) grid
Each estimate represents an average rainfall amount over the entire 4 x 4 km2 area
NEXRAD rainfall estimates compare well with point rain gage measurements (r2 ~ 0.9)
From Bedient

13. Streamflow and Hydrologic Measurements
Learning Objectives
Be able to describe runoff processes and components of a runoff hydrograph (p43-45)
Be able to describe the impact of watershed shape and land alterations on a runoff hydrograph (p46)
Be able to list ways that atmospheric parameters are measured (p47)
Be able to describe how streamflow is measured and construct a rating curve (p48-54)
CEE 3430 – Spring 2011

14. Physical Processes involved in Runoff Generation
Rainfall Runoff Processes
Figure 1. Physical Processes involved in Runoff Generation.
Physical Processes involved in Runoff Generation

15. Figure 2-3

16. Watershed effects on hydrograph shape
Figure 1-12

17. f1 f0
Figure 7. Rainfall, runoff, infiltration and surface storage during a natural rainstorm. The shaded areas under the rainfall graph represent precipitation falling at a rate exceeding the infiltration rate. The dark grey area represents rainfall that enters depression storage, which is filled before runoff occurs. The light grey shading represents rainfall that becomes overland flow. The initial infiltration rate is f0, and f1 is the final constant rate of infiltration approached in large storms. (from Dunne and Leopold, 1978)

18. Runoff Generation Mechanisms
(a) Infiltration excess overland flow
(also called Horton overland flow) P qo f
(following Beven, 2001)
See infiltration excess runoff generation animation (ch 2)

19. (c) Saturation excess overland flowP
Variable source area P P qo qr qs
(following Beven, 2001)
See saturation excess runoff generation animation (ch 2)

20. Evaporation Measurement
Eddy Covariance
Pan
Photo:

21. Shielded and Unshielded weighing Precip (Rain and Snow Gages
Snow Pillow

22. Streamflow Measurement
Figure 1-14

23. Rating Curve
Figure 1-15

24. Summary
The atmosphere is the major link between oceans and continents and drives the hydrologic cycle through precipitation
Precipitation variability is summarized statistically in intensity-duration-frequency curves used for design
Spatial averaging methods are used to calculate precipitation over a watershed
Runoff due to precipitation involves overland flow, infiltration, interflow, the direct runoff hydrograph and baseflow
Measurement methods for Precip, ET and streamflow
CEE 3430 – Spring 2011