In cooperation with the Texas Department of Transportation
Areal-Reduction Factors for the Precipitation of the 1-Day Design Storm in Texas
By William H. Asquith
U.S. Geological Survey
Water-Resources Investigations Report 99–4267
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Contents
Abstract
Introduction
Purpose and Scope
Daily Precipitation Data Sources
Background and Previous Studies
Approach
Annual Maxima-Centered Areal-Reduction Factors
Watershed Precipitation Volume
Depth of Effective Precipitation and Areal-Reduction Factors
Empirical Depth-Distance Relations From Sample-Ratio Calculation
Areal-Reduction Factors
Database Evaluation
Point-Process Evaluation
Areal-Process Evaluation
Empirical Depth-Distance Relations Near Selected Localities
Frequency Considerations
Seasonal Considerations
Estimation of Areal-Reduction Factors
Application of Techniques
Summary
Selected References
Appendix I. Summary Statistics of Intra-Network Sample Ratios for Austin, Texas
I–1. |
Summary statistics of intra-network sample ratios surrounding any annual precipitation maxima near Austin, Texas |
I–2. |
Summary statistics of intra-network sample ratios surrounding the 2-year or greater annual precipitation maxima near Austin, Texas |
I–3. |
Summary statistics of intra-network sample ratios surrounding the 5-year or greater annual precipitation maxima near Austin, Texas |
Appendix II. Summary Statistics of Intra-Network Sample Ratios for Dallas, Texas
II–1. |
Summary statistics of intra-network sample
ratios surrounding any annual precipitation maxima near Dallas, Texas |
II–2. |
Summary statistics of intra-network sample ratios surrounding
the 2-year or greater annual precipitation maxima near Dallas, Texas |
II–3. |
Summary statistics of intra-network sample ratios surrounding
the 5-year or greater annual precipitation maxima near Dallas, Texas |
Appendix III. Summary Statistics of Intra-Network Sample Ratios for Houston, Texas
III–1. |
Summary statistics of intra-network sample
ratios surrounding any annual precipitation maxima near Houston, Texas |
III–2. |
Summary statistics of intra-network sample ratios surrounding
the 2-year or greater annual precipitation maxima near Houston, Texas |
III–3. |
Summary statistics of intra-network sample ratios surrounding
the 5-year or greater annual precipitation maxima near Houston, Texas |
III–4. |
Summary statistics of intra-network sample ratios surrounding
the 10-year or greater annual precipitation maxima near Houston, Texas |
Figures
1. |
Map showing location of study areas in Texas | |
2–4. |
Maps showing location of stations for | |
2. |
Two precipitation-monitoring networks near Austin, Texas | |
3. |
Two precipitation-monitoring networks near Dallas, Texas | |
4. |
Three precipitation-monitoring networks near Houston, Texas | |
5. |
Empirical depth-distance relation and a subset of intra-network sample ratios for any annual precipitation maxima near Dallas, Texas | |
6–8. |
Comparison of empirical 2-year or greater depth-distance relations for | |
6. |
Two Austin precipitation-station networks, National Weather Service (NWS) and City of Austin (AUS) | |
7. |
Two Dallas precipitation-station networks, National Weather Service (NWS) and City of Dallas (DAL) | |
8. |
Three Houston precipitation-station networks: National Weather Service (NWS), Harris County Office of Emergency Management (HAR), and Houston Urban Program (HURP) | |
9–11. |
Empirical depth-distance relations for selected recurrence intervals for | |
9. |
Austin, Texas | |
10. |
Dallas, Texas | |
11. |
Houston, Texas | |
12–13. |
Empirical 2-year or greater depth-distance relations for | |
12. |
Winter and summer for Austin, Dallas, and Houston, Texas | |
13. |
Austin, Dallas, and Houston, Texas | |
14–16. |
Estimated 2-year or greater depth-distance relations for | |
14. |
Austin, Texas | |
15. |
Dallas, Texas | |
16. |
Houston, Texas | |
17–18. |
Areal-reduction factors for 2-year or greater 1-day design storms for | |
17. |
Large circular watersheds for Austin, Dallas, and Houston, Texas | |
18. |
Small circular watersheds for Austin, Dallas, and Houston, Texas |
Tables
1. |
Stations for two precipitation-monitoring networks near Austin, Texas |
2. |
Stations for two precipitation-monitoring networks near Dallas, Texas |
3. |
Stations for three precipitation-monitoring networks near Houston, Texas |
4. |
Abbreviated example of intra-network sample ratios surrounding any annual precipitation maxima near Dallas, Texas |
5. |
Abbreviated summary statistics of intra-network sample ratios surrounding any annual precipitation maxima near Dallas, Texas |
6. |
Summary of observed cumulative annual probabilities for each precipitation-monitoring network |
7. |
Equations that define the estimated 2-year and greater depth-distance relation and the areal-reduction factor for circular watersheds for Austin, Dallas, and Houston, Texas |
8. |
Example areal-reduction factor calculation for a hypothetical, linear watershed in the Austin area |
Abstract
The reduction of the precipitation depth from a design storm for a point to an effective (mean) depth over a watershed often is important for cost-effective design of hydraulic structures by reducing the volume of precipitation. A design storm for a point is the depth of precipitation that has a specified duration and frequency (recurrence interval). The effective depth can be calculated by multiplying the design-storm depth by an areal-reduction factor (ARF). ARF ranges from 0 to 1, varies with the recurrence interval of the design storm, and is a function of watershed characteristics such as watershed size and shape, geographic location, and time of year that the design storm occurs. This report documents an investigation of ARF by the U.S. Geological Survey, in cooperation with the Texas Department of Transportation, for the 1-day design storm for Austin, Dallas, and Houston, Texas. The "annual maxima-centered" approach used in this report specifically considers the distribution of concurrent precipitation surrounding an annual precipitation maxima. Unlike previously established approaches, the annual maxima-centered approach does not require the spatial averaging of precipitation nor explicit definition of a representative area of a particular storm in the analysis. Graphs of the relation between ARF and circular watershed area (to about 7,000 square miles) are provided, and a technique to calculate ARF for noncircular watersheds is discussed.
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