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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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