Prepared in cooperation with the Brazos River Authority
Analysis of Minimum 7-Day Discharges and Estimation of Minimum 7-Day, 2-Year Discharges for Streamflow-Gaging Stations in the Brazos River Basin, Texas
By Timothy H. Raines and William H. Asquith
U.S. Geological Survey
Water-Resources Investigations Report 97–4117
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Contents
Abstract
Introduction
Purpose and Scope
Physical Setting
Methods of Analysis
N-Day Analysis
Statistical Analysis
Frequency Analysis
Acknowledgments
Analysis of Minimum 7-Day Discharges
Mann-Kendall Test
Wilcoxon Rank-Sum Test
Estimation of Minimum 7-Day, 2-Year Discharges
Stations With Unregulated Discharge Only
Stations With Regulated Discharge Only
Stations With Both Unregulated and Regulated Discharge
Summary
References Cited
Plate
| 1. | Map showing locations of streamflow-gaging stations in the Brazos River Basin, Texas |
Figures
| 1. | Map showing location of the Brazos River Basin, Texas | |
| 2. | L-moment diagram for unregulated and regulated streamflow-gaging stations, Brazos River Basin, Texas | |
| 3–10. | Graphs showing annual minimum 7-day discharges for station: | |
| 3. | Graphs showing annual minimum 7-day discharges for station: | |
| 4. | 08083100 Clear Fork Brazos River near Roby, Texas, water years 1963–93 | |
| 5. | 08150000 Big Creek near Needville, Texas, water years 1948–95 | |
| 6. | 08093100 Brazos River near Aquilla, Texas, water years 1941–95 | |
| 7. | 08082000 Salt Fork Brazos River near Aspermont, Texas, water years 1940–95 | |
| 8. | 08091000 Brazos River near Glen Rose, Texas, water years 1924–95 | |
| 9. | 08096500 Brazos River at Waco, Texas, water years 1897–1995 | |
| 10. | 08091500 Paluxy River at Glen Rose, Texas, water years 1948–95 | |
Tables
| 1. | Reservoirs with normal storage capacities exceeding 5,000 acre-feet, Brazos River Basin, Texas |
| 2. | Streamflow-gaging stations with at least 10 years of unregulated or 10 years of regulated discharge data, Brazos River Basin, Texas |
| 3. | Mann-Kendall test results of annual minimum 7-day discharges for streamflow-gaging stations with at least 10 years of unregulated or 10 years of regulated discharge data |
| 4. | Wilcoxon rank-sum test results of annual minimum 7-day discharges for streamflow-gaging stations with at least 10 years each of unregulated and regulated discharge data |
| 5. | Estimated minimum 7-day, 2-year discharges for streamflow-gaging stations, Brazos River Basin, Texas |
Abstract
The minimum 7-day, 2-year discharge (7Q2) was estimated for each of 97 streamflow-gaging stations in the Brazos River Basin, Texas, with at least 10 years of unregulated or 10 years of regulated discharge data using the Pearson Type III distribution fitted by L-moment statistics of the annual minimum 7-day discharges. Forty-six stations have at least 10 years of unregulated discharge data only, 26 stations have at least 10 years of regulated discharge data only, and 25 stations have at least 10 years of unregulated and 10 years of regulated discharge data.
Statistical tests were applied to the annual minimum 7-day discharges to determine if the unregulated and regulated data have trends; and to determine if the data before regulation are significantly different from the data after regulation. Results of the Mann-Kendall test indicate a significant trend at the 95-percent confidence level for 8 of the 46 unregulated-only stations, for 6 of the 26 regulated-only stations, and for 5 stations before regulation and for 10 stations after regulation of the 25 stations with both unregulated and regulated data. Results of the Wilcoxon rank-sum test indicate a significant difference between the unregulated and regulated annual minimum 7-day discharges at the 95-percent confidence level for 16 of the 25 stations with both types of data.
The 7Q2 is zero for 31 of the 46 unregulated-only stations and for 9 of the 26 regulated-only stations. The 7Q2 also is zero for 9 stations before regulation and for 3 stations after regulation of the 25 stations with both unregulated and regulated data. The 7Q2 ranges from 33 to 631 cubic feet per second for the eight regulated-only stations on the mainstem Brazos River. For the seven stations on the mainstem Brazos River with at least 10 years of unregulated and 10 years of regulated discharge data, the 7Q2 ranges from 0 to 568 cubic feet per second before regulation and from 0.30 to 670 cubic feet per second after regulation. The 7Q2 increased during regulation for 19 of the 25 stations with both unregulated and regulated data. The effect of regulation for most stations generally was an increase in the magnitude of the 7Q2.
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