|Oklahoma Water Science Center
By Robert L. Tortorelli, Teresa J. Rasmussen, and Charles A. Perry
U.S. Geological Survey Scientific Investigations Report 2004-5192
This report is available as a pdf.
Evidence that the magnitudes of the annual peak flow for some streams in Oklahoma are changing led to an investigation by the U.S. Geological Survey, in cooperation with the Oklahoma Department of Transportation, of trends in the magnitude of annual peak flows and mean annual flows in Oklahoma. Trends in peak flow are of particular interest to State and Federal highway agencies because trends might indicate changing levels of risk to highway structures in flood plains. The Kendall’s tau test was used to identify trends and LOWESS trend lines were used as a graphical exploratory technique for trends. A peak-flow analysis used the entire period of record from 80 streamflow-gaging stations within and near Oklahoma that had a minimum of 36 years of record. Records from 3 streamflow-gaging stations indicated statistically significant upward trends in peak flows, while records from 12 stations indicated statistically significant downward trends. The records with upward trends were from streamflow-gaging stations scattered in the central and northeastern part of the study area, while the significant downward-trend stations were all located in the western part of the study area.
A peak-flow analysis used a recent 36-year period of record, 1968–2003, from 63 stations within and near Oklahoma. Seven station records had significant downward trends, and one station record had a significant upward trend. The significant downward-trend stations were located in the western part of the study area. The significant upward-trend station was located in the central part of the study area.
A peak-flow analysis used various 30-year periods separated by 5-year increments through the available periods of record from 63 stations within and near Oklahoma. From that analysis it is possible to identify time periods within each station record when peak-flow trends were occurring. Peak-flow trends generally were downward during 1956-85 and upward in 1966-95.
A mean annual-flow analysis used the entire period of record from 80 stations within and near Oklahoma that had more than 36 years of record. A regional pattern similar to the peak-flow analysis resulted, except more upward trends were significant. Twenty-eight records (35 percent) exhibited a trend; 22 streamflow-gaging stations indicated statistically significant upward trends in peak flows, while records from 6 stations indicated statistically significant downward trends. The significant downward-trend stations were located in the northwestern part of the study area. The LOWESS trend lines indicated an increase in streamflow at the end of the 20th century, around 1980-2000, for two-thirds of the stations analyzed.
A mean annual-flow analysis used a recent 36-year period of record, 1968–2003, from 63 stations within and near Oklahoma. Eighteen station records showed significant trends; 14 station records had upward trends, and 4 records had downward trends. The significant downward trend stations were located in the northwestern part of the study area.
Changes in precipitation patterns, long-term declines in ground-water levels in some stream basins, and increased water use may be contributing to peak-flow trends. To evaluate possible causes of the peak-flow trends, the Kendall’s tau test was applied to total annual precipitation within and near Oklahoma, and to ground-water levels in Oklahoma. The lack of significant precipitation trends and presence of downward trends in ground-water levels in western Oklahoma indicated that declining water tables may be a factor contributing to downward trends in peak streamflow. Declining water tables maybe caused by ground-water withdrawals and other factors such as construction of ponds and terraces. Water use could not be used in the trend analyses due to a lack of reliable historic record. Estimates of total freshwater withdrawals in Oklahoma available on a 5-year basis from calendar year 1950 to 2000 were shown.
Peak-flow records containing trends may introduce statistical error into flood-frequency analysis. The effects of significant trends on flood-frequency analysis were investigated by adding hypothetical trends to four streamflow-gaging station records that had no significant trends. The added trends resulted in changes in the 100-year flood magnitudes of as much as 91 percent.
Purpose and Scope
Kendall’s Tau Test
LOWESS Trend Line
Streamflow Trend Analyses
Entire Period of Record
Recent Period, 1968-2003
Mean Annual Flow
Entire Period of Record
Recent Period, 1968-2003
Evaluation of Trend Causes
Factors Affecting Peak Flow
Precipitation Trend Analysis
Water-Table Trend Analysis
Effects of Trends on Flood-Frequency Analysis
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For information about water-resource studies in the State of Oklahoma, please visit our Web site at http://ok.water.usgs.gov/.
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