Trend and Shift Statistics on Annual Maximum Precipitation in Ohio River Basin over the Last Century

Abstract

The National Weather Service Hydrometeorological Design Studies Center is updating the Rainfall Frequency Atlas of the United States (Technical Paper 40, 1961) for the Ohio River Basin states (TP 40 Update). As a part of the data quality control for the TP 40 Update, a statistical investigation for trend and shift in mean and variance of the Annual Maximum Precipitation (AMP) was carried out using data for the last 100 years. Over 1797 stations with records equal to or greater than 50 years from 22 states within the Ohio River Basin were evaluated. The initial findings are: 1) nearly 16 percent (or 282 of 1797) stations exhibit a significant linear trend in the mean at the 90 percent significance level; 2) about 80 percent of the 282 stations indicate a continuous going-up tendency of the mean over the past century while 20 percent are going-down; 3) in a comparison of two sub-periods, 1920-1958 and 1959-1998, 17 percent or 91 of 532 stations demonstrate a significant shift in mean between the two sub-periods. Eighty percent of the shift in mean was upward with an average increase of 13 percent in the recent four decades (1959-1998) in comparison with the earlier four decades (1920-1958); 4) with regard to variance, 55 percent or 229 of 417 stations with records of at least 80 years (1919 or earlier - 1998) showed a significant increase in variance in the recent four decades (1959-1998) in comparison with the earlier 4-6 decades (1919 or earlier - 1958). More than 90 percent of the shift in variance was an increase (an average increase of 23 percent in standard deviation).

These findings suggest a linear trend (while not common, i.e., in 16 percent of the stations analyzed) , that exhibited a significant tendency to grow (80 percent) or to decrease (20 percent) steadily year to year over the last century in annual maximum precipitation in the Ohio River Basin. And there was a significant shift in mean or variance, or both, between the past four decades (1959 - 1998) and the previous 4-6 decades. The average shift was 13 percent for the mean and 23 percent for the standard deviation. This indicates that the annual maximum precipitation in the Ohio River Basin varied over a wider range - between extreme highs and extreme lows - for the recent four decades than the earlier period.

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