Ichetucknee River Flow 1929 – 2010: Statistical TrendBin Gao, Kathleen McKee, Wendy Graham

Fig. 3 Flow (cfs) at US 27 on the Ichetucknee River from 1929 to 2010 showing piecewise trends with a break at 1966. (A) First period: Linear Model p-value = 0.153, slope = 1.288 cfs/year

Mann-K p-value = 0.191, slope = 1.142 cfs/year(B) 2nd Period: Linear Model p-value = 0.0025, slope = -2.264 cfs/year

Mann-K p-value = 0.0014, slope = -2.492 cfs/year

IntroductionFlows in the springs and river of the Ichetucknee system are important for ecosystem function and there is much discussion of those flows decreasing over time. The only long term site for flow is in the river at the US 27 bridge downstream from the nine named springs. We thought it was important to statistically test whether discharge in the Ichetucknee river was going down over time so we performed monotonic and piecewise statistical trend analyses of available flow data.

MethodsAnnual Data Estimates: Instantaneous flow data from the Ichetucknee river at US 27 provided by Megan Wetherington (SRWMD) was averaged on a calendar year basis. From 1931 to 1942 and from 1998 to 2009 there were nearly monthly values (Fig. 1). Most years had bi-monthly or quarterly values. Between 1976 and 1995, there were only one or 2 values per year. Because there is little difference in monthly flow statistics (Fig. 2), we felt that one or 2 measures per year may adequate to estimate the annual average flow. No measures were recorded in 1988, 1992, 1993, 1994, 1996. A value for 1993 was found in USGS field measurements. For the purposes of this analysis, other missing year averages were interpolated from previous and subsequent years.

Trend Analysis: S-plus statistical software and Extraction Management Tool (EMT) written by the SJRWMD were used to conduct monotonic and piecewise trend analysis on the time series. No significant monotonic trend was detected over the whole time series. A graphical evaluation with a locally weighted scatter plot smoothing (LOWESS) was used to determine whether the time series had a piecewise trend. The LOWESS analysis showed a break at 1966 so two time series were tested for trends using the Mann-Kendall test on residuals of each time series transformed using an autocorrelation filtration. Statistical significance of trends were determined based on the p-value and the slope using Linear Model and the Mann Kendall tests.

ResultsNo significant trend was detected over the whole time series (p-value=0.067). Piecewise trend analysis showed a significant downward trend in the flow data since 1966. (Linear Model p-value = 0.0025 and Mann Kendall p value = 0.0014) (Fig. 3). However it should be noted that this is the period where significantly fewer measurements were available to estimate annual average flow values (Fig. 2) . No statistically significant trend was detected before 1966 (p value =0.191).

Fig. 1 Flow (cfs) at US 27 on the Ichetucknee River from 1929 to 2010 showing number of measures per year (black) and interpolated values (red).

Fig. 2 Flow (cfs) per calendar month at US 27 on the Ichetucknee River from 1898 to 2010 showing average, maximum, minimum and standard deviation, illustrating little variability in flow statistics among months.

(A) (B)

cfs

February 16, 2011