Ichetucknee River Flow 1929 – 2010: Statistical Trend
Bin Gao, Kathleen McKee, Wendy Graham
Introduction
Flows 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.
cfs
Methods
Annual 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.
Results
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).
No 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).
(A)
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.
(B)
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
February 16, 2011