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When the entire record for a given time scale was used for trend analysis, the trend (if significant) was determined to start either around 1994 or in another year (Table 2 and Figures 6(a) and 7(a)). This implies that the commonly used trend analysis methods, including the modified and sequential Mann–Kendall test techniques, cannot always differentiate an abrupt change point from trend start year. Alternatively, the approach proposed by this study and illustrated in Figure 2 can overcome this issue. The post-1994 downward trends started from a year between 1997 and 2001 (the last column of Table 2), depending on time scale. The post-1994 annual downward trend was determined to start from 1999 (Figure 6(b)), which was probably triggered by the downward trends of August–November that started in or before 1999 (Table 2). The downward trends of these four months also triggered the downward trends in summer (Figure 7(b)) and fall (Figure 8(a)). The downward trends in April and June started later, while the downward trend in May continued throughout the record year. The downward trend in spring (Figure 8(b)) started from 2000, which was likely triggered by the combined downward trends of April and May.
Table 2

Start years of the significant trends in Table 1 at a significance level of α = 0.05a

Entire recordc
Pre-1994
Post-1994
Time scaleSpecific scaleStart yearDue to the abrupt change?Start yearStart year
Annual Hydrologic yearb 1999 No – 1999 
Seasonal Spring 1993 Yes – 2000 
Summer 2000 No – 1999 
Fall 1999 No – 1999 
Monthly March – – – – 
April 1994 Yes – 2007 
May 1982 No – none 
June 1999 No – 2001 
July – – – – 
August 1999 No – 1998 
September 1994 Yes – 1999 
October 1994 Yes – 1999 
November 1994 Yes – 1997 
Entire recordc
Pre-1994
Post-1994
Time scaleSpecific scaleStart yearDue to the abrupt change?Start yearStart year
Annual Hydrologic yearb 1999 No – 1999 
Seasonal Spring 1993 Yes – 2000 
Summer 2000 No – 1999 
Fall 1999 No – 1999 
Monthly March – – – – 
April 1994 Yes – 2007 
May 1982 No – none 
June 1999 No – 2001 
July – – – – 
August 1999 No – 1998 
September 1994 Yes – 1999 
October 1994 Yes – 1999 
November 1994 Yes – 1997 

a‘–’ means insignificant trend.

bFrom 1 September to 31 August.

cCalendar years from 1959 to 2011.

Figure 6

Plots showing the sequential Mann–Kendall test statistics u(ptk) (Equation (11)) and u(rtk) (Equation (14)) of annual streamflow for the: (a) record year, and (b) hydrologic years from 1994 to 2011. Hydrologic year is defined as from 1 September to 31 August. The intersects between u(ptk) and u(rtk) curves indicate the start years of significant trends at a significance level of α = 0.05.

Figure 6

Plots showing the sequential Mann–Kendall test statistics u(ptk) (Equation (11)) and u(rtk) (Equation (14)) of annual streamflow for the: (a) record year, and (b) hydrologic years from 1994 to 2011. Hydrologic year is defined as from 1 September to 31 August. The intersects between u(ptk) and u(rtk) curves indicate the start years of significant trends at a significance level of α = 0.05.

Figure 7

Plots showing the sequential Mann–Kendall test statistics u(ptk) (Equation (11)) and u(rtk) (Equation (14)) of seasonal streamflow in summer for (a) record year and (b) hydrologic years from 1994 to 2011. Hydrologic year is defined as from 1 September to 31 August. The intersects between u(ptk) and u(rtk) curves indicate the start years of significant trends at a significance level of α = 0.05.

Figure 7

Plots showing the sequential Mann–Kendall test statistics u(ptk) (Equation (11)) and u(rtk) (Equation (14)) of seasonal streamflow in summer for (a) record year and (b) hydrologic years from 1994 to 2011. Hydrologic year is defined as from 1 September to 31 August. The intersects between u(ptk) and u(rtk) curves indicate the start years of significant trends at a significance level of α = 0.05.

Figure 8

Plots showing the sequential Mann–Kendall test statistics u(ptk) (Equation (11)) and u(rtk) (Equation (14)) of seasonal streamflow in (a) fall and (b) spring, during the hydrologic years from 1994 to 2011. Hydrologic year is defined as from 1 September to 31 August. The intersects between u(ptk) and u(rtk) curves indicate the start years of significant trends at a significance level of α = 0.05.

Figure 8

Plots showing the sequential Mann–Kendall test statistics u(ptk) (Equation (11)) and u(rtk) (Equation (14)) of seasonal streamflow in (a) fall and (b) spring, during the hydrologic years from 1994 to 2011. Hydrologic year is defined as from 1 September to 31 August. The intersects between u(ptk) and u(rtk) curves indicate the start years of significant trends at a significance level of α = 0.05.

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