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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 = 0.05^a

		Entire record^c		Pre-1994	Post-1994
Time scale	Specific scale	Start year	Due to the abrupt change?	Start year	Start year
Annual	Hydrologic year^b	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 record^c		Pre-1994	Post-1994
Time scale	Specific scale	Start year	Due to the abrupt change?	Start year	Start year
Annual	Hydrologic year^b	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.

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Figure 6

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Plots showing the sequential Mann–Kendall test statistics u(pt_k) (Equation (11)) and u(rt_k) (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(pt_k) and u(rt_k) curves indicate the start years of significant trends at a significance level of α = 0.05.

Figure 7

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Plots showing the sequential Mann–Kendall test statistics u(pt_k) (Equation (11)) and u(rt_k) (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(pt_k) and u(rt_k) curves indicate the start years of significant trends at a significance level of α = 0.05.

Figure 8

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Plots showing the sequential Mann–Kendall test statistics u(pt_k) (Equation (11)) and u(rt_k) (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(pt_k) and u(rt_k) curves indicate the start years of significant trends at a significance level of α = 0.05.

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