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The results of our analysis indicate statistically significant negative trends in annual precipitation in the Nyong basin (Table 4). Pettitt's test shows a major rupture in the series of annual precipitation in 1975–1976 (Figure 4). Compared to the whole period average, the deficits associated with this rupture is –2.2% (Figure 4). A drop in precipitation started during the 1970s and remained uninterrupted until the 2000s (Table 5).

Table 4

Results of annual and seasonal rainfall, discharges and runoff coefficient analysis using Mann–Kendall's test

Variables	Annual		Spring		Summer		Autumn		Winter
Variables	p-value	Evolution	p-value	Evolution	p-value	Evolution	p-value	Evolution	p-value	Evolution
Rainfall	0.03547	–	0.03877	–	0.32	+	0.08	–	0.147	–
Discharges	0.3782	–	0.008722	–	0.4246	–	0.8688	–	0.8418	–
Ke	0.6645	+	0.01127	–	0.1909	–	0.4488	+	0.3018	–

Variables	Annual		Spring		Summer		Autumn		Winter
Variables	p-value	Evolution	p-value	Evolution	p-value	Evolution	p-value	Evolution	p-value	Evolution
Rainfall	0.03547	–	0.03877	–	0.32	+	0.08	–	0.147	–
Discharges	0.3782	–	0.008722	–	0.4246	–	0.8688	–	0.8418	–
Ke	0.6645	+	0.01127	–	0.1909	–	0.4488	+	0.3018	–

Note: The ‘ + ’ and ‘–’ signs indicate positive and negative trends, respectively. Calculated p-values below the significance level (α = 0.05) indicate that there is a significant trend in the series.

Ke, runoff coefficient.

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Table 5

Deviations (%) of the decadal annual and seasonal averages of precipitation, discharges and runoff coefficients compared to their interannual mean scores in the studied basin

Variables	Decades	Annual	Spring	Summer	Autumn	Winter
Precipitation	1950	2.5	7.3	−21.5	4	14.1
	1960	3.8	2.5	−2.2	4	27.3
	1970	0.9	0.1	2.8	−0.2	8.9
	1980	−2.7	−3.9	11.8	−3.4	−27.0
	1990	−2.7	−6.5	11.4	−1.6	−19.4
	2000	1	0.1	2	1.2	1.9
	2010	−3.0	0.4	−6	−4.8	−9.1
Discharges	1950	−5	9	−0.5	−14.4	−4.3
	1960	17.6	25.9	20.4	12.7	15.9
	1970	−4.2	−1	−14.2	0.5	−12.1
	1980	1.5	−4.5	−9.3	8.4	−1.5
	1990	−	−	−	−	−
	2000	−7.4	−15.6	−1	−4.4	−10.9
	2010	−2.8	−12.3	−2.1	−5.6	9.8
Ke	1950	−9.9	8.3	46.1	−31.2	−13.0
	1960	7.2	24	12.7	5.2	6
	1970	−7.2	−3.2	−18.8	16.5	−11.2
	1980	14	6.9	−32.3	32.3	17
	1990	−	−	−	−	−
	2000	−13.8	−16.1	−11.2	−10.5	−13.9
	2010	9.9	−14.0	7.6	−8.4	18

Variables	Decades	Annual	Spring	Summer	Autumn	Winter
Precipitation	1950	2.5	7.3	−21.5	4	14.1
	1960	3.8	2.5	−2.2	4	27.3
	1970	0.9	0.1	2.8	−0.2	8.9
	1980	−2.7	−3.9	11.8	−3.4	−27.0
	1990	−2.7	−6.5	11.4	−1.6	−19.4
	2000	1	0.1	2	1.2	1.9
	2010	−3.0	0.4	−6	−4.8	−9.1
Discharges	1950	−5	9	−0.5	−14.4	−4.3
	1960	17.6	25.9	20.4	12.7	15.9
	1970	−4.2	−1	−14.2	0.5	−12.1
	1980	1.5	−4.5	−9.3	8.4	−1.5
	1990	−	−	−	−	−
	2000	−7.4	−15.6	−1	−4.4	−10.9
	2010	−2.8	−12.3	−2.1	−5.6	9.8
Ke	1950	−9.9	8.3	46.1	−31.2	−13.0
	1960	7.2	24	12.7	5.2	6
	1970	−7.2	−3.2	−18.8	16.5	−11.2
	1980	14	6.9	−32.3	32.3	17
	1990	−	−	−	−	−
	2000	−13.8	−16.1	−11.2	−10.5	−13.9
	2010	9.9	−14.0	7.6	−8.4	18

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Figure 4

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Evolution of rainfall, discharges and runoff coefficients of the Nyong catchment at annual and seasonal time steps, according to the Pettitt test. The continuous horizontal lines in orange colour indicate the interannual averages. Decreases and surpluses are calculated compared to the whole period average. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/hydro.2023.116.

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