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Flood vulnerability assessment of donstream area in Mono basin in Yoto district, south-eastern Togo

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par Abravi Essenam KISSI
University of Lome - Master 2014
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3.3.3.1. Trend Analysis of Rainfall And River Discharge to Assess Climate Change.

This study examines trend in river discharge and rainfall in the downstream part of the Mono basin, using Mann-Kendall statistic test. Athieme flow gauging station (downstream) and Tabligbo rainfall gauging station were selected "Table 4". Each station had a long record of 40 years (1971-2010) of data to determine whether or not there have been any significant changes in those variables over the downstream part of the river basin using Mann-Kendall test run at 5% significance level on time series data. Available monthly rainfall and daily river discharge data were first grouped into monthly, seasonal and annual average data. Missing data were filled through linear interpolation of the same months data of the contiguous years on either side of the missing value.

Table 4: Reference of meteorological and hydrological stations

Stations

Latitude

Longitude

Altitude

Creation date

Data period

Tabligbo

06°30' N

01°37' E

70 m

1937

1971-2010

Athieme

06°34'44'' N

01°39'53 E

8.2 m

1944 

1971-2010

- Mann-Kendall Test

Mann-Kendall test was formulated by Mann (1945) as non-parametric test for trend detection and the test statistic distribution was given by Kendall (1975) for testing non-linear trend and turning point. This test, is widely employed in various studies to ascertain the presence of statistically significant trend in hydrologic and climatic variables with reference to climate change (Yu et al.1993; Douglas et al. 2000; Hess et al.2001; Burn and Elnur 2002; Yue et al. 2003; Burn et al.2004; De Toffol et al., 2008; Singh et al. 2008). There are two advantages of using this test. First, it is a non-parametric test and does not require the data to be normally distributed. Second, the test has low sensitivity to abrupt breaks due to inhomogeneous time series. According to this test, the null hypothesis H0 assumes that there is no trend and under the alternate hypothesis, it is assumed that a significant change has occurred over time, or that an increasing or decreasing trend is evident in the time series.

In this study, trend analysis has been done by using non-parametric Man-Kendall test together with the Sen's Slope Estimator (Qi) for the determination of trend and slope magnitude to find out the annual and monthly variability of rainfall and discharge data over the Mono basin.

The null hypothesis is tested at 95% confidence level for both rainfall and discharge data. If the p value is less than the significance level á (alpha) = 0.05, H0 is rejected. Rejecting H0 indicates that there is a trend in the time series, while accepting H0 indicates no trend was obtained.

Positive value of Qi indicates an upward or increasing trend and a negative value of Qi gives a downward or decreasing trend in the time series. Statistical Mann-Kendall test and Sen's Slope Estimator Test were performed, using Addinsoft's XLSTAT 2014 software.

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