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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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This chapter presents and discusses results of the research based on the primary and secondary data collected. The results presented in this chapter have been arranged in sections which include finding on trend in rainfall and river discharge of the study area, determinants of vulnerability and Flood Vulnerability index and mapping of vulnerable areas results.

4.1. Empirical Finding on Trend and Variability Analysis

4.1.1. Precipitation Time Series Analysis

Statistical properties of the annual and monthly rainfall series were tested and presented in "Annexe 1". The result shows that April, May, June and October represent the smallest Coefficient of Variation (CV): 0.486, 0.388, 0.353, and 0.439, respectively which means that they were the homogenous months in terms of rainfall variations during the period of record. On the other hand, December, January and February show the largest CV with 2.122, 1.586 and 1.06, respectively. The rest of the months present similar rainfall pattern representing similar variation during the study period. The annual maximum rainfall occurred in the year 1999 with the total precipitation of around 1341.5 mm approximately and the minimum rainfall occurred in the year 1977 with the total of around 674 mm.

On running the Mann-Kendall test on precipitation data, the Sen's slope shows an evidence of a positive trend in annual series. The rate of annual rainfall change is about 3.434 mm/year. The result indicates that the null hypothesis was accepted for the annual rainfall trend (p-value= 0.159). Thus, statistically significant positive trend is not found for annual rainfall over the time period.

On plotting the linear trend line for the 40 years rainfall data, the following results in "Figure 7" were obtained.

Annual rainfall-plot

y = 3.434x + 943.6
R² = 0.053

Rainfall (mm)

Linear fit

Figure 6: Linear trend line corresponding to rainfall data (1971-2010:Tabligbo station)

The "figure 7" represents the graph for the twelve (12) months average rainfall for the time period (1971-2010). It shows two peaks in the year one in June (159.647mm) and another one in October (130.06 mm) which reveals the bimodal pattern of rainfall in the study area.

Monthly Average Rainfall

Figure 7: Monthly Average Rainfall (1971-2010)

In the Mann-Kendall test, the Sen's slope estimator reveals the trend of the series for 40 years for individual 12 months from January to December which are -0.073, -0.560, -0.069, 0.353, 0.285, 1.134, 0.980, 0.153, -0.480, 1.307, 0.699, and -0.191, respectively. For April, May, June, July, August, October and November, there is an evidence of a rising trend while the result is displaying negative trend in January, February, March, September and December. Thus Sen's slope estimator shows a positive trend for eight months and for other four months it shows a negative one representing almost non-significant condition. The Null hypothesis was accepted for all the twelve (12) months "annexe 2". Therefore, statistically significant trends are not found for precipitation on monthly basis, at 95 % confidence level, even though there are negative and positive trends for the record of period (1971-2010) considered.

Annual Rainfall Cumulative Deviation

Figure 8: Annual rainfall cumulative deviation (1971-2010) (Tabligbo station)

To be able to determine normal, wet and dry years, cumulative deviation from mean of rainfall pattern were computed for the periods of record. "The figure 8" reveals that a cyclic pattern of variations with alternating drier and wetter years is suggested. Two main phases of different lengths were detected (1971-1977 and 1978-2010). The first phase covering eight (8) years shows six negatives anomalies that let to conclude a dry phase. It is followed by a long period from 1978-2010 characterized by variations with alternating drier and wetter years. This result explains rainfall variability over the study area during the period under examination.

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