Investigating the variation of Intra-seasonal rainfall characteristics in Sierra Leone
Abstract
The rainy season in Sierra Leone is unimodal from April to October; an understanding of the
inter-annual variability of rainfall in Sierra Leone is of importance to economic sectors such as
fisheries, agriculture, infrastructure, hydro-electric power generation (HEP) and water resources.
This study investigated the variation of the intra-seasonal rainfall characteristics in Sierra Leone.
Daily rainfall data from 4 synoptic stations was obtained from the Sierra Leone Meteorological
Department from 1990-2014.
The relationship between onsets, cessation, and duration to seasonal totals was determined using
Pearson correlation, variability of intra-seasonal rainfall characteristics was determined using
coefficient of variation. The magnitudes of the intra-seasonal rainfall characteristics were
obtained from the slopes of linear regression lines and the statistical significance was obtained by
non parametric method (Mann-Kendall).
The results from the study showed that the onset of the rainy season starts early and ends late in
the Freetown and Lungi stations resulting to longer durations. The seasonal total of rainfall was
also higher at Freetown ranging from 3000-3500mm. On the other hand, at BO station the rainy
season starts a bit late and ends late while at Daru, the onset start late and ends early indicate a
short duration with seasonal totals of rainfall ranging from 1500-2500mm. The observed
phenomenon can be attributed to the fact that Freetown and Lungi are near the coastline hence
there is enough source of moisture, while BO and Daru are further inlands hence somehow dry.
The results for the correlation analysis indicated negative and significant coefficients for the
onsets at Daru. Implying that early onsets results to higher amounts of rainfall and late onsets
results to lower amount of rainfall. Both BO and Daru stations depicted positive and significant
correlation coefficients between duration of the rains and seasonal rainfall totals. This suggests
that as the duration increases the seasonal rainfall amounts increases and vice versa.
The spread of onset dates at BO is considerably larger compared to the other three stations hence
it is variability is slightly higher (12.4%) compared to the three other stations. The onsets are
skewed to the right that is most of the onsets occurs much later. At Freetown the variability of
cessation is considerably larger compared to the other three stations. The cessation dates are
skewed to the right that is most of the cessation dates occur much later. The spread of duration at
Daru is considerably lower with a variability of 4.4%. The variability of seasonal rainfall at
Freetown is slightly higher than the other three stations, with a variability of 9.9%. It is also
further noted that the variability of the intra-seasonal rainfall characteristics at BO station was
slightly higher as compared to the other stations though not significant.
The observed trends for onset and cessation dates, as well as duration of the season were not
statistically significant except for cessation dates at BO station. The seasonal rainfall totals
depicted insignificant decreasing trends for all the stations except Freetown station which
indicated an insignificant increasing trend.
The results from this study will be useful in addressing the problems associated with late onset
and early cessation of rains which has a detrimental effect to farmers in the area of agriculture in
terms of food security in the country and also combating the problem of water shortage in
industries, dams, reduced stream flow in rivers and watercourses and many more among others.
Publisher
University of Nairobi
Rights
Attribution-NonCommercial-NoDerivs 3.0 United StatesUsage Rights
http://creativecommons.org/licenses/by-nc-nd/3.0/us/Collections
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