Article Variabilities in Rainfall Onset, Cessation and Length of Rainy Season for the Various Agro-Ecological Zones of Ghana (original) (raw)
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Climate, 2015
This paper examines the onset and cessation dates of the rainy season over Ghana using rain gauge data from the Ghana Meteorological Agency (GMet) over the period of 1970-2012. The onset and cessation dates were determined from cumulative curves using the number of rainy days and rainfall amount. In addition, the inter-annual variability of the onset and cessation dates for each climatic zone was assessed using wavelet analysis. A clear distinction between the rainfall characteristics and the length of the rainy season in the various climatic zones is discussed. The forest and coastal zones in the south had their rainfall onset from the second and third dekads of March. The onset dates of the transition zone were from the second dekad of March to the third dekad of April. Late onset, which starts from the second dekad of April to the first dekad of May, was associated with the savannah zone. The rainfall cessation dates in the forest zone were in the third dekad of October to the first dekad of November, and the length of the rainy season was within 225-240 days. The cessation dates of the coastal zone were within the second and third dekad of October, and the length of rainy season was within 210-220 days. Furthermore, the transition zone had cessation dates in the second to third dekad of October, and the length of the rainy season was within 170-225 days. Lastly, the savannah zone had cessation dates within the third dekad of September to the first dekad of October, and the length of rainy season was within 140-180 days. The bias in the rainfall onset, cessation and length of the rainy season was less than 10 days across the entire country, and the root mean square error (RMSE) was in the range of 5-25 days. These findings demonstrate that the onset derived from the cumulative rainfall amount and the rainy days are in consistent agreement. The wavelet power spectrum and its significant peaks showed evidence of variability in the rainfall onset and cessation dates across the country. The coastal and forest zones showed 2-8-and 2-4-year band variability in the onsets and cessations, whereas the onset and cessation variability of the transition and savannah zones were within 2-4 and 4-8 years. This result has adverse effects on rain-fed agricultural practices, disease control, water resource management, socio-economic activities and food security in Ghana.
Assessment of Long-Term Spatio-Temporal Rainfall Variability over Ghana using Wavelet Analysis
Climate
Rainfall variability has strong impact on food security, livelihood and socioeconomic activities as farming in West Africa is mainly rain-fed. The annual, seasonal and decadal rainfall variability over Ghana has been studied and their periodicities analysed using wavelet analysis. A rainfall time series from 1901-2010 from the Global Precipitation Climatology Center (GPCC) was used in this analysis. It was observed that high mean annual rainfall totals ranging from 900-1900 mm are recorded over the entire country. In addition, very high totals between 1500-1900 mm are recorded at the SouthWestern part of the country whereas low totals (900-1200 mm) are recorded in the Savannah and East coast of the country. In general, a decreasing trend was observed for the annual rainfall over all the agro-ecological zones except for the coastal zone, where a slight increasing trend of 0.1600 mm per year was seen. The seasonal trend analysis revealed a significant decreasing trend at 0.01 significance level in all the agro-ecological zones except for the Savannah during the DJF season indicating an intensification of the Harmattan. The Coastal zone recorded the lowest mean rainfall values for all seasons with the highest of about 150 mm in MAM. The Forest zone on the other hand recorded very high rainfall values for all seasons with the maximum of about 200 mm in JJA. The Transition zone, however, recorded almost quite stable rainfall amount for all seasons except for DJF. On the decadal time scale, below normal rainfall values were observed between the 1901-1920 and 1980-2010 periods for almost all the agro-ecological zones except for the Savannah which showed above normal rainfall values within the 1901-1940 period. Indicating that, the decreasing trend observed in recent years is not solely due to antropogenic factors but have a strong contribution from a natural climate variability. The wavelet analysis also revealed a strong annual periodicity over all the agro-ecological zones except for the Coastal and Forest zones where the annual periodicity was accompanied by 4-8 months signal. The results of both the 5 year moving average and the decadal anomaly confirm a significant decrease in rainfall amount. This will have negative consequences on agricultural practices, water resource management and food security.
Comparison of Rainy Season Onset, Cessation and Duration for Ghana from RegCM4 and GMet Datasets
2016
The socioeconomic sector of West African countries is rain-fed agriculture driven. Information regarding the onset, cessation and duration of the rainy season is thus, very essential. In this paper, a comparison of the onset, cessation and duration of the rainy season has been carried out using simulated rainfall data from the fourth generation Regional Climate Model (RegCM4) and rain gauge measurements from Ghana Meteorological Agency (GMet), covering a period of 1998 to 2012. Similar onset and cessation dates were seen in both the simulated and guage rainfall measurements for the various agro-ecological zones, resulting in similar duration of the rainy season. The average duration of the rainy season were less than 200 days for the savannah and coastal zones whereas the duration of the rainy season were beyond 200 days for the forest and transition zones. The bias of these comparisons was less than 30 days and the root mean square error (RMSE) values were less than 15 days for all stations, except Saltpond. The Pearson's correlation (r) typically ranged between 0.4 and 0.8. However, negative correlations were observed for Tamale in the savannah zone, and the entire coastal zone. These findings are indications that RegCM4 has the potential to clearly simulate the movement of the rain belt, and thus, could fairly determine the onset, cessation and duration of the rainy season. The findings have significant contributions to effective water resource management and food security in Ghana, as the thriving of these sectors depend on the dynamics of the rainfall seasons.
Spatiotemporal dynamics of rainfall in Upper East Region of Ghana, West Africa, 1981–2016
SN Applied Sciences, 2020
There has been an increasing concern about effects of global warming on rainfall. Negative impacts on rainfall affect the environment and socioeconomic activities of nations, globally. Rainfall characteristics at the Upper East Region (UER) of Ghana in terms of temporal and spatial variability are investigated from 1981 to 2016 using rainfall data from Ghana Meteorological Agency (GMet) and Climate Hazards Group Infrared Precipitation with Station. Using cumulative residual analysis, Mann-Kendall (MK), Sen's slope, wavelet transform (WT) and principal component analysis (PCA) tests, the rainfall variability in UER was examined. Results showed that 1981, 1999, 2002 and 2013 were years of major changes in rainfall variation. The MK and Sen's slope showed that 55.71% of UER have decreasing monthly rainfall with 27.34% significant trend. Months April, May and June showed decreasing rainfall trends. Months July, August and September showed significant increasing rainfall trend. WT revealed a significant variation in the annual rainfall. PCA revealed that the spatial variability of rainfall in UER is very diverse with 33.76% of the variability located in the northeastern part. The findings serve as benchmark in providing in-depth understanding of rainfall variation in UER for water resource managers, agriculturalists and drought mitigation.
The Spatio-Temporal Variability of Rainfall over the Agro-Ecological Zones of Ghana
Rainfall variability plays an important role in many socioeconomic activities such as food security, livelihood and farming in Ghana. Rainfall impact studies are thus very crucial for proper management of these key sectors of the country. This paper examines the seasonal and annual rainfall variability in the four agro-ecological zones of Ghana from the CHIRPS V2 rainfall time series spanning a period of 1981-2015. The rainfall indices were computed with the aid of the FClimDex package whereas the trends of these indices were further tested using the Mann Kendall trend test. The results show good agreement (r ≥ 0.7) between CHIRPS V2 and gauge in almost all portions of country although high biases were observed especially in DJF season over parts of the Northeastern (NE) portions of the country. The mean seasonal rainfall climatology over the country is observed to be in the range of 20-80 mm, 60-200 mm, 100-220 mm and 40-180 mm in DJF, MAM, JJA and SON seasons respectively with high intensities of rainfall dominating Southwestern portions of the country. The trend analysis revealed positive trends of consecutive dry days in the Transition, Forest and Coastal zones and negative trends in the Savannah zone of the country. Decreasing trends of consecutive wet days are observed over the Savannah, Transition and Coastal zones whereas increasing trends dominate the Forest zone. Savannah, Forest and Transition zones show weak increasing trends of the number of heavy rainfall days whilst weak decreasing trends are observed over the Coastal zone of the country. Similarly, weak increasing trends of the number of very heavy rainfall days are observed over all the agro-ecological zones except in the Transition zone. It is observed that the annual wet day rainfall total has increasing trend in the Savannah and Forest zones of the country whereas decreasing trends cover the remainder of the zones. The trends of these indices in the agro-ecological zones were all significant at a significant value of 0.05. This paper assessed the performance of the CHIRPS V2 rainfall data over the region
Changes in expectations and extremes in the rainfall climatology of Accra, Ghana, 1895–2005
Applied Geography, 2014
This paper re-examines one of the longest continuous monthly rainfall records in West Africa (1895 e2005) for evidence of longer-term periodic behavior of annual and seasonal rainfall totals through the use of a sliding, rather than discrete, thirty year window. The approach provides greater flexibility in defining periods of shifting behavior. The temporally changing probabilities of experiencing rains considered "normal", "above normal" and "below normal", as defined by terciles of the entire 111 year record, are investigated. Simple time series plots of 30 year running means and standard deviations indicate non-stationarity, particularly in the mean. Based on these observations and wavelet analysis, three major eras are defined and properties within each considered. Within each era, lag cross correlations with global sea surface temperatures, shed light on the changing dominance of major causes of interannual variability of seasonal rains, El Niño-Southern Oscillation (ENSO) and tropical Atlantic sea surface temperatures (SST). Effects of ENSO are most pronounced at the height of the major rains (May June), which are twice as large on average in cold phases of ENSO than warm phases during 1895 e1920, and about 30% higher in the most recent period. Rains during the short dry spell (July August) and minor rainy (September October) seasons appear to be controlled by SSTs in the Gulf of Guinea and to a lesser extent the equatorial Atlantic.
Theoretical and Applied Climatology, 2020
Improved information on the distribution of seasonal rainfall is important for crop production in Ghana. The predictability of key agro-meteorological indices, namely, seasonal rainfall, maximum dry spell length (MDSL) and dry spell frequency (DSF) was investigated across Ghana (with an interest on the coastal savannah agro-ecological zone). These three variables are relevant for local agricultural water management. A dynamical model (i.e. European Centre for Medium-Range Weather Forecasts (ECMWF) System 4 seasonal forecasts) and a statistical model (i.e. response to sea surface temperatures (SSTs)) were used and analysed using correlation and other discrimination skill metrics. ECMWF-System 4 was bias-corrected and verified with 14 local stations' observations. Results show that differences in variability and skills of the agro-meteorological indices are small between agro-ecological zones as compared to the differences between stations. The dynamic model System 4 explains up to 31% of the variability of the MDSL and seasonal rainfall indices. Coastal savannah exhibits the highest level of discrimination skills. However, these skills are generally higher for the below and above normal MDSL and seasonal rainfall categories at lead time 0. Similarity in skills for the agro-meteorological indices over the same zones and stations is found both for the dynamical and statistical models. Although System 4 performs slightly better than the statistical model, especially, for dry spell length and seasonal rainfall. For dry spell frequency and longer lead time dry spell length, the statistical model tends to perform better. These results suggest that the agro-meteorological indices derived from System 4′ updated versions, corrected with local observations, together with the response to SST information, can potentially support decision-making of local smallholder farmers in Ghana.
Water
Drought and wetness events have become common due to global warming, warranting the need for continuous analysis and monitoring of drought and wet events to safeguard people’s livelihoods. In this study, the Standardized Precipitation Evapotranspiration Index (SPEI) was utilized to analyze the spatiotemporal characteristics of drought and wetness events in the coastal Savannah agroecological zone from 1981 to 2021. Climate data from 14 locations across the zone were used to characterize drought and wetness events at the 3 and 12 month timescales. Except for September 1995 and November 2002, when changepoints occurred, the results revealed the homogeneous nature of temperature and rainfall in the zone. More drought events were observed in the dry and minor seasons, while the wet season had more wetness events under both the SPEI-3 and SPEI-12 timescales. The results also showed that, while moderate-to-severe drought events were common for most years, extreme drought events were more ...
Atmosphere
Rainfall onset and cessation date greatly influence cropping calendar decisions in rain-fed agricultural systems. This paper examined trends of onsets, cessation, and the length of growing season over Northern Ghana using CHIRPS-v2, gauge, and farmers’ perceptions data between 1981 and 2019. Results from CHIRPS-v2 revealed that the three seasonal rainfall indices have substantial latitudinal variability. Significant late and early onsets were observed at the West and East of 1.5° W longitude, respectively. Significant late cessations and longer growing periods occurred across Northern Ghana. The ability of farmers’ perceptions and CHIRPS-v2 to capture rainfall onsets are time and location-dependent. A total of 71% of farmers rely on traditional knowledge to forecast rainfall onsets. Adaptation measures applied were not always consistent with the rainfall seasonality. More investment in modern climate information services is required to complement the existing local knowledge of fore...
The role of agriculture cannot be over emphasized in the lives of the people of the guinea savannah. Their largely subsistent cultivation on small farm holdings is completely dependent on rainfall. Three rainfall characteristics of onset, cessation and length of rainy season were analysed in this study. Monthly rainfall data spanning a period of 21 years (1998-2017) was obtained for three locations (Plateau, Nasarawa and Benue) from the head office of the Nigerian Meteorological Office (Nimets). Walter (1967) formula for calculation of onset and cessation was adopted. The least square regression model was further used to establish the mean and direction of change for the studied variables. Findings show that in spite of the variability in onset and cessation dates for the period and location, the regression technique established 7 th , 20 th , and 10 th April as onset dates for Plateau, Nasarawa and Benue states respectively. Interestingly, mean cessation date for all locations fell on the 16 th of October. The LRS showed a marked variability with locations. The implications of these findings were discussed and the research concluded that in view of the changing pattern observed and linked to climate change, there is the need for government to introduce hybrid varieties that are quick yielding and low water dependent.