Ghana’s economy heavily relies on agriculture, which is predominantly rainfed across its agro-ecological zones. As a result of this, it is vulnerable to rainfall variability, which tends to have a major impact on the industrial and agricultural production sectors of the country. This study investigates the variations occurring across two major farming areas (Wenchi and Saltpond) within the Transition and Coastal Savannah agro-ecological zones of Ghana respectively. Rainfall variations are studied with rainfall data from 1968-2011 from Ghana Meteorological Agency (GMet). The rainfall analysis is done over two Climatological Periods (CP), 1968-1989 as CP1 and 1990-2011 as CP2. This study uses two climatic extreme indices as well as rainfall amounts and onset over these two agro-ecological zones to investigate the changes that have occurred in rainfall. The study found that in the Coastal agro-ecological zone, CP1 had a decreasing rainfall trend as compared to CP2 with higher variations in Saltpond. In the Transition agro-ecological zone, Wenchi, CP1 also exhibited a decreasing trend as compared to CP2. In addition, onset of rains in Saltpond mostly occurred in May for CP1 but for CP2, it oscillated between April and May. For Wenchi, onset of rains was in March for CP1 and predominantly April for CP2. In going forward, farmers in these agro-ecological zones should be supported to practice effective adaptation and mitigation measures so as to improve their yields in this challenging climate.
Rainfall has major influence on industrial and agricultural production in any agro-ecological zone in the world. Changes in the amount and pattern of rainfall over Ghana are therefore, a major concern. These changes may affect the onset and length of rainfall, which has implication for Ghana’s rain-fed agriculture. Ghana’s economy relies greatly on agriculture and it accounts for the rise in the Gross Domestic Product (GDP) of the country in most years. This sector also employs almost half of the economically active population [
Countries in Africa, especially those in West Africa are most prone to the impact of climate change [
The study reported that extreme events would very likely intensify and much more frequent. These increments may yield to an increase in extreme events, which may be destructive to both human and natural systems. However, it can be realized that whereas some places will experience an increase in their annual rainfall, others may experience a decrease, which may even lead to drought. Nicholson [
The impacts of climate change is manifesting through changes in extreme events such as droughts and floods. Such extremes can be detrimental to human life, and hamper economic development. Unfortunately, assessments of future climate change are often limited to mean temperature and precipitation. Less attention has been given to future changes in extremes particularly for Africa until recently, e.g. over southern Africa [
The variations observed in the amount and pattern of rainfall make it challenging for easy prediction, however a prior knowledge of the variations will help the farmer plan for possible maximization of crop yield. Information on the changes in onset or the trend of rainfall over a particular period is used to make decisions and policies on rainfall dependent sectors. The start of rains marks the start of the main farming season in Ghana. The rainfall season is usually considered to be the planting period for some of the main staple crops, such as maize, beans and potatoes in Ghana [
The paper aims to study the changes in the mean rainfall, extremes and onset dates within the transition and coastal savannah zones of Ghana for two climatic periods, 1968-1989 and 1990-2011. In the paper, analysis of rainfall intensity, consecutive wet days, consecutive dry days and onset dates over the two climatological periods are presented and discussed. This paper will inform farmers on the pattern and changes that are occurring in rainfall onset and amount in the Transition and Coastal Savannah agro-ecological zones of Ghana. It will also inform appropriate interventions for farming in the two agro-ecological zones to reduce the impact of the changes in the climate.
Ghana’s climate is classified as tropical and is characterized generally by two rainfall seasons. Saltpond is in the Coastal Savannah Zone, which is called Zone A according to the classification of Ghana Meteorological Agency (GMet). The zone experiences two rainfall seasons, the major season occurs between March and July and the minor season occurs around September to November. The annual total rainfall is about 700 to 800 mm. The vegetation along the coast is the coastal savannah with grassland and few trees.
Wenchi is in the Transition Zone, which is identified as Zone C according to the GMet. The zone experiences two rainy seasons: a minor (April to July) and a major (September to November). There is not much distinction between the amounts of rainfall in the two rainy seasons and so they appear as one continuous season [
The station data used in this analysis were obtained from the Ghana Meteorological Agency. The data used for this analysis represent two synoptic stations (Wenchi and Saltpond), which represent the Transition and Coastal Savannah zones of Ghana. It should be noted that other stations close-by to these two synoptic stations exhibited similar trends and as such one in each of the two agro-ecological zones was chosen for the analysis. The climatological variable used was daily rainfall data spanning from 1968 to 2011. The data for the selected stations were quality controlled using RClimdex which uses the R statistical package. Periods without data for all the climatological variables were set to
a missing value of −99.9. With this threshold setting, RClimdex identifies values that lie outside 4 standard deviations of the mean of the time-series. The quality control on the data was conducted to eliminate errors in the data. Errors emanating from manual keying and negative daily precipitation amount were corrected after cross checking from the original hard copy.
The data was analyzed over two climatic periods, 1968-1989 and 1990-2011, hereafter called CP1 and CP2 respectively. The climatological periods were chosen in order to have a past and current comparison of the available data to discuss the trend of rainfall over the past years. The periods chosen have the years with most complete data with less than 10% missing data. The intensity of rainfall and rainfall onset for each station was determined for CP1 and CP2 using the InStat-plus version 3.05 programming tool from GraphPad Software, San Diego California, USA.
The Consecutive Dry Days (CDDs) and Consecutive Wet Days (CWDs) were computed to study the daily characteristics of extreme events over the chosen climatological periods. CDD is defined as the count of number of consecutive days where daily rainfall amount is less than 1mm. CWD is defined as the count of number of days where daily rainfall amount is equal to or more than 1 mm. These rainfall characteristics will suggest possibilities of increased floods or drought spells occurrences for the CP1 and CP2.
Rainfall onsets in both the Transition (Wenchi) and Coastal Savannah (Saltpond) over CP1 and CP2 were determined using InStat-plus to study the differences that has occurred in recent years relative to the past. The definition for rainfall onset is borrowed from Raman (1974) who defined onset as the first occasion of 7-day total rainfall exceeding 25 mm and includes at least 4 rainy days.
Over the CP2 period, Saltpond recorded an overall increasing trend with high variation in the 1990s than in the 2000s (
Annual rainfall amount over Wenchi for CP1 (
decreasing trend from 1968 to 1989. Rainfall amount ranged from 1387 mm to about 1427 mm in the 1960s and range from 849 mm to 1726 mm in the 1970s and 1980s with most years recording 1150 mm. Annual variations in the 1960s and 1980s are higher than in the 1970s. Similarly for Wenchi in the CP2 period (
Consecutive Dry Days (CDD) shows an increasing trend over Saltpond (
Similarly over Wenchi, there is a significant evidence of increasing CDD (
For Consecutive Wet Days (CWD), a consistent decreasing trend is observed over both Saltpond and Wenchi during the two climatological periods (Figures 8-11). However during the CP1 (CP2) for both study areas, a non-significant decrease was observed with a p value of 0.18 >>> 0.05 (0.576 >>> 0.05) and 0.112 > 0.05 (0.643 >>> 0.05) for Saltpond and Wenchi respectively. This agrees as inversely proportional to the increasing trend of CDD and depicts the possibility of increasing droughts, which will be detrimental to farming practices.
For Saltpond, rainfall onset mostly occurred in May for CP1 but oscillated closely between April and May in CP2. For Wenchi, the table shows that the
CP1 Onset dates | CP2 Onset dates | ||||
---|---|---|---|---|---|
Years | Saltpond | Wenchi | Years | Saltpond | Wenchi |
1968 | 23-May | 2-Apr | 1990 | 16-Jun | 11-May |
1969 | 9-May | 5-Mar | 1991 | 30-Apr | 17-Apr |
1970 | 11-May | 11-Mar | 1992 | 8-May | 9-Apr |
1971 | 4-Jun | 6-Apr | 1993 | 20-Apr | 30-Apr |
1972 | 11-Jun | 30-Mar | 1994 | 25-May | 30-Mar |
1973 | 10-May | 28-Mar | 1995 | 11-Mar | 13-Apr |
1974 | 6-May | 11-Mar | 1996 | 11-Apr | 23-May |
1975 | 20-Mar | 6-May | 1997 | 26-Mar | 3-Apr |
1976 | 16-Apr | 20-Feb | 1998 | 10-May | 6-Apr |
1977 | 30-Apr | 11-May | 1999 | 3-Jun | 1-Apr |
1978 | 27-Apr | 5-Apr | 2000 | 21-May | 18-Apr |
1979 | 30-Apr | 16-Apr | 2001 | 20-Apr | 1-Apr |
1980 | 30-Apr | 11-Apr | 2002 | 20-Feb | 26-Mar |
1981 | 20-May | 31-Mar | 2003 | 13-May | 15-Apr |
1982 | 22-Apr | 1-Apr | 2004 | 14-Apr | 1-Feb |
1983 | 29-May | 7-Apr | 2005 | 21-May | 9-Apr |
1984 | 10-May | 11-Mar | 2006 | 13-May | 14-Feb |
1985 | 6-Jun | 4-Apr | 2007 | 14-Mar | 4-Jun |
1986 | 27-May | 1-Apr | 2008 | 5-Apr | 24-Apr |
1987 | 4-May | 15-Mar | 2009 | 18-Apr | 25-Mar |
1988 | 2-Jun | 2-Mar | 2010 | 28-Apr | 26-Apr |
1989 | 12-May | 18-Mar | 2011 | 20-May | 27-Apr |
rainfall onset mostly occurred in March in CP1 whereas in CP2 the onset of the rains was mainly in April.
Similarly, the onset of rainfall in both Saltpond and Wenchi in CP2 (
This study used rainfall data from Saltpond and Wenchi representing the Coastal Savannah and Transition Savannah agro-ecological Zones of Ghana. In the study, two climatological periods CP1 (1968-1989) and CP2 (1990-2011) were considered for the changes in the trend of rainfall. Annual rainfall amounts, the onset of rainfall and extreme daily characteristics were compared for the two periods.
We report in this paper that the average rainfall amount has decreased over both agro-ecological zones but there was an increasing trend in rainfall pattern at present than in the past. Both study areas recorded larger rainfall amounts in CP1 relative to CP2. Both ecological zones depicted similar trend of CDDs and CWDs for the periods considered. A significant decrease in CDD was observed in CP1 and a non-significant increase in CDD was observed in CP2 over Saltpond and Wenchi. A decreasing CWD, which was not significant over both periods was observed for both study areas but the decreasing trend depicts the possibility of droughts in the present years which will be detrimental to farming practices in this area.
The results also showed that the onset of rains, which should normally start in March in Saltpond, [
Indeed there have been notable changes in rainfall pattern in both the Transition and Coastal Savannah agro-ecological Zones with regard to developments after the 2000s. It is therefore imperative for farmers to be encouraged to undertake better field management practices in other to improve their yields since changes in rainfall pattern are known to overall have negative impact on yields. Effects such as the shortening of farming seasons and irregular planting dates will lead to food insecurity which in turn affects the country’s economy which relies heavily on these agro-ecological zones as its “bread-basket”. In going forward, other farming practices such as irrigation should be introduced to help the farmers in these agro-ecological zones beat the challenges of the changing climate. We also recommend the establishment of enough early warning systems so as to help farmers in these areas with first-hand information to enable them withstand the variations in the climate of their agro-ecological zone. It should be noted that these findings have consequences on rain-fed agricultural practices as well as the socio-economic growth and the food security of Ghana.
This research was completed at the Department of Physics, University of Cape Coast, Ghana under the PEER Science Project with the kind support of the USAID.
Quagraine, K.A., Klutse, N.A.B., Nkrumah, F., Adukpo, D.C. and Owusu, K. (2017) Changes in Rainfall Characteristics in Wenchi and Saltpond Farming Areas of Ghana. International Journal of Geosciences, 8, 305-317. https://doi.org/10.4236/ijg.2017.83015