Changes in length of rainy season and rainfall extremes under moderate greenhouse gas emission scenario in the Vea catchment, Ghana


 The economic implications of extreme climate changes are found to impact sub-Saharan Africa negatively. This study aimed to analyze projected changes in length of rainy season (LRS), and rainfall extreme indices at the Vea catchment, Ghana. The analysis was performed using high-resolution simulated rainfall data from Weather Research and Forecasting (WRF) model under moderate greenhouse gas emission scenario for the period 2020–2049 relative to 1981–2010 period. LRS was computed from the difference between rainfall onset and cessation dates, and its trends were assessed using Mann–Kendall test and Sen's slope estimator. Annual rainfall intensity and frequency indices were computed. Results showed an increase in mean LRS from 168 to 177 days, which was at a rate of 1 day/year in the future (2020–2049). The LRS increase would be more significant at northern and south-western parts of the catchment. Rainfall intensity and frequency indices are projected to increase at spatial scale across the catchment. Projected changes in rainfall extremes could increase the frequency and intensity of drought and flood events. Thus, it is necessary to integrate suitable climate change adaptation measures such as rainwater harvesting, flood control measures, and development of early warning systems in the planning process by decision-makers at the catchment.


INTRODUCTION
The IPCC report reveals that extreme weather events will continue to increase in many parts of the world (Reay et al. ). For instance, in Africa, by the end of the year 2020, it was projected that between 75 million and 250 million people would be affected by increasing water stress as a consequence of climate change and climate variability extremes (IPCC ). With high confidence, the IPCC reports climate change to increase stress on water resources in Africa (IPCC ). As demand for water resources increases, the lives and livelihoods of many people in Africa will be adversely affected. Comparing Africa to the other continents in terms of vulnerability to climate change extremes, the IPCC () notes that 'Africa is one of the most vulnerable continents because of multiple stresses and low adaptive capacity'. Water resources in Africa have been subjected to the highest hydro-climatic variability over space and time relative to other continents (IPCC ).
This has negative implications on the continent's continuous economic development. Adaptation strategies in Africa have been noted to be insufficient considering the projected changes in climate over the region. The impact of climate change over the African continent varies.
In West Africa, annual rainfall has been revealed to be on the decrease since the 1960s with a recorded decrease of 20 to 40% around the 1990s (IPCC ). The decrease in annual rainfall has caused an estimated 25-35 km southward shift of the 'Sahelian, Sudanese and Guinean ecological zones' with more projected impacts by 2030 (IPCC ). As the IPCC () puts it, 'seventeen countries in West Africa share 25 trans-boundary rivers and many countries within the region have a water-dependency ratio of around 90%'. The 90% recorded water-dependency ratio in West Africa is the highest on the African continent. This justifies the selection of West Africa for this study.
Additionally, research into the spatiotemporal pattern of the length of the rainy season and rainfall extremes, for instance, can aid in the development of country-specific adaptation and resilient mechanisms to be put in place relative to situations where these climatic extremes are not predicted (Willems et al.   This is in support of the IPCC () recommendation that 'strategies that integrate land and water management, and disaster risk reduction, within a framework of emerging climate change risks, would bolster resilient development in the face of projected impacts of climate change'. The aim of this study was, therefore, to analyze the projected changes in the length of the rainy season, and rainfall extreme indices (frequency and intensity) in the Vea catchment in the UER of Ghana using high-resolution datasets.

Study area
The Vea catchment is one of the sub-catchments within the White Volta Basin in Ghana and is located between latitudes 10 30 0 N-11 08 0 N and longitudes 1 15 0 W-0 50 0 E  In this study, six of the rainfall indices categorized into intensity and frequency indices were adopted and computed using RClimdex (Larbi et al. ). These indices were selected due to their direct impact on water resources and agriculture (Hadri et al. ). Table 1 Table 2).

Annual changes in extreme rainfall
The projected changes and time series of the intensity (PRCPTOT, RX5day, and R95p) and frequency (CWD, CDD, and R20 mm) rainfall indices in the Vea catchment are shown in Table 3 and Figure 4, respectively. All three intensity indices are projected to increase in the near future relative to the historical period (Table 3). The mean annual total wet-day precipitation (PRCPTOT) is projected to increase by 40.2 mm, while annual maximum consecutive 5-day rainfall (RX5day) and very wet days (R95p) are projected to increase by 30.6 mm and 86.1 mm, respectively.
The maximum number of consecutive wet days (CWD) and consecutive dry days (CDD) would increase in the near future by 18 and 41 days, respectively. On the other hand, the annual mean value for the number of heavy rainfall days (R20 mm) is projected to decrease by 2 days in the catchment (Table 3).   The projected increase in the CDD in the northern and southern parts of the Vea catchment suggests a dry ten-  The LRS and rainfall extreme assessment in this study was only based on one regional climate model, which when compared to multi-model mean will have high uncertainty in the future projection, making it a limitation of this study. Further studies need to be conducted in this field using multi-model mean and also under extreme climate change scenario (i.e., RCP8.5 scenario).