**1. Introduction**

Many regions worldwide have experienced modifications in their hydrological regimes due to changing global climate [1]. The changing climate significantly impacts the availability and quantity of water in individual river basins and, on a global scale, has long been an international concern [2]. Freshwater availability in West Africa is paramount to economic development and social well-being [3]. In most West African countries, agriculture is the crucial sector supporting about 60% of the region's population [4]. Agriculture production in Nigeria and other West African countries depends majorly on water availability. Most importantly, wetlands watersheds contribute in diverse ways to the livelihoods of millions of people in this region [5, 6]. However, due to climate change, West Africa is already experiencing sea-level rise with severe coastline erosion, increased temperatures, unpredictable rainfall, dwindling water resources and many more. Accordingly, it is critical to examine future consequences of climate change on river discharge seasonality for a more explicit and quantitative understanding of the available water for various functions. Knowledge about river discharge seasonality is necessary for understanding important interannual hydrological dynamics [7].

There exist intricate interactions between streamflow and other climate variables (e.g. precipitation, temperature, and evapotranspiration) in a catchment [8, 9]. Therefore, the effects of climate change on streamflow across a given region are often assessed using hydrological models forced with global climate models [10]. Many studies that have examined climate change impacts on discharge seasonality globally can be found in the literature. Dettinger and Diaz [11]; explored on a global scale, the seasonality and variability of streamflow. They concluded that local seasonal cycles of necessary climatic inputs influence streamflow seasonality. On a regional scale, Aich et al. [12] compared the impacts of climate change on streamflow regimes in four large African river basins and found visible impacts of climate change on high and low flows and mean discharges. Eisner et al. [13] investigated climate change impacts on streamflow seasonality, over eleven large river basins, using an ensemble of hydrological and climate models. They found an increasing/decreasing tendency for high and low flows in many of the basins. Hirpa et al. [14] estimated the response of future streamflow in the greater Horn of Africa, using a distributed hydrological model driven by an ensemble of climate models. The study found a reduction in streamflow in the major rivers in Ethiopia and increasing streamflow projections in the equatorial zone towards ending of the century. WaleWorqlul et al. [15] found that climate change would significantly influence the hydrology of two subbasins in the upper Nile in Ethiopia. Li and Jin [16] in their study also found increasing streamflow variability attributed to increasing rainfall variability in the Jing River of China. Ficklin et al. [17], in their study, projected an arid-conditions by the 2080s for many subbasins in the Upper Colorado River Basin of the southwestern United States when they investigated the impacts of Climate Change on Streamflow and hydrology of the basin. Vano et al. [18] identified seasons susceptible to future climate change in the Pacific Northwest using the Variable Infiltration Capacity hydrology mode forced with an ensemble of ten global climate models. These studies and many others all show that climate change plays an increasingly important role in river discharge seasonal flow; hence, it is crucial to know how the climate changes would affect seasonal variations of a basin's streamflow.

#### *Future Climate Change Impacts on River Discharge Seasonality for Selected West African River… DOI: http://dx.doi.org/10.5772/intechopen.99426*

The vulnerability of most West African countries to climate change originates from high reliance on climate-based economic activities and large populations in coastal urban areas [19]. Climate change may undoubtedly modify the river flow regime of most West African basins. It may cause a temporary increase of flow in some places or reduce river flows in other areas [20]. Few studies have addressed climate change impacts on the seasonal river discharge regime in West Africa, especially over Nigeria. Ayeni et al. [20] assessed the impacts of climate changes on three basins of southwestern Nigeria for the period (1961–2007), using the Pitman rainfall-runoff model forced with CSIRO Mark3.5, MIROC3.2-medres and UKMO-HadCM3 GCMs. Ndulue and Mbajiorgu [21] quantified the hydrological changes due to climate impacts and land use in the Upper Ebonyi river watershed, south-eastern Nigeria, for the present period (1985–2014) and the future period (2020–2030 and 2040–2050), using the SWAT forced with the CSIRO-Mk3–6-0 climate model.

The availability of observation data has limited research in this region; the majority of the river basins are ungauged [21]. Also, studies in west African tend to be fragmentary [22], as they use different; climate models; bias correction approach, emission pathways; meteorological forcings; period considered (future and historical). Furthermore, as water availability is critical to economic and social well-being, it is vital to understand the relationship between climate change and streamflow and how it will affect streamflow to establish appropriate adaptation policies. These reasons, coupled with the region's high vulnerability, have made it crucial to provide reliable future projections for river discharge seasonality.

This study, therefore, aims to assess the climate change impacts on far-future river discharge seasonality. We used the PCR-GLOBWB model, forced with five consistent climate models, considering two future emission scenarios (RCP4.5 and RCP 8.5). This study is anticipated to provide useful information to policymakers and river basin development authorities, leading to the improvement of water management within the context of climate change. Our study will also help concerned river basin authorities to improve water management and future socio-economic development for river basins in Nigeria.
