Authors: Mouhamadou Bamba Sylla , Jeremy S. Pal, Aissatou Faye, Kangbeni Dimobe & Harald Kunstmann
Site of publication: www.nature.com
Type of publication: Article
Date of publication: 26 September 2018
In West Africa, rain fed agriculture is the most prominent instrument for securing income and overcoming poverty. Agricultural water requirements in the region are largely met by rainfall that is associated with the West African monsoon occurring in the boreal summer. Irrigated agriculture represents about 4% of the cultivated land in Sub-Saharan Africa and remains largely undeveloped due to a lack of sufficient economic resources and political will.
Consequentially, changes in monsoon intensity, timing and spatial patterns as well as temperatures have the potential to both positively and negatively impact agricultural productivity in West Africa and consequently the wellbeing of its population. In recent decades, drought, population increase and water withdrawals have increased water stress in the major river basins of West Africa.
Previous studies of climate change impacts on crops in the region generally project yields to decrease even by 2050 due to increased growing season temperatures and changes in the monsoon precipitation variability, thereby amplifying food insecurity in an already vulnerable region.
Crop Water Demand and Irrigation Need
CWD is related to the amount of water needed by a crop for optimal growth and is defined here as ET0. While in reality, some crops experience CWD higher than ET0 and others less, this analysis provides a general understanding of how demands are projected to change in a warmer climate. Two methods (Hamon and Hargreaves) are used here to assess the projected change for the sake of robustness (see Analysis Approach).
In the 2 °C global warming scenario, warmer temperatures result in an average increase in CWD by up to 10% to 15% with respect to the reference period in all major West African river basins.
While the Hargreaves formulation shows uniform changes throughout all basins (8% to 10%), the Hamon formulation shows more spatial variability. If the future global warming is held to 1.5°C, the projected changes are reduced by as much as 50% with the smallest changes in the Gulf of Guinea basins.
Previous studies of climate change impacts on crops in the region generally project yields to decrease even by 2050 due to increased growing season temperatures and changes in the monsoon precipitation variability, thereby amplifying food insecurity in an already vulnerable region
As indicated above, CWD in West Africa is generally met through precipitation (i.e. effective precipitation). Deficits in CWD could potentially be mitigated through the development of large-scale irrigation. IWN measures crop water needs under optimal growing conditions solely in terms of additional water requirements and provided that other conditions are optimal (e.g. diseases and pest free, favourable soil conditions).
Under the 2 °C global warming scenario, IWN is projected to increase by more than 15% and 30% of reference period values, respectively, for the Hargreaves and Hamon formulations over most basins. While the Hargreaves formulation displays the greatest changes (up to 20%) in western basins (i.e. Gambia, southern Senegal and Western Niger), Hamon formulation also shows extensive changes (up to 30%) in the Gulf of Guinea basins (Sassandra, Bandama and Volta).
Limiting global warming to 1.5 °C reduces the increase to 10–20% over these regions. Although IWN is largely projected to increase over the region, it is not obvious whether future water availability will compensate for or exasperate these increases.
Projected Water Availability and Basin Irrigation Potential
The projected WA changes under 2 °C of global warming are regionally heterogeneous with both significant increases and decreases depending on the region.
Limiting global warming to 1.5 °C, decreases the negative changes and their spatial extent and strengthens and increases the spatial extent of positive changes in the Chad and Niger basins by as much as 60%.
In the 2 °C global warming scenario, warmer temperatures result in an average increase in CWD by up to 10% to 15% with respect to the reference period in all major West African river basins
In addition, in Senegal and Gambia, the magnitude of negative changes is limited to 15% and in Sassandra, Bandama and the adjacent section of the Niger basin, the increases are limited to 10%.
In this study we assessed the hydroclimatology of the 10 major West African river basins under 1.5 °C and 2 °C global warming scenarios using a comprehensive ensemble of CORDEX regional climate model experiments.
The results indicate that, under a relatively moderate rise of global temperature of 2 °C, the combined increases in CWD and smaller mixed changes in ET cause IWN to increase considerably regardless of the ET0 formulation.
As climate warms, the potential to sustain irrigated agriculture or other activities that require large amounts of water such as hydropower is projected to decrease. A relatively small decrease in global warming from 2 °C to 1.5 °C significantly reduces the gap between IWN and WA greatly reducing the negative consequences of climate change on water resources.
Limiting global warming to 1.5 °C, decreases the negative changes and their spatial extent and strengthens and increases the spatial extent of positive changes in the Chad and Niger basins by as much as 60%
It should be emphasized that although some uncertainties are present in these projections, they are mostly in the magnitude of the changes as most of the models project negative changes.
As population and urbanization intensify and economies grow, increasing municipal and industrial water needs are likely to amplify the overall water demands in West Africa.
This study thus suggests that effective mitigation should be pursued to cap the global warming at 1.5°C hoped for by COP21. Notwithstanding, West Africa will likely continue to face water limitations under climate change.
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