Figure 2. Unmet water demand of the Ouémé-Bonou catchment in percent of total water demand for the socio-economic scenarios B1, B2 and B3.
The water balance for the Ouémé-Bonou catchment was calculated for the period from 2002 to 2025, with 2002 as the base year for reference (Höllermann et al.2009 and 2010). The results show that the different climate scenarios A1B and B1 have a strong impact on water availability. A significant decrease in catchment inflow is observed for the period 2015 to 2025 in climate scenario A1B, while this effect is mitigated in climate scenario B1. The decreas-ing inflows affect the accessible groundwater and reservoir storage.
While the groundwater aquifers tend to refill completely during the rainy season until 2014, the recharge rate from 2015 on is not high enough, and a decrease in storage is observed, leading to lower groundwater levels during the dry and rainy seasons (Höllermann et al. 2010). Less catchment inflow aggravates the refill capacity of larger reservoirs. Furthermore, growing water demand increases the pressure on reservoir water. This increase is more significant in cli-mate scenario A1B under the IMPETUS socio-economic scenario B1, presenting the highest water extraction from reservoirs (Höllermann et al. 2009 and 2010).
According to the effects of the climate scenarios, water scarcity is highest in the last decade of the study period. This becomes appar-ent from the above-average increase in unmet demand in the period 2015–2025 (see Figure 2), especially amongst demand sites that rely on water from rivers or reservoirs.
As the availability of surface water follows a monthly variation due to the changes from rainy to dry season, the shortages solely oc-cur during the dry season. While the effects of the dry season with unmet demands can be found for about 8 months per year in the period of 2002–2014, these effects extend to 10 months per year for the period of 2015–2025 (see Figure 3).
The water balance results of WEAP imply that water supply secu-rity depends strongly on the water sources used. While the
avail-Figure 3. Monthly water demand of the socio-economic scenario B1 averaged over the periods 2002-2014 (a) and 2015-2025 (b). The water demand is distinguished into covered (blue) and unmet (red) demand.
able amount of groundwater is potentially high enough to satisfy demand, users relying on surface water from rivers and reservoirs experience shortages. As a result, the different user types relying on surface water are competing with each other. Especially in the northern parts of the study area, a distinctive competition between domestic water users and livestock watering is observed. Other com-petitors for surface water are the peri-urban irrigation sites, which double their demand during the dry season. The situation worsens with the projected changes in climate because the months with no unmet demand decrease from four months (2002–2014) to two (2015–2025). By contrast, the WEAP results imply that the water security for users relying on groundwater is high. However, these results must be treated with caution. Due to the simple groundwater modelling approach the uncertainties concerning the total amount of groundwater are quite high.
Furthermore WEAP does not simulate the socio-economic and in-stitutional factors that prevent optimal exploration of groundwater.
Even though uncertainties and constraints exist, the WEAP results offer a solid basis for assisting planners in developing recommenda-tions for future water resources management. The scenario analysis with WEAP has revealed potential conflicts over water, the occur-rence of shortages, and the development of mitigation strategies.
For example, increases in surface water efficiency as well as techni-cal and organizational improvements in the rural infrastructure (e.g., reliable pumps, sufficiently deep wells, and improved management structures) can mitigate current and future shortages in water supply and therefore increase water supply security. However, one has to keep in mind that an improved infrastructure for exploring ground-water might also further deplete the groundground-water.
Conclusions
The scenario analysis for the Ouémé catchment and its sub-catch-ment has shown that Climate Change, as well as the population growth and socio-economic changes will have a strong impact on the water resources. The increase of temperature and decrease in rainfall cause a decrease in total available water for both scenarios for the period 2001-2025/2049. With the WEAP water management model, it was possible to identify future problems of water supply in the Ouémé catchment by linking water availability with water de-mand. For the simulated climate scenarios, users relying on surface water from rivers and reservoirs, especially, will experience short-ages in the future, while the available amount of groundwater is potentially high enough to satisfy the demand. Here the problems of poor access to the available groundwater must be kept in mind.
The results of the presented studies are useful for supporting wa-ter management planning activities in Benin. As the IWRM-strategy is actually implemented in the water policy of Benin, reliable data concerning the development of available water resources and water demand on a catchment level are required. With the results of the water management model WEAP these data are provided for plan-ners and decision makers in Benin in a user-friendly way.
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