Infos climat FLASH INFO du Portail du CILSS dans le domaine du Changement Climatique et de la Gestion Durable des Terres en Afrique de l'Ouest

Water management

Impact of climate change on water resources
Some adaptation strategies

Introduction
The key functions of water management will be discussed in this article. In a geography where water resources are scarce and their use, so crucial, we will also interest to monitor the evolution of water levels and rivers discharge, the impacts of climate change on groundwater and surface water as well as at the sea level (land-ocean interactions). Finally, some examples of adaptation will be proposed such as the adaptation of cropping calendar for irrigated rice, hydrological forecasts and the integrated use of groundwater reserves).
Impact of climate change on water resources

1. A. groundwater
The effects of climate variability are manifested rapidly in surface waters, but they are often difficult to be detected for groundwater because they are deferred.
Therefore, it is difficult to detect and quantify the magnitude and time of occurrence of the consequences of climate variability and change on aquifers.
Groundwater is affected in terms of supply process (reduced recharge therefore fluctuations in the level of water tables), the type of interactions that exist between groundwater and surface water and changes in the use of water (irrigation, for example).
Shallow aquifers are more responsive to these stress factors than deeper aquifers, which tend to be more isolated from prevailing conditions at the surface.

1. B. Surface water
The West and Central Africa recorded in recent years heavy rains that caused flooding. Some figures:
In 2010, floods affected: 180,000 ha in Nigeria, 133,000 ha in Benin, 108,000 ha in Chad and 23,000 ha for Burkina Faso.
It is important to adapt to excess water by adopting irrigation techniques that control better the water slide and by adjusting cropping calendars.

Figure 1: compared annual hydrographs of River Niger in Niamey.

The total volume of discharge at the Niamey hydrometric station from 1 June to 31 October 2010 was high (15.83 billion m3) and was never recorded according the NBA.
The increased discharges in basins tributaries of the right bank as a result of exceptional rains and the breaking of certain reservoirs have favored that of the local flood in Niamey.

Figure 2: Daily discharge of River Niger in Niamey

The superposition of hydrographs of six years shows that local flooding evolves saw tooth. The peak (1,920 m3 / s) for the 2003-2004 hydrological year is slightly higher than the Malian flood (1,910 m3 / s). This means that it is the opposite phenomenon that occurs in recent years.
Increased runoff in the sub-basins is due to the degradation of vegetation cover and soil (upward trend in runoff coefficients) (Amani and Nguetora, 2002 Descroix et al, 2012);
In some areas, flooding is caused by the rising of the water table.
1. C. coastal zones
The rising of ocean levels, proven by in situ measurements, leads to submersion and coastal erosion. Thus, there is an increase of floodable areas and salinity in estuaries and coastal aquifers.
According to experts, 2 cm increase per year would be enough to devastate vast swathes of land in the fragile coastal areas of the West Africa.
2. Some adaptation strategies
Faced with the potential and future impacts of climate change, there are various techniques of adaptation and some of which are mentioned here for illustrative purposes.
2. A. Adaptation of the irrigated rice cropping calendar to the onset of high and low water: Saga irrigated area/ Niamey / Niger
The decrease in water discharge (and hence the water height) in the river during low discharge due to the change of regime has implications for the pumping duration.

Decrease in the TMH (Total Manometric Height) therefore decrease in discharge. Simulations of shifting cropping calendars are then needed.

2.B hydrological forecasts

Establish a multi-scale forecasting tool chain in the basin.

It concerns:

-        Flood Forecasting and low discharge (daily, decadal, monthly)

-        Simulation and calibration of discharge (especially by CRA with GeoSFM model) at the sub-basins outlets (Experimental Stage).

Figure 3: Map of flood risk in the city of Niamey/NIGER

2. C integrated use of groundwater reserves
The shallow groundwater and monitoring of water bodies dynamics allow a mapping of the hydraulic potential of drowned shallows and cuirasses.
For example, manual or motorized drillings at lower cost and at shallow depth such as the "enterprise work" experience in Burkina Faso, Mali and Niger with discharges up to 20 m3 per hour were obtained in different localities of Burkina Faso.
These systems have allowed an extension of truck farming. Information systems on the dynamics of water bodies for recessional agriculture can be developed: recession cropping of sorghum in Chad, Mauritania...
2.D Valuation of mangrove soils for agricultural production:
Experience in Guinea Bissau and Guinea Conakry.
The mangrove is a meeting point of two types of water: salt water and fresh water with a tree that is characteristic of this environment: the mangrove tree.
Mangrove areas are found in the major maritime plains, especially around a river delta in lands located at a distance more or less far away from the seaside and where coexist, alternately the two types of water during the year. These are among the most productive ecosystems in the world.
The limit of two waters, salty water and freshwater, is called front.

Technique Advantages
Inflows of freshwater and saltwater in the plots are used alternately by using contour dikes:
-    Weeds are killed by salt water during the water inlets in the dry season, sea water brings more silt,
-    Then a leaching by several floods and drainage of rainwater is favored until the soil becomes suitable for rice,
-    The water has thus the following functions:
-    regulating  soil acidity(PH) in the dry season,
-    regulating salinity in the cropping season

- zero input

- Increased yields if the operation is maintained over 5-6 years (the average yield of the mangrove system closed by bunds is 1570 kg / ha or a nutritional capacity of 10 adults per year and it can reach up to 3.5 t / ha). However, at least salt tolerant seeds must be used.

 











 

Conclusions:
Integrated management of water resources remains essential in a context of scarcity and intensification in the use of the resource. Practices that enable good management exist in the field. They must be disseminated by large information campaigns, and supported by policies at local, national and sub-regional and continental levels.

 

 

 

 

 

 

 

 

 

 

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