Use of Remote Sensing and GIS in Water Resources Management: North Western Sahara Aquifer System (NWSAS)

The North-Western Sahara Aquifer System, which stretches over an area of more than one million square kilometers, is shared by Algeria, Tunisia and Libya. Due to the structural configuration and climate type of the region, the recharge of the NWSAS is very limited. These aquifer systems represent geological reserves whose natural outlets (springs and foggaras) led to the development of oases where people lived, for centuries ago, in perfect harmony with the Saharan ecosystem.

Agriculture represents the primary source of livelihoods for a large part of the African population. Agriculture in Africa is diverse and ranges from small subsistence to large commercial farming systems.

About 90% of water withdrawals in this region are consecrated to irrigation. Intensified water abstraction has led to serious problems: regular lowering of the water level, increased cost of pumping, reduction in artesianism, drying up of natural outlets, poor water quality due to salinization, etc.

An effective management of agriculture in Africa requires deep knowledge as science plays a key role in understanding its dynamics. Remote sensing techniques provide recurrent information on vegetation cover and water resources management in various spatial and temporal resolutions and represent therefore a valuable source of data and information.

Groundwater is vital to most nations and worldwide. For instance, irrigated agriculture is the largest user of ground water. Groundwater resource development has had great social and economic benefits by low-cost, drought-reliable and high quality water supplies for urban areas, rural populations and crop irrigation (90% of rural water-supply in Sub-Saharan Africa). The economies of these countries highly depend on groundwater resources.

Groundwater withdrawals in the three concerned countries have remarkably increased over the last 50 years.  Agriculture accounts for about 90% of the total withdrawals and projections indicate that a further increase of water withdrawals in the three countries of focus is more likely to happen. For instance, water withdrawal (extracted from more than 16,000 water points) increased from 0.6 billion m3 in 1950 to about to 2.8 billion m3 in 2012, compared to a recharge level of about 1 billion m3/year in total. Based on hydrogeological studies coordinated by UNESCO then OSS, a consultation mechanism for the North-Western Sahara Aquifer System (NWSAS) was set up in 2002. In June 2007, Algeria, Libya and Tunisia decided to set up its permanent secretariat at OSS premises. The activities of this mechanism are fully supported by the three member countries and can be considered similar to a Basin authority.

Due to a rapid increase of groundwater use for irrigated agriculture in arid zones, Algeria, Libya and Tunisia face the challenge to set-up an agriculture development action plan that takes into account the sustainability of this unique water resource (The North Western Sahara Aquifer System (NWSAS). The actual increase of water extraction for irrigation will have serious consequences on the integrity of these non-renewable groundwater reservoirs in terms of quantity, accessibility and quality. In order to provide water management institutions with efficient tools to manage this resource, OSS, together with other international partners, has developed a set of methodologies based on a hydrogeological model and earth observation (EO) based services.

The North-Western Sahara basin has registered a significant increase in water abstractions, going from 300 million m3/year in 1950 to almost 2.8 billion m3/year in 2012. See Figure 2 and Figure 3. 

Water points are not equipped with meters, so data is mainly gathered using survey collection techniques based on information provided by water users and water volume approximations (generally every 5 to 10 years, as the survey is very costly).

Despite the accumulation of considerable non-renewable water resources estimated at 30 000 billion m3, water withdrawal for agricultural use resulted in an overall situation on the basin characterized by:

• A drop in artesianism;
• A decrease in groundwater levels and an increase in pumping costs;
• Water and soil salinisation.

Moreover, an increased exploitation of the NWSAS for the coming few years is more likely to happen due to the following reasons:

• Demographic growth: the population is expected to double, increasing from 4 million to 8 million by 2030;
• Intensive transfer of water to supply periphery zones of the basin which are highly vulnerable and sensitive to climate change. Thus, added to the growing demand of water caused by population growth, the basin resources are also exploited to feed the other populations living on the peripheries of the basin.

The common challenge nowadays for the three countries concerned is to ensure a sufficient water abstraction for the development of the region without inducing negative impacts on the resource.

Agriculture and vegetation cover on this aquifer are represented generally by small patches (small areas compared to the overall area of 1 million km2), or a concentrated agriculture in the oases. See Figure 4 and Figure 5.