Understanding the hydrogeology of Anjouan and the impact of deforestation on the availability of water resources

Arnaud Charmoille, PhD in Hydrogeology and volunteer for the NGO AVSF, came to study the groundwater resources of Anjouan and our intervention around the forest area Moya for two weeks during the month of August 2012. The objective of the study was to better understand the problems of deforestation and its impact on the availability of water on the island. The full report (in French) with summary entitled “Outline of the hydrogeological functioning of the island of Anjouan (Comoros): Typology of available water resources and discussion on the impact of deforestation” is available through this link.

In August 2012 Arnaud Charmoille, hydrogeologist and volunteer for the NGO AVSF has collected testimonies, data and field observations on the island of Anjouan to better understand the hydrogeology of Anjouan and the impact of deforestation on the availability of water

In August 2012 Arnaud Charmoille, hydrogeologist and volunteer for the NGO AVSF has collected testimonies, data and field observations on the island of Anjouan to better understand the hydrogeology of Anjouan and the impact of deforestation on the availability of water

As mentioned on several occasions in this blog, the island of Anjouan is facing a supply problem for drinking water and reduction of its surface water resources. Deforestation is systematically evoked to explain the apparent decrease in the flow of rivers, by the public and the various authorities of the island.

My name is Arnaud Charmoille, hydrogeologist and volunteer for the NGO AVSF. I have been sent on this mission to work with the ECDD project to give an opinion on the specific water problematic of Anjouan. The question, the expectations of the population, being the first to do this kind of work in Anjouan were prime motivating factors to carry out this mission.
My work began with a visit to Anjouan in August 2012, I collected testimonies from the different water actors and collected data and field observations. These data were hydrological, hydrogeological, geological, hydrochemical, geomorphological and geographical.

This visit was followed by a major work of interpretation of the acquired data and observations in the field. In particular, I compared the results with data available in the literature dealing with volcanic islands that are contextually approaching the island of Anjouan.

Example of analysis of hydrochemical results. Samples are represented depending of their sodium and chloride concentration. This kind of graph allows the differentiation of shallow aquifers from deep aquifers.

Example of analysis of hydrochemical results. Samples are represented depending of their sodium and chloride concentration. This kind of graph allows the differentiation of shallow aquifers from deep aquifers.

This analysis allowed me to draw a diagram of the hydrogeological and hydrological functioning of the island. Once this was done I could analyze how the deforestation could have an impact on the water resources of the island.

A cloud forest in Anjouan.  This forest type is formed on the peaks, slopes and ridges, the reliefs are often bathed in fog

A cloud forest in Anjouan.
This forest type is formed on the peaks, slopes and ridges, the reliefs are often bathed in fog

Originally, a part of the Anjouan natural forest is a forest type “cloud forest”. This forest type is formed on the peaks, slopes and ridges and is often bathed in fog. In these forest types, specific plant species develop that are capable of capturing the fog droplets. This phenomenon is frequently observed on islands of small area and high relief which is the case on the island of Anjouan. In tropical region, the provision of additional water produced by cloud forests increases the amount of water available for infiltration and maintains high flows during the dry season. This ecosystem also prevents flood during rainy season, two points that are currently lacking on the island of Anjouan!

Impact of cloud forest mutation and disparition on hydrogeological cycle (Foster, 2001)

Impact of cloud forest mutation and disparition on hydrogeological cycle (Foster, 2001)

At the isle of Anjouan deforestation, except in some areas, has completely removed the original vegetation cover; part of the cloud forest has been transformed into a habitat consisting of species introduced by man as coconuts, clove, banana, etc. However even if there is a certain vegetation cover, these introduced species do not exhibit a good intercept of fog droplets. There is no provision of intercepted additional water by the fog. Groundwater that feed streams is therefore seeing its rate decrease significantly during dry season. Considering that the majority of primary forest has disappeared, we can imagine that the situation is being stabilized…

These findings, which are the result of the early work of this type carried out on the island, must of course be supplemented by work and further investigations.

Although a reduction in the flow of streams exists and can be attributed to deforestation, it seems, however, from the results and observations made during this mission that water resources are sufficient for Anjouan. It looks more that the provision to the population is a real problem. In the future it will be hard to envisage that if the population continues to grow, to limit the water distribution system in catchments such gravity. Exploitation of groundwater seems inevitable in certain sectors of the island that are provided with little surface water.

Finally, I wish to thank all people, which directly or indirectly helped me to carry out this mission. Special thanks to the ECDD project team that left me with unforgettable memories of the island!

For those interested in reading the article (in French), you can download it here.

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