Influence of Human Activity on Dune Fields Development in Southern Morocco

Andrzej Barczuk¹, Maciej Dluzewski<sup>2

1</sup> Institute of Geochemistry, Mineralogy and Petrology, Faculty of Geology, Warsaw University,
e-mail: a.barczuk@uw.edu.pl
² Department of Geomorphology, Faculty of Geography and Regional Studies, Warsaw University
 e-mail: dluzewski@uw.edu.pl
 

Abstract

Influence of environmental changes on dune fields development is of a special interest for  numerous scientific researches. The changes are usually directly or indirectly caused by human activity (Tsoar, Moller 1986, Mainguet 1991, Goudie 1993). The rate of dune fields development is exactly proportional to the scale and range of these changes. A detailed analysis especially regards a place of human intervention (within certain area of out of the area) and determines the basic factors responsible for the dune fields development. In the case of such research it is important to detect the main factor responsible for deflation in the source area. Lowering of a ground water table (drying of a superficial ground layer) or agricultural activity (intensive grazing, wrong land cultivation leading to degradation of vegetation) are usually considered as such factors. Proper identification of factors responsible for the dune fields development requires identification of the source area of the dune material. The next step is to analyse all environmental changes which took place in the research area and to identify reasons for the changes.

The research was undertaken in southern Morocco between years 2000 and 2006 (Coude du Dra region and Tafilalt - Erg Chebbi) and was aimed to describe dynamics of the dune fields, to identify the source area of the dune material and to determine factors responsible for the dune fields development. Changes of the area occupied by the dune fields were analyzed on the basis on topographic maps and aerial photographs. The source area of the dune material was identified by analysis of textural features of the dune and substratum materials. Granulometric indices (Folk, Ward 1957) were calculated for the dune material and mineral-lithological composition was determined with use of optical and electron microsocopy, as described by Barczuk (1992). These analyses are thought to be most important in identification of the source area and assessment of the length of transport of the dune material (Kuenen 1960, Barczuk, Dłużewski 2003).

It was found that dune fields spreading in the Coude du Dra region is very fast. During the last 100 years the area occupied by the dunes increased several times. Granulometric indices show that local substratum is the most important source of the dune material. This is inferred from low aeolization of the dune material and similar mineral-lithological composition of the dune and substratum materials. The dune material is characterized by a high share of ferrous-clayey clasts which are not resistant to mechanical abrasion and could not have been transported for a long distance. Such particles originated in shallow water and are characteristic for the local substratum. Therefore, the dune fields develop because the physico-geographical conditions changed and deflation of the local substratum became possible. From own research and literature (Zainabi 2004) it was found that the ground water table has been lowered  in this region because of construction of dams in the Dra river valley and especially the biggest dam of Mansour Eddahbi, located in the vicinity of Ouarzazate, which has been in operation since 1971-72.

In the Tafilalt region there are two types of dune fields. The first one is the old dune field of Erg Chebbi composed of 13 star dunes, and the second one – barchans or barchanoidal dunes which are several metres high. The Erg Chebbi is a mature dune field and its area changed very little during the last century. Granulometric indices show that aeolization of the dune material is high and therefore the source area of the material was distant. The dunes consist mostly of quartz grains which are highly resistant to mechanical abrasion. It is concluded that local environmental changes do not markedly influence development of this dune field. In a contrast to the Erg Chebbi, the dune fields of the second type develop very fast. The mineral-lithological analysis showed that despite short transport (which is inferred from granulometric indices) the dune material does not originate from the local substratum of sediments deposited in an environment of low energy, but it originates from fluvial material. Such source material was deposited in an environment of high energy which results in a high content of quartz. This mineral is resistant to mechanical abrasion and therefore dominates in sediments episodically transported by dynamic water flow in ueds.wadi.  It is concluded that a decrease of discharge in ueds.wadi is the reason for the dune fields development in the Tafilalt region. This is linked to lowering of a local ground water table - a result of the fast growth of local demands for water connected with development of tourist industry. Operation of the  Hassan Abdahil dam which crosses the Ziz uedwadi near Errachidia plays a secondary role. As a matter of fact, its construction lead to drying the Dajet Srji located west of the Erg Chebbi, but the research proved that the dry floor of this former lake is not a source of the dune material in the analysed dune fields.

Final conclusions is that similar human activities (construction of dams on main rivers) do not necessariy have the same effect in a rate of dune fields development. In the case of the Coude du Dra region, the main reason for the fast dune fields development lies outside the region. In the Tafilalt region, local human activity is a direct cause of development of the dune fields. Construction of the dam near Errachidia is a secondary factor.  Intensive consumption of ground water in this region could be partly attributed to the fact that water from the nearby Ziz uedwadi became inaccessible.