Mankind and Deserts 2

The wild beauty of deserts has always been a source of fascination the world over. Mankind and Deserts 2 – the second of three volumes – focuses on water, its absence or indeed its extreme scarcity, as well as on the ways in which salts come to be formed in areas such as these. Aridity of the climate does not exclude rainfall, after which deserts flourish; wet mists, dew, exceptional events separated by years of total drought. Water flows into temporary and disorganized networks but, occasionally, large rivers cross the deserts, giving rise to vibrant civilizations: the Nile, Tigris and Euphrates, Niger, to name a few. Temporary or permanent lakes collect water in basins without outlet to the ocean, referred to as endorrheic basins, such as Lake Chad. This results in salt accumulation and evaporitic formations. A large variety of salts crystallize, in addition to halite, among which is potash. Halite – common salt – is an essential resource and its trade leads to the creation of salt caravans, used to exchange it with gold, even on a 1-1 weight basis, generating subsequent wealth. From ancient, almost mythical, exploration to modern scientific studies, deserts have come to be better known yet still hold great appeal. This book traces the history of their knowledge while providing a basis for understanding their features and the tools needed for their protection, in an ever-changing world.

Foreword ix Yvette Dewolf Introduction: Water in Deserts xiii Fernand Joly Chapter 1. Water Falling onto Soil and the Effects It Produces 1 Fernand Joly 1.1 The arrival of water 1 1.2 Weathering 3 1.3 Runoff 15 1.4 Geodynamic and geomorphological effects of runoff in arid zones 24 1.5 Groundwater 26 1.6 References 33 2 Chapter 2. Temporary Water Bodies and Lakes 39 Fernand Joly 2.1 Waterbodies 39 2.2 Lakes 43 2.3 The principal features of lakes in an arid zone 75 2.4 References 82 Chapter 3. Hydrographic Networks 87 Fernand Joly 3.1 Endorrheism–arheism 87 3.2 The disruption of hydrographic networks 94 3.3 Allogenous rivers and streams 97 3.4 References 118 Chapter 4. Salts in Deserts 121 Guilhem Bourrié 4.1 The nature of the salts 123 4.2 The origin of salts and evaporite sequences 125 4.3 Evaporation of seawater 128 4.4 Evaporation of continental freshwater 129 4.5 Systems mixing surface water and deep water 131 4.6 Atmospheric origin of nitrate deposits 132 4.7 Mankind and salts 133 4.8 References 135 List of Authors 139 Index 141