1 Groundwater modelling in arid and semi - arid areas : an introduction

Water resources globally face unprecedented challenges, but these are at their greatest in the world’s arid and semi-arid regions. Recent IPCC estimates (Kundzewicz et al., 2007) state that between 1.4 and 2.1 billion people live in areas of water stress; those numbers are expected to increase significantly under the pressures of population growth and climate change. By definition, arid and semi-arid regions have limited natural water resources, and precipitation and runoff have very high variability in space and time. Traditional societies recognised these characteristics and developed sustainable water management solutions. In higher rainfall areas, for example in the mountains of northern Yemen and Greek islands such as Cephalonia, rainwater was harvested from roofs and paved surfaces and stored for household or community use. In desert areas, such as Arabia’s ‘empty quarter’, with infrequent, spatially localised rainfall, nomadic communities would follow rainfall occurrence, using water from surface storage or shallow groundwater for a few months to support themselves and their livestock, before moving on. For agriculture, terraced systems were developed to focus infiltration to provide soil moisture for crop water needs (as in the mountains of Northern Oman and Yemen), and earth dams were built to divert flash floods onto agricultural land for surface irrigation (as in South West Saudi Arabia). In parts of the Middle East, groundwater was extracted sustainably using qanats (Iran) or afalaj (Oman), ancient systems of tunnels for gravity drainage of groundwater, developed over centuries or longer. The twentieth century has seen pressures of increased population, increased social expectations for domestic use and increased agricultural water use, and the general result has been unsustainable use of water, from the developed and wealthy economies of Southwestern USA, to poorer countries of South America, Africa, Asia and the Middle East. This has led to declining groundwater levels, reduced (or non-existent) surfacewater flows and loss of wetlands, such as the Azraq oasis in North East Jordan – a RAMSAR wetland site now totally dry due to over-pumping of groundwater. There is a range of adverse effects associated with this over-exploitation, for example, loss of important habitats, deteriorating water quality, including ingress of saline water in coastal aquifers, and land subsidence. In the twenty-first century we face the added challenge of climate change. It is essential therefore that we recognise the need for sustainable use of water, balancing the long-term use with the long-term water availability, and learn to use best practice from traditional and new methods of water management. This requires social recognition of water scarcity, the political will to confront difficult societal choices, and good science and engineering to develop and support sustainable management solutions. This book has been developed under UNESCO’s G-WADI programme and aims to provide state-of-the-art guidance for those involved in water science and management in arid and semi-arid areas concerning the modelling methods that are needed to characterise water resource systems and their sustainable yield, and to provide protection from pollution. In an earlier book (Wheater et al., 2008), G-WADI mainly addressed the issues of surface-water systems. Here we consider groundwater.