Groundwater is generally a reliable and good quality water source, and with modern technology for drilling, electrification and pumping, it is widely accessible throughout most parts of the world today. In fact, these technological advances are primarily accountable for the recent, remarkable increase in global abstraction of groundwater. The history of global intensive groundwater use is less than 50 years old and much of the modern increase in global water use has been contributed by groundwater. Surface water use has remained constant or increased at a slower rate, simply because resources are running out or the feasibility of capturing and storing them is low. What is also remarkable about today's groundwater use is that the increase is continuing on a global scale, with only patches of declining or stagnating trends. Global aquifers hold an enormous water reserve that is several times greater than surface water resources (UN/WWAP, 2003). Groundwater could, in principle, be exploited at an aggregate level that is higher than it is today. However, the overriding limitations to further groundwater use in the future will continue to be environmental problems associated with the desiccation of aquifers and the socio-economic problems related to increasingly unequal access, especially in developing countries, to the resource as the groundwater levels decline and the aquifers become contaminated as a side effect of intensive use and generally increased pressure on natural resources.
In addition, the classical problem of uneven geographical distribution of surface water resources also applies to groundwater, at least at more regional scales, and the general mismatch between the location of high demand (highpopulation, potential-intensive agricultural areas) and groundwater availability is very real and relevant. Often, and logically, groundwater is developed and in further demand in dry and semiarid areas where surface water is scarce or seasonal. But such regions are typically underlain by non-replenishable or slowly replenishable aquifers unfit for intensive exploitation, putting a natural break on unlimited growth in use. Likewise, half of the world's population today lives in coastal areas (Post and Lundin, 1996) where groundwater traditionally provided secure and adequate water supply. However, these areas are increasingly threatened by deterioration of water quality due to salt water ingress from widespread and intensive groundwater extraction (Kaushal et al., 2005).
It is essential to focus on the agriculture in the context of global groundwater use for the simple reason that volumes used in this sector significantly exceed other uses, e.g. industrial and domestic, at the global scale. Especially in many arid and semiarid regions of the world that coincide with nations in development, such as India, North China and Pakistan, groundwater use is critical for food security. Here the management challenges are manifold in the sense that a balance between securing groundwater-dependent livelihood and ensuring the long-term environmental sustainability is required. But even in more developed countries in less arid regions such as the USA, Australia and Mexico, as well as in Mediterranean countries like Spain, groundwater supplies significant water for agricultural use and its management presents great challenges. High dependence on groundwater also occurs in humid countries, but more for industrial and domestic uses (e.g. Japan, the former USSR and north European countries like Denmark and the Netherlands (Margat, 1994) ). Here, the volumes drawn are generally not threatening the resource base from a quantity point of view - it is more water quality issues that present the major challenges.
Yet another category of countries includes those that potentially could benefit from an intensification of groundwater use for agriculture and associated development, such as parts of sub-Saharan Africa (see Masiyandima and Giordano, Chapter 5, this volume), Nepal and eastern India (see Shah, Chapter 2, this volume). In these cases, present limitations to such development seem to be associated with poor energy access, lack of infrastructure and market access, lack of credit possibilities and possible cultural or demographic barriers.
Groundwater is now surpassing surface water in importance in many regions of the world, in terms of water supply for irrigation. The 2005 FAO AQUASTAT database on irrigated area lists the countries Algeria, Bangladesh, India, Iran, Libya, Saudi Arabia, Syria and Yemen, as those depending more on groundwater than surface water for their irrigation. Expanded groundwater use in a global context can be seen as a second step in the continued and accelerated quest for water for human development. Basically, surface water was accessed, appropriated and allocated first, as this resource was more visible and readily available and most human settlements confluenced with rivers and streams where water was traditionally secured. As surface water resources are being exhausted and strained in terms of quality and the options to dam them have diminished, groundwater has become the second-generation resource to be captured and appropriated.
Taking this analysis further and linking it with the general hydrological cycle, there is now a trend towards focusing on rainwater as the 'new' water source to capture for direct use as well as for storing and optimizing. What is interesting in this scheme is that in a sense we are moving progressively backwards, or upstream, in the hydrological cycle to look for water, because rainwater feeds groundwater and groundwater feeds surface water (Fig. 17.1). More importantly, there is a tendency, amongst lay people but also water professionals, to look at water sources independently and consider them as isolated, new resources that can be explored without affecting the others. One example is the artificial recharge movement in India that attempts to capture rainfall and runoff for local replenishment of aquifers for the improvement of livelihood and to counteract groundwater declines (see Sakthivadivel, Chapter 10, this volume). However, referring to Fig. 17.1, it is obvious that these sources are intricately interlinked, and capturing one will diminish the availability of the downstream sources. Realizing this and respecting this simple, but fundamental, upstream-downstream and mass balance concept is crucial to any kind of water resources management. Following these arguments, it is also clear that groundwater cannot be managed in isolation and that integration of all water resources needs to be considered in overall assessment and planning.
Hence, the issue of groundwater use today is essentially not technical but managerial - how to balance the benefits of use with the associated negative
impacts? Although this is increasingly being realized by water managers, practitioners and scientists, it is proving to be one of the most challenging tasks for humankind as we move into the 21st century as population increases, demand for higher living standards in the developing world and climate change with associated increases in extreme events all mix together to put higher pressures and threats on already strained resources, including groundwater.
While paradigms for groundwater management are slowly emerging and various models are investigated and tested (see Schlager, Chapter 7; Kemper, Chapter 8; and Moench, Chapter 9, this volume), it is also clear that from actual cases that management based on strict control of groundwater development and use (demand management) is generally difficult to implement and enforce, perhaps especially in developing countries (see Shah, Chapter 2; and Wang et a/., Chapter 3, this volume). This is to a large extent attributable to the fact that groundwater has many 'open-access' properties, leaving little incentive for users to curtail their use because they cannot fully capture the associated benefits (see Schlager, Chapter 7, this volume). The generally easy access to the resource for individuals combined with this fact actually presents the core dilemma in groundwater management, and as of today there seems to be very few examples of solutions addressing this dilemma.
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