Spatial distribution of population in South Asian countries. Data taken from a global population model of LandScan 2007 ( ). 

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... groundwater is recognized as an important natural resource with great economic value. In many developing nations, groundwater abstraction has accelerated resource development over the past 20 years and led to major social and economic benefits. 1 Estimates show that freshwater represents nearly 2.5 percent of the Earth’s total water content, of which around 30 percent is groundwater and the rest includes ice and glaciers, surface water, and soil and atmospheric water. 2 Thus groundwater represents a significant proportion of the Earth’s freshwater content, and in many countries groundwater is the only reliable source of freshwater. Approximately one-fifth of the Earth’s total freshwater resources can be found in South Asia – the home of around 1.7 billion people (Figure 1). During the monsoon surface water is abundant throughout the region; during the dry season surface water scarcity is common. A vast amount of freshwater is stored as groundwater beneath the densely populated floodplains of the Ganges, Brahmaputra and Indus River systems. In the dry season or when the monsoon is delayed, this storage is critical. It can also be a safer alternative to often-polluted surface water year-round. For these reasons, groundwater is the main source of domestic, industrial and irrigation water supplies throughout South Asia. A safe and sustainable water supply is essential for improving public health, and achieving economic growth and food security in the region. Currently, groundwater resources are facing degradation due to a range of problems, such as overexploitation, mismanagement, and natural and anthropogenic contamination. The strategic importance of groundwater for global water and food security will further intensify under climate change. 3 Groundwater-fed irrigation has become the mainstay of irrigated agriculture over much of India and Bangladesh, Punjab and Sindh provinces of Pakistan, and the Terai plains of Nepal. 4 Traditionally, surface water from ponds and rivers had been used to provide both drinking and irrigation water supplies in all South Asian countries. However, over the last few decades groundwater has largely replaced surface water-fed water sources. In Bangladesh, currently 97 percent of drinking water and nearly 80 percent of irrigation water come from groundwater (Figure 2). The use of groundwater for irrigation in India and Pakistan is approximately 60 and 35 percent respectively. By volume, India is the biggest groundwater user in the world. A recent estimate shows that in India, Bangladesh, Pakistan and Nepal combined the annual groundwater withdrawal is nearly 250 km 3 – approximately 35% of the world’s total groundwater withdrawal. A substantial proportion of this groundwater is used to produce rice, the staple food of South Asia. Recently, Bangladesh has made significant progress towards becoming self-sufficient in food grains, primarily through groundwater- sustained agriculture. It has long been taken for granted that shallow groundwater used for irrigation and drinking water supplies in Bangladesh is fully recharged during the monsoon season. However, recent studies reveal that the volume of groundwater storage is rapidly declining in many parts of Bangladesh and India because groundwater is not being recharged at the same rate as it is used. 5,6 Intensive and unsustainable use of groundwater in South Asia, particularly in northern India and central and northwestern Bangladesh, has led to rapid depletion of aquifers in recent years. NASA’s GRACE (Gravity Recovery and Climate Experiment) satellite observations have been used to show that northern India has lost approximately 109 km3 of groundwater between 2002 and 2008 (Figure 3). 6,7 Over the same period, India’s neighbor Bangladesh, which has 4.5% of India’s landmass, has lost nearly 3 km 3 of its groundwater due to over-abstraction. 8 It is reported that sustained groundwater depletion has contributed substantially to global sea-level rise 3 ; groundwater depletion in Asia is estimated to have contributed to a global rise of 2.2 millimeters over the period 2001 to 2008. Recent sea-level rise in the Bay of Bengal has been attributed, at least in part, to over-abstraction of local groundwater to supply irrigation and municipal water over the last few decades. 5 Another concern is the deterioration of groundwater quality due to both natural processes and anthropogenic activities. In large parts of Bangladesh and several northeastern states of India, shallow groundwater is contaminated with high concentrations of naturally occurring arsenic. Nearly 100 million people in the Indian sub- continent are currently exposed to dangerous levels of arsenic in their drinking water supply. 9 High concentrations of naturally- occurring fluoride is another threat to public health affecting nearly 66 million people in southern and northwestern India. 10,11 Although, arsenic and fluoride contamination is not as big a problem in coastal Bangladesh, highly saline groundwater is a major public-health concern, particularly for maternal health. 12 Similar concerns exist in other deltaic areas of South Asia. Although sources of high salinity in coastal groundwater are difficult to identify, it has been shown that the reduction of flow through the lower Ganges and rising sea levels are partly responsible. 13 How will climate change affect South Asia’s groundwater resources in future? Unlike surface water, groundwater is more resilient to climate change and slow to respond to any change. 14 However, some specific aspects of climate change can greatly influence the timing and magnitude of groundwater recharge and quality, such as a shift in monsoon season, heavy rainfall events, increased evaporation, increased runoff and rising sea levels. 3,15 Elsewhere, it has been shown that episodic heavy rainfall events favor more rapid groundwater recharge in central Tanzania. 15 Heavier rainfall events are also projected to occur in South Asia but the potential impact on groundwater recharge remains unanswered. As mentioned above, sea level rise can cause coastal fresh groundwater at shallow depths to be gradually replaced by saltwater. This process can accelerate through over abstraction of groundwater in many of the growing coastal cities of South Asia. The degradation of groundwater resources by human development and climate change is increasingly disturbing drinking and irrigation water supplies globally. The problem is not exclusive to South Asia, but it is perhaps most critical in what is the world’s most densely populated region. Public health, food security, industrial growth, and ecosystems all are currently at greater risk than ever before. More public investment will be needed to manage the growing demand for drinking, industrial and irrigation water supplies. Alternatives are needed and improved efficiency of use is required. Many past development projects in South Asia did not take into consideration the declining state of groundwater. Governments need to recognize the social and economic importance of protecting aquifers from further deterioration. Public awareness and education are also essential. Lastly, more scientific research is necessary, particularly in complex coastal environments. Continually improving our knowledge of groundwater systems in South Asia, and the threats they face, is a key step in protecting this precious natural ...