B S Sukhija

National Geophysical Research Institute, Hyderabad-500 007, India (e-mail:[email protected])

Abstract: Extreme climate conditions are expected in the twenty-first century in the form of higher maximum temperature (with more hot days) resulting in frequent droughts. The continents of Africa and Asia are anticipated to be extremely vulnerable to droughts. In the impending extreme climate conditions, humanity's sustenance hinges on groundwater as it forms the world's largest freshwater resource. Adaptive and mitigation measures entail well planned strategies for sustained groundwater through extreme climate conditions including droughts. In this paper two such strategies are discussed to overcome the problem of droughts: (i) artificial groundwater recharge using percolation ponds; and (ii) identifying and characterizing deep aquifers resilient to droughts through detailed geophysical, hydrogeological and isotopic studies.

Percolation ponds act as artificial recharge structures which are constructed across monsoon streams with the purpose of harvesting surface runoff caused by monsoon streams. Conventional and tracer methods were developed in India to determine how effective these artificial recharge structures could be. From studies carried out on percolation ponds located in diverse geological formations such as granites, basalts and sandstones, it was concluded that these structures are quite useful for overcoming droughts in semi-arid and arid regions, and it was demonstrated that the role of geology outweighs the effect of climate on such structures. It has been shown that in a similar climatic environment, the percolation ponds in sandstones were far more efficient (efficiency 60%) than those in basalts (efficiency 20-30%).

Recently it has been realized that certain deep aquifers can yield a good quantity and quality of water even during extreme climate events. The Neyveli aquifer in southern India has been demonstrated to be such a representative aquifer for mitigation of droughts. Very extensive and intensive hydrogeological and isotopic studies on the aquifer revealed that the aquifer has distinct characteristics, namely: (i) distinct recharge area; (ii) extensive groundwater regime with high degree of recharge rate; (iii) wide span of radiocarbon ages from Modern to >30 000 years bp indicating modern as well as palaeorecharge; and (iv) minimal changes in groundwater quality despite very heavy and continuous withdrawal during the last four decades. All these criteria provide the necessary ingredients for drought resilient aquifers which can be used to identify similar aquifers elsewhere in the world.

Climate change has always had a profound impact on human society. History has recorded flourishing human society and economy in case of moderate climate and vanishing civilization in the case of extreme climatic conditions. In the context of climate extremes, the responsive sensitivity of human society and its adaptive capacity are the two factors that determine the vulnerability and the degree to which human society succumbs to the adverse effects of climate change. The adap-tiveness or adaptive capacity is the ability of a system to adjust to climate change including climate variability and extremes. Such extremes can be expected in every subsystem of the Earth system, including water cycle, sea levels, ocean circulation, ice cover, air quality and even biodiversity. This could lead to altered oceanic circulation, vertical mixing, and reduced sea-ice cover. These changes have a direct effect on society in coastal regions by way of submergence of coastal areas, erosion of shores and associated habitat, increased salinity in estuaries and freshwater aquifers, changes in sediment and nutrient transport. The coastal ecosystem is particularly at risk, for example saltwater marshes, mangrove ecosystem, coastal wetlands, coral reefs, sea-atolls and river deltas. In extreme climatic situations, human welfare hinges mainly on demand and supply of water, food and energy. In the global scenario, climate change will have a significant impact on sustainable development in different parts of the world and in different measures. Human society will rise to meet the challenges induced by climate variability by appropriate adaptation and mitigation to utilize the natural resources to the fullest benefit of society. In cases where human endeavor fails to harness the natural resources then the gap between the 'haves' and the 'have-nots' widens, which further acts as a vehicle for escalating human suffering.

It is anticipated (McCarthy eí al. 2001) that there are very likely to be extreme climate phenomena during the twenty-first century in the form of higher maximum temperature with more hot days and heat-waves over nearly all land areas, and more intense precipitation events leading to increased flood runoff, resulting in frequent droughts and floods. It is anticipated that as a result of increased temperature there will be widespread and accelerated glacier retreat, thus shifting the timing of stream flow and increasing inflows in certain regions. Consequently the water cycle will become intensified with more frequent extreme conditions including increased evaporation. Moreover, perceptible changes will take place such as high variability in quantity of water in time and space, and water quality degradation due to increased water temperature. The adverse effect of increased water temperature shows up on biogeo-chemical processes lowering the dissolved oxygen concentration of water. This effect will be offset in the case of enhanced stream flow, resulting in dilution of chemical concentration and vice versa in the case of lower stream flow.

The vulnerability of human populations to climate change differs considerably on a regional scale primarily because of differences in the base line climate and different exposures to climate stimuli in different regions. Also the severity of vulnerability depends on the adaptive capacity of the population and their management skills and resource availability. For example, the two continents of Africa and Asia are extremely vulnerable to climate adversity, and are additionally affected due to population growth notwithstanding the diminishing water resources. Africa is the continent with the lowest conversion factor of precipitation to runoff, which averages 15%, mainly due to predominant aridity in the region. Groundwater recharge measurements in India indicate the recharge rate to be as low as 3-5% of rainfall in the arid/semi-arid region (Sukhija eí al. 1996a). Thus, climate change in Asia, which has 60% of the world's population, will have a significant impact as the region is naturally dependent on the monsoon system and is economically weaker resulting in lesser adaptability. Groundwater resources, which form world's largest freshwater resource, become increasingly important in a changing climate scenario. Aquifers have a large capacity and are capable of storing large amounts of runoff water due to increased flood events, are less vulnerable to droughts and in fact are important resources of drinking water supply during calamities as they are less vulnerable to contamination, do not require large-scale engineering structures for their exploitation and even for their augmentation, and are less prone to evaporation. However, as a result of changing climate and human interventions, in certain areas of the world there is a general fall in groundwater levels. However, groundwater provides an opportunity for several adaptation methods by way of artificial recharge or managed aquifer systems to overcome the ill effects of climate change. Artificial recharge (Asano 1985) is carried out with the following purposes for circumventing the problems created by the vagaries of changing climate: (1) to reduce, stop or reverse the declining trend of groundwater levels caused due to drought; (2) to protect the groundwater in coastal aquifers against seawater intrusion; and (3) to store surplus surface runoff floodwater to augment the ground reservoir and to utilize the wastewater for future use.

The purpose of this paper is to review the adaptive methods for sustaining groundwater through droughts, and discuss in detail two methods of adaptive and mitigation measures, namely: (i) artificial recharge through percolation ponds; and (ii) identification, assessment and characterization of deep confined aquifers which are resilient to droughts.

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