Tuesday, Jan 06, 2004 OVER MANY parts of Tamil Nadu, Karnataka, and Andhra Pradesh, exceptional stresses on water resources have led to rain harvesting as a means of intercepting flows that may otherwise be "lost" as runoff to the ocean, and put them to immediate beneficial use. While attractive in the short-term, intensive rain harvesting over long periods of time has the potential to seriously impact groundwater levels, aquatic habitats, and sediment supplies. In principle, harvesting consists of intercepting rainfall where it occurs, saving the water in various storage structures, and beneficially using it locally. The reasoning is that water that runs off in streams towards the oceans is wasted, and that it makes sense to capture the water forthwith, for beneficial use. To judge whether this reasoning is credible, one has to comprehend how our Earth functions. In the universe, life is known to exist nowhere except the Earth. Modern science unequivocally believes that life cannot exist without water. Life has sustained on our Earth for 3.5 billion years because of some extraordinary attributes of the Earth. One such attribute is the hydrological cycle, by which a small fraction of one per cent of all freshwater in the Earth is circulated over and over again. One part of rain falling on land flows in rivers and streams towards the ocean, and one part of it goes back to atmosphere as evaporation, and as transpiration by plants. The remaining part circulates underground as soil water and groundwater. It is within this simple, and yet profound framework that rain harvest needs to be understood. Because they are situated in the rain shadow of the Western Ghats, large areas of Tamil Nadu, Karnataka, and Andhra Pradesh receive limited rainfall. Also, because of the hot climate, evaporation rates from these areas are very high. Thus, the quantity of rainfall as well as the quantity of water transferred as evaporation and transpiration are beyond human control. Surface water that flows in streams plays an extremely important role in transporting sediments and nutrients, as well as sustaining the habitats of many plants and animals, collectively referred to as ecosystems. Water that infiltrates underground partly remains in the soil, sustaining plant life, and partly remains below the water table in the form of groundwater. This groundwater feeds natural springs and the wells that supply our domestic, agricultural, and industrial needs. Observations made in California a century ago have shown that groundwater gets significantly recharged by rainwater along the foothills, where highly permeable sands and gravels abound, and along stream channels downstream of foothills. It is apparent from the foregoing that undue interception and storage of rainfall over large areas can significantly affect stream flows, leading to reduced transport of sediments and nutrients, destruction of habitats, and reduction of groundwater recharge. In short, intensive rain harvesting over large areas can significantly disrupt the hydrological cycle. It is this potential for serious disruption of the hydrological cycle that needs to be borne in mind as governmental and non-governmental agencies vigorously pursue rain-harvesting with best intentions of ameliorating the water crisis. Over the past more than a century, thousands of tanks have been constructed throughout Tamil Nadu, Karnataka, and Andhra Pradesh, successfully harvesting rain. One impact of these impoundments has been that rivers such as the Palar are now practically dry, although they are known historically to have appreciable flows. Geologically, large tracts of Tamil Nadu, Karnataka, and Andhra Pradesh are underlain by hard rocks such as granite, which have very little ability to store groundwater. A bulk of the available groundwater is stored in the weathered, decomposed material overlying massive rock. This weathered rock material is usually just a few meters to a few tens of meters thick. Therefore, the groundwater reservoir in these areas is markedly shallow, with limited water holding capacity, and the quantity of water in the reservoir is vulnerable to seasonal fluctuations in rainfall. In this context, some water management experiences from the arid lands of western United States are of value. Starting from the middle of the 19th century, vast quantities of groundwater were pumped for irrigated agriculture and other uses in California, Arizona, Nebraska, Texas, and elsewhere. For all practical purposes, groundwater was treated as the property of the overlying land owner, and groundwater pumping was uncontrolled. In addition, many dams were built to divert water for irrigation, flood control and other purposes. For several decades, economy flourished, thanks to human control of water. However, over the past three decades, it has come to be recognised that surface water, groundwater, wetlands, and ecosystems are intimately interlinked. Vigorous water exploitation has led to substantial reduction in stream flows, groundwater mining, and destruction of life-sustaining habitats. Inevitably, laws are being enacted for mandatory, integrated management of surface water and groundwater. The legal basis for such legislation is a doctrine known as public trust, according to which the state owns all the water in trust for the people. Water may be used only for beneficial purposes, and water shall not be wasted. It has also been recognised that integrated water management of surface water and groundwater is best achieved with watersheds as management units. The State of Nebraska, for example, has divided its domain, for water management, into Natural Resource Districts, which coincide with watersheds of important streams. The wisdom of constructing dams to permanently solve water resources problems is being seriously questioned. Maintenance of critical levels of stream flow downstream of dams is becoming part of routine water management. These experiences are relevant to the people of Tamil Nadu, Karnataka, and Andhra Pradesh, as they grapple with their water crisis. Because of physiography, geology, and climate peculiar to their land, they have to accept the reality of limited water resources, and make a conscious decision to use the available resources wisely, and equitably. Science and technology can help understand the hydrological cycle, so as to derive its benefits without unduly disrupting it. Contrary to common reliance on the powers of modern technology, the hydrological cycle cannot be subdued by technology. Wise utilization of water resources by the people of Tamil Nadu, Karnataka, and Andhra Pradesh will require social discipline, and a willingness of society to adapt to Nature's constraints. Given these, what can be done? Even as rain-harvesting is implemented to meet the water crisis in the short-term, governments at various levels need to formulate long-term plans for integrated water development. Carefully chosen watersheds of suitable size would constitute ideal units of management. Based on climatic, stream flow, soil, groundwater, and other data, a water budget would help assess overall water availability over the watershed. To beneficially manage the available water, surface water, artificial recharge, groundwater withdrawal, and water conservation practices will become components of integrated water management. Such management cannot be based exclusively on science, technology, or economics. It has to be guided by local values, traditions, and institutions. This coming together of science and human values, in a situation of limited resources, presents extraordinary challenges and unprecedented opportunities for shared living. It may surprise many to learn that the Silicon Valley of California, so well known for Information Technology, boasts of a thoughtful water supply system, integrating surface water impoundments, artificial recharge structures, groundwater pumping, water conservation, and water education, over a well-defined watershed of the Santa Clara Valley. Between 1920 and 1940, this system evolved through a gradual realisation by the local people of the importance and of the attributes of the hydrological cycle. The development of this remarkable water supply system came about because of credible science interacting with intense public debate and referendums. At a time of preoccupation with the bounties of computer technology, perhaps the people of Tamil Nadu, Karnataka, and Andhra Pradesh may wish to gain from America's water experience. (The writer is Professor, Materials Science and Engineering Environmental Science, Policy and Management, University of California at Berkeley.)