Gonsalves, Julian

Langan, Simon J.

Nicol, Alan

Victor, Michael

Gonsalves, Julian F.

CGSpace

Managing Water 3Flexible Water Storage Options and Adaptation to Climate ChangeAgriculture is by far the largest human use of water. It uses 70% of global freshwater withdrawals, mainly for irrigation to supplement water for rainfed crops and livestock. Natural variability in rainfall and temperature means that, in many places, access to freshwater is already unpredictable. How climate change will alter this ‘natural’ variability is the subject of considerable study.For many millions of smallholder farmers, reliable access to water is the difference between plenty and famine. The classic response is to store water behind dams or in tanks or ponds when it is abundant and where it can be conserved for times of shortage. Water storage spurs economic growth and helps alleviate poverty by making water available when and where it is needed. Today, many developing countries, even those with abundant water, have insufficient water storage capacity.Inadequate storage leaves farmers vulnerable to the vagaries of climate. Ethiopia is one such example. Ethiopian farmers are heavily reliant on rainfed subsistence agriculture. The lack of storage infrastructure means farmers have limited ability to cope with droughts and floods. These limitations are estimated to cost the economy one-third of its growth potential. The Ethiopian case is a good illustration of the urgent need for appropriate investments in water storage to increase agricultural productivity and to ensure that farmers have options for adjusting to the coming climate changes. Water-Smart Agriculture in East Africa122of significant social and environmental costs and have adversely affected poor people. For most of the world’s large dams, downstream economic and environmental consequences have been given little attention in design and operation. Most dams were constructed with the emphasis on maximizing the economic returns from the dam itself, with little understanding of the long-term consequences of changing river flow patterns downstream.Over the last 40 years, there has been an increasing understanding of how dams modify riparian ecosystems. Using dams to regulate flow has been found to cause serious degradation of ecosystems and the natural resources and services upon which many people living downstream of the dam depend. Concerns about the negative social and environmental impacts led to reduced investment in large dams in the 1990s. More recently, there has been a reevaluation of the role of dams and though the controversy continues, investment in large dams in Africa and Asia is increasing again.Other forms of water storage and water use can also have negative environmental impacts, affecting river ecosystems and wetlands. Pumping from aquifers lowers the water table and can reduce dry-season flows and spring discharges and can cause wetlands to dry up. Even storage in small tanks and in the soil can modify flow regimes if scaled up over large areas. Dams are one of the many surface and below-surface water storage options for agriculture. Others include natural wetlands, water stored in the soil, and rainwater-harvesting ponds. Historically, irrigation depended heavily on water in rivers or naturally stored in lakes, floodplains, and wetlands.Groundwater provides much of the water used for irrigation. In India, more than 19 million pumps withdraw 230 km3 of groundwater annually. In Spain, northern China, and California, crop production is almost entirely dependent on groundwater. All groundwater originates as rainfall that percolates down through the soil into aquifers. In some places, the groundwater in these aquifers came from rains that fell many thousands of years ago when rainfall patterns were very different. Libya, for example, is currently exploiting vast reserves of water stored beneath the Sahara Desert, where almost no rain falls today. Water from these ancient aquifers is sometimes called ‘fossil water.’ Pumping fossil water is like pumping oil; once used, there is no more. Even where groundwater is recharged, if pumping exceeds the rate of recharge, water levels will fall until the aquifer is exhausted or until it becomes uneconomical to pump. This can be devastating for poor farmers as can already be seen in a number of places, including Gaza, northern China, and California. Artificial recharge of groundwater aquifers is possible (for example, using recharge ponds) and is an element of water storage that should not be neglected.Some effective methods for storing water are also relatively simple and cheap, bearing in mind that in some regions such as Ethiopia, even simple ponds and tanks are beyond the financial means of the poorest. Ponds and tanks built by individual households or communities can store water collected from microcatchments and rooftops. Individual ponds and tanks may be small in volume, but, in some places, this water is vital to supplement domestic water supplies, household gardens, rainfed crops, and livestock.Water storage and the environmentOf all the choices available for water storage, large dams are the most controversial. Many large dams contribute significantly to economic development. However, it is also true that inappropriate construction and operation have been the cause Flexible Water Storage Options and Adaptation to Climate Change 123The importance of ecosystem services is now widely recognized. Providing water to support those services is increasingly viewed as an essential use of water, along with water for agriculture, industry, and domestic use. In many countries, national legislation now makes explicit provisions to safeguard flows in rivers to protect the environment and support basic human needs.Different types of water storage also have a unique carbon footprint. Tropical hydropower reservoirs produce greenhouse gas (GHG) emissions from the decomposition of flooded vegetation and primary production. Under certain circumstances, these GHGs may exceed that of comparable fossil fuel power stations. Pumping from deep groundwater aquifers takes a lot of energy, usually in the form of electricity or diesel fuel.IWMI’s partners and research collaborators estimate that the groundwater irrigation in India accounts for about 4% of the country’s total GHG emissions.Population growth, rising incomes, and urbanization are just some of the drivers increasing the demand for water in cities and industry. Part of the problem in supplying these needs is that the pattern of demand is seldom the same for all users. For example, hydropower demand is more or less constant through the year with diurnal variations, whereas irrigation water is needed only at specific times of the year. For flood control, water levels in a reservoir need to be lowered, while irrigation requires that a reservoir be kept as full as possible. These differences are often a source of competition for, and conflict over, stored water. To reduce conflicts, it is important that everyone with a stake in the storage (including local people) participate in decisionmaking processes pertaining to the water and its use.Impact of climate change on water storage optionsClimate change will increase rainfall variability and average temperatures, affecting both the supply and demand side of the irrigation equation. In some areas of the world, annual precipitation will decline, decreasing river flows and groundwater recharge. In other places, total precipitation may increase but it will fall over shorter periods with greater intensity so that dry spells are longer. Higher temperatures will increase evaporation so that crops will use more water. Although the effects will vary from place to place, farmers will generally need to adapt to less soil moisture and higher evaporation. This means larger volumes and more frequent use of supplemental water.All storage options are potentially vulnerable to the impacts of climate change. For example, less rainfall and longer dry periods mean that soil water conservation measures may fail to increase soil moisture sufficiently for crops. Groundwater recharge may be reduced if infiltration decreases. Many near-coast aquifers will be at risk from saltwater intrusion as a result of sea level rise. Ponds, tanks, and reservoirs may not fill enough to support agriculture or may be at risk of damage from more extreme floods. Larger, more intense floods could also cause catastrophic large dam failures.The externalities created by different storage types are also likely to be affected by climate change. For example, water storage tanks, ponds, and reservoirs create breeding grounds for mosquitoes and can lead to increases in malaria and other water-borne diseases. The higher temperatures expected with climate change may worsen the situation. Similarly, adverse environmental impacts, arising from changes in the flow regimes of rivers, may be exacerbated by climate change. Factors such as these must be considered in the future planning, design, and operation of water storage schemes.Water-Smart Agriculture in East Africa124Role of water storage in climate change adaptationWith increased uncertainty, higher demand, and greater competition, water storage is only one component of a multipronged approach for adapting agriculture to climate change. Future water resource management must also include reallocation of water between users and increasing water productivity wherever possible. There is no doubt that providing more and diverse physical storage infrastructure is an imperative for securing reliable supplies of water for agriculture and other uses.Each type of storage has its own niche in terms of technical feasibility, socioeconomic sustainability, impact on health and environment, and institutional requirements. Each needs to be considered carefully within the context of its geographic, cultural, and political location. With so much uncertainty in climate change scenarios, the best option is to focus on flexibility in storage systems, wherever possible combining a variety of types to take advantage of their unique characteristics.Poor farmers already struggle to cope with changing and unpredictable weather patterns and this will be worsened by climate change. As climate change becomes a greater threat to water systems and agriculture, variety in the types of water storage systems used will provide an important mechanism for adaptation. However, the types of storage must be tailored to the specific needs and socioeconomic conditions of an area. Planners need to start taking climate change into account when they design and manage integrated storage systems.Type of farming system Possible biophysical risks associated with climate changeReservoirs  6 Reduced inflow, resulting in longer periods between filling 6 Higher evaporation, increasing the rate of reservoir depletion 6 Infrastructure damage as a result of higher flood peaks 6 Improved habitat for disease vectors (e.g., mosquitoes) 6 Increased risk of eutrophication and salinization 6 Increased siltationPonds and tanks  6 Reduced inflow, resulting in longer periods between filling 6 Higher evaporation, increasing rates of pond/tank depletion 6 Infrastructure damage as a result of higher flood peaks 6 Improved habitat for disease vectors (e.g., mosquitoes) 6 Increased risk of eutrophication and salinization 6 Increased siltationSoil moisture  6 Reduced infiltration resulting from modified rainfall intensities 6 Waterlogging resulting from modified rainfall intensities and duration 6 Longer dry periods resulting from altered temporal distribution of rainfall 6 Depleted soil moisture arising from higher evaporative demand 6 Soil erosion resulting from modified rainfall intensities and duration 6 Reduced soil quality (including water-holding capacity and nutrient status) resulting from modified rainfall and temperatureAquifers  6 Reduced recharge resulting from modified rainfall intensities 6 Reduced recharge resulting from land-cover modification and increased soil moisture deficits 6 Saline intrusion in near-coast aquifers 6 Increased percolation through frequent floodingNatural wetlands  6 Reduced rainfall and runoff inputs resulting in wetland desiccation 6 Higher flood peaks resulting in wetland expansion and flooding of fields and homes 6 Improved habitat for disease vectors (e.g., mosquitoes) 6 Retreat of glaciers due to higher temperatures and altered precipitation patterns Flexible Water Storage Options and Adaptation to Climate Change 125McCartney, M.P. 2007. Decision support systems for large dam planning and operation in Africa. Colombo, Sri Lanka: International Water Management Institute. 47p. (IWMI Working Paper 119). www.iwmi.org/Publications/Working_Papers/working/WOR119.pdfMolden, D. (ed.). 2007. Water for food, water for life: a comprehensive assessment of water management in agriculture. London, UK: Earthscan and Colombo, Sri Lanka: International Water Management Institute. 645 p.Shah, T. 2009. Climate change and groundwater: India’s opportunities for mitigation and adaptation. Environ. Res. Lett. 4 035005. 13 p.Zomer, R.J., Trabucco, A., van Straaten, O., Bossio, D.A. 2006. Carbon, land and water: a global analysis of the hydrologic dimensions of climate change mitigation through afforestation/reforestation. Colombo, Sri Lanka: International Water Management Institute. 44 p. (IWMI Research Report 101). www.iwmi.org/Publications/IWMI_Research_Reports/PDF/pub101/RR101.pdfSourceFlexible Water Storage Options: for adoptation to climate change, Water Policy Brief, Issue 31. 2009 International Water Management Institute (IWMI). Colombo, Sri Lanka: IWMI. 5 p. ReferencesInternational Water Management Institute, IWMI-TATA Water Policy Program. 2003. Rethinking tank rehabilitation: issues in restoring old tanks to their original state in irrigation structure. Vallabh Vidyanagar, Gujarat, India: International Water Management Institute (IWMI). 8 p. (IWMI Water Policy Briefing 7). www.iwmi.org/Publications/Water_Policy_Briefs/PDF/wpb07.pdfKeller, A., Sakthivadivel, R., Seckler, D. 2000. Water scarcity and the role of storage in development. Colombo, Sri Lanka: International Water Management Institute. 27 p. (IWMI Research Report 39). www.iwmi.org/Publications/IWMI_Research_Reports/PDF/Pub039/Report39.pdfKim, U., Kaluarachchi, J.J., Smakhtin, V.U. 2008. Climate change impacts on hydrology and water resources of the Upper Blue Nile River Basin, Ethiopia. Colombo, Sri Lanka: International Water Management Institute. 27 p. (IWMI Research Report 126). www.iwmi.org/Publications/IWMI_Research_Reports/PDF/PUB126/RR126.pdf

Managing water: Flexible water storage options and adaptation to climate change

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Managing water: Flexible water storage options and adaptation to climate change

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