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4 Water Resource Availability Water is essential to human survival and critical to the success of agri- cultural systems. This chapter examines what is known about the quantity of water available to farmers in sub-Saharan Africa (SSA) and South Asia (SA) and the projected impact of climate change on water availability in these regions. Technologies for increasing the quantity of water available to agriculture and for increasing the ability to manage and conserve water resources are discussed in this chapter. Water and soil are tightly linked and conservation of those combined resources is addressed in Chapter 5. Water Resources in sub-Saharan Africa SSA is a region of diverse climate, from tropical humid to arid, and much of the continent is influenced by the monsoon season. The continent and its climates are criss-crossed by numerous rivers (Figure 4-1). In the humid regions, rivers and groundwater networks overlap. In arid regions, groundwater resources are not connected with surface rivers, so ground- water recharge (by rain) is more important than in humid areas. The base flow of rivers in the arid regions is also low, and evaporation is very high. Thus, some countries have less water flowing out than flowing in. In SSA, a horizontal band of countries forms the Sahel, an arid transi- tional zone between the desert to the north and the tropical regions to the south. Water resources in the Sahel are limited and unevenly distributed. The Nile River flows through the east Sahel; the west is served by the Niger. The Niger is also the major river system of the tropical, monsoon-influenced climate of the Gulf of Guinea on Africaâs west coast, an area that makes up 123
124 Emerging Technologies to Benefit Farmers FIGURE 4-1â Major rivers of Africa. SOURCE: From de Wit, M., and J. 4-1.eps Stankiewicz. 2006. Change in Surface Water Supply Across Africa with Predicted Climate Change. Science 311:1917-1920. Re- bitmap image printed with permission from AAAS. one-fourth of Africaâs water resources. East of the Gulf of Guinea, the coun- tries of humid, tropical central Africa are served by two major riversâthe Congo and the OoguuÃ©âthat make up almost half the continentâs water resources. In eastern Africa, the climate ranges from semi-arid to tropical humid, and the major river in this subregion is the Nile. Although the water resources in this region are limited, Africaâs largest lake, Lake Victoria, is there. The climate in southern Africa is diverse, from subtropical humid to arid. The Zambezi, Limpopo, and Orange Rivers serve the region, but the water resources are modest, and some groundwater reserves are not renew- able (FAO, 2003), although two large inland lakes, Lake Malawi and Lake Tanganyika, are in the region.
Water Resource Availability 125 Water Resources in South Asia SA is also a region of diverse climate and water resources. The western parts of SAâAfghanistan, Pakistan, and northwestern Indiaâare char- acterized by dry climates, whereas very humid climates prevail in eastern India, Bangladesh, Bhutan, Nepal, and Sri Lanka (Figure 4-2). Rainfall in the entire region comes primarily during the annual monsoon, when rain falls for a period of 3 months or less, often very intensely, and leads to se- vere runoff over sloping land and inundation of flat terrain. The variability and unpredictability of the monsoon rains in time and space create great difficulties for farmers, who base their planting, fertilizing, and other pro- duction decisions on their expectations of the timing and amount of rain. Vastly more water falls on the humid and subhumid regions of India and Bangladesh during the monsoon than on the arid regions of the west. The annual precipitation in India, including snowfall, is nearly 4 trillion cubic meters, and 75 percent of that is from the monsoon (Mohapatra and Singh, 2003). In contrast, the arid regions of northwestern and western AFGHANISTAN IRAN NEPAL PAKISTAN BHUTAN DRY ZONE HUMID ZONE INDIA BANGLADESH 90 Limit of 90-day LGP Boundary between dry and humid zones SRI LANKA FIGURE 4-2â The South Asia region showing the approximate boundary line at 4-2.eps which rainfall or soil moisture is adequate to support a 90-day-long growing period for crops. replaced type SOURCE: Food and Agricultural Organization, 1994. Reprinted with permission. Â© 2006 by the Food and Agriculture Organization of the United Nations.
126 Emerging Technologies to Benefit Farmers SA, which experience severe water shortages during the 9-month-long dry season, receive 80 percent of their water from the seasonal melting of snow in the Hindu Kush mountain range. Most of the entire SA region has high temperatures during the summer, when soil temperatures exceed 45Â°C at a depth of 1 cm (Gupta and Gupta, 1986). Thus, the rate of water evapora- tion is high (Lal, 2006). Demand on Water Resources in subâSaharan Africa AND South Asia Having a sense of the water resources of a region is important, but it is more relevant to know how accessible the water is and how rapidly it is used relative to the rate at which it is recharged. SSA and SA are very dif- ferent from each other in both regards. Since the 1960s, SA has emerged as the worldâs largest user of groundwater for irrigation (Shah et al., 2006). Indeed, groundwater is used in over 75 percent of the irrigated areas in some parts of India, Pakistan, the Terai region of Nepal, and Bangladesh. In contrast, only 6 percent of Africa is under irrigation, and far fewer wells and irrigation systems exist to bring water to crops. Not surprisingly, the limited accessibility of water makes a huge difference in agricultural productivity. Grain yields are linked to rainfall in countries where almost all the agriculture is rain-fed. In Ethiopia, the interannual oscillations of national grain production mirror variation in rainfall, and so does the gross domestic product (see Figure 4-3). The huge irrigation demands on water resources in SA are competing with industrial and other uses, and excessive withdrawal of groundwater is leading to reduction in water levels, drying up of wells, and increase in problems of water quality, including the problem of arsenic in groundwater (Reddy et al., 2000). It is estimated that as many as 25 percent of farmers in India are overtapping the aquifers and withdrawing water faster than it is being recharged (Pearce, 2006; Postel, 2006). From 20 to 30 percent of the renewable water resource in SA is with- drawn each year for various uses, in contrast with SSA, where total water withdrawal as a percentage of renewable water is only around 3 percent (Table 4-1). Water availability can also be described as a function of the size of the population. The total renewable water resource per capita in South Asia is about 1,591 m3/year. Given population growth and other demands, renewable per capita freshwater resources in SA are likely to be severely constrained in the future, especially in Afghanistan, India, and Pakistan. In contrast, Africa has only 13 percent of the worldâs population, and its total renewable water resource per capita is 5,000 m3/year. Because of low population density, even countries in the Sahel, on the average, have
Water Resource Availability 127 Percent rainfall variation around the mean Percent change in GDP growth 80 25 20 60 15 40 10 20 5 0 0 -5 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 -20 -10 Year -40 -15 rainfall variation around the mean -20 -60 GDP growth -25 -80 -30 FIGURE 4-3â Rainfall and growth in gross domestic product in Ethiopia, 1982-2000. 4-3.eps SOURCE: World Bank, 2006. Reprinted with permission from D. Grey and C. Sadoff. adequate per capita renewable water resources. The problem is that the water is not always accessible to the population. Of the 47 countries of SSA, six (Comoros, Eritrea, Lesotho, Malawi, Somalia, and Zambia) experience a moderate deficit of renewable water resources of about 1,500 m3/year per capita, and six others (Burkina Faso, Cape Verde, Djibouti, Kenya, TABLE 4-1â Total Water Withdrawal by Volume and as Percentage of Renewable Water Water Withdrawal Water Withdrawal (km3) (% of Renewable water) 1995 2010 2025 1995 2010 2025 Region Baseline Projection Projection Baseline Projection Projection India 750 750 815 30 33 36 South Asia 353 391 421 18 20 22 (excluding India) Sub-Saharan Africa 128 166 214 2 3 4 World 3,906 4,356 4,772 8 9 10 SOURCE: Rosegrant et al., 2002. Adapted and reproduced with permission from the Inter- national Food Policy Research Institute (www.ifpri.org) and the International Water Manage- ment Institute (www.iwmi.cgiar.org).
128 Emerging Technologies to Benefit Farmers Rwanda, and South Africa) have severe deficits of 1,000 m3/year or less per capita (UNESCO, 2006). Water Resources and Climate Change Climate change models predict scenarios for SSA and SA that may drastically affect the availability of water in parts of these regions. In most climate change projections, most of SSA will probably be warmer than today, and the rate of warming will be more than the global average. In fact, Africa has already been getting warmer, at the rate of about 0.05Â°C per decade (IPCC, 2001). Projected changes in rainfall are much less clear. Two analyses of how a theoretical 10 percent decrease in rainfall would affect the flow of Africaâs surface waters found that the flow in rivers in Africaâs wettest regions would decrease by 17 percent (Nyong, 2005) and that the semi-arid zones would experience a disproportionately larger decrease of 25 to 77 percent, with the most severe decreases in southern Africa and the Sahel (de Wit and Stankiewicz, 2006). If those projections are realized, the change in water availability will be devastating in some regions. In SA, climate change promises to be just as ominous, although most climate change models predict that central and eastern Asia will become wetter. However, irrigation systems in Pakistan and India are served by four rivers (Indus, Ganges, Brahmaputra, and Yamuna) that originate in the glaciers and snow of the western Himalayas. About half the annual snow and glacier melt from the high mountains is used in irrigation (Winiger et al., 2005). Therefore, it is of great concern that snowfall amounts in the Himalayan region have been progressively declining while melting has increased and caused floods. Quantitative comparisons of satellite images of 1972, 1989, and 2000 reveal a decline in the annual average surface snow rate and the deposition of dust over the snow and glaciers. From 1972 to 2007, the observed surface temperatures have increased by 8 kelvin (Prasad and Singh, 2007). The more than 700 million people who live in the Indo- Gangetic Basin are likely to face a future with less water (Brown, 2001). To add to the gloomy picture, severe weather events are expected to increase in both SSA and SA and to result in floods and droughts in an un- predictable pattern. Recently, more than 340,000 people were displaced in southern Africa because of flooding that began in December 2007. Erratic monsoon rains have led to the loss of crops, livestock, property, and human lives throughout SA. The average area annually inundated by floods is 7.6 million hectares, and 33 million people are affected in India alone (Mo- hapatra and Singh, 2003). The problem is even more serious in the densely populated deltaic region of Bangladesh. Joining the rivers of the north to
Water Resource Availability 129 those of the south and west has been considered a long-term solution to the flooding, but an objective and careful analysis of such a plan is necessary. Technologies for water management Anticipating changes in water resources, planning for shortages (or excesses) of water, managing the use of the resource, and finding ways to increase the availability of water in SSA and SA should have high priority for nations in these regions. In the industrialized world, a great deal of ac- tivity is under way to deal with anticipated constraints on water resources. The technologies described here are some that are worthy of exploration in the context of SSA and SA. On-Farm Integrated Water Management As noted earlier, SSA and SA differ dramatically with respect to ir- rigation. In SA, large areas of cropland are irrigated, from 30 percent of cropland in Afghanistan to 80 percent in Pakistan (WRI, 2005; Lal, 2007). India uses water at 200 km3/year for irrigation, the equivalent of 3 times the flow of water in Chinaâs Yellow River. But the water-related productiv- ity of irrigated rice in India is comparatively low, averaging grain yields of just 0.4 g/kg of water used compared with up to 7 g/kg of water in the Philippines (Kar et al., 2004). In contrast, only about 5 percent of the potentially irrigable land in SSA is under irrigation, and two countries (Sudan and Madagascar) account for about 60 percent of the irrigated land. However, if irrigation in SA is inefficient, so are the rain-fed farms of SSA, where evaporation and runoff lead to large losses. In SSA, the priority need is to expand the land area for water capture and irrigation. The integration of techniques to manage water on the farm is one of the most important set of technologies that exist to transform agriculture in SA and SSA. On-farm water management includes techniques for water capture, storage, pumping, transfer, field application, and drainage tech- nologies. On the scale of the small farmer in Africa or India, water capture has typically involved small dams to pool and store surface water. An im- portant development for small farmers has been the drilling of small-scale, individual tube wells, typically powered by treadle pumps, and the develop- ment of small lined reservoirs for local storage of water. The delivery of water to plants via irrigation is a critical step in water and soil conservation. Flood or furrow irrigation, the predominant method in India, is the most inefficient form of irrigation, although land leveling (essentially the removal of high and low spots of land) using animals or
130 Emerging Technologies to Benefit Farmers tractors tied to a leveling bucket that is dragged across the earth, can im- prove irrigation efficiency (Jat et al., 2006). Over the last decade, micro-irrigation, primarily drip irrigation, has been increasingly adopted because of the availability of low-cost tubing and pipe systems (Wallace, 2000; Panigarhi et al., 2001; Viswanathan et al., 2002; Aujla et al., 2005). Studies comparing drip irrigation to conventional surface irrigation in a variety of cropping systems revealed water productiv- ity gains ranging from 91 to 149 percent (Molden, 2007). Water can be used most efficiently if it is applied to the active root zone of plants. Subsurface drip irrigation (SDI), which consists of buried plastic tubes containing regularly spaced, embedded emitters (pores), is a technol- ogy with some potential for farmers in arid regions of the world, if the costs of the technology could be addressed. Although SDI has existed for over 20 years in the United States, SDI has been used on less than 25,000 hectares, primarily in Arizona and California. One of the reasons that SDI is not used more widely is that until recently, design requirements to match system characteristics with soils and crops had not been well defined. The initial use of SDI has been to irrigate annual row and field crops and per- manent crops, like citrus. Drip tubes have been typically located 130 to 210 cm apart, and 15 to 25 cm below the soil surface. However, emerging design standards will permit SDI to be used for any crop, including those that are planted neither in beds nor in rows. SDI can be adapted to grow any crop (e.g., corn, wheat, rice, sorghum, soybean, cassava, yam) and can produce the âultimateâ in water use efficiency for open field agriculture, resulting in water savings of 25-40 percent in comparison with flood irriga- tion. The typical efficiency of SDI is 90 percent compared with 60 percent for conventional furrow 80 percent for furrow with valve and also for low-pressure sprinkler. However, SDI also has limitations, primarily its cost and the possibil- ity of clogging over time. Work is under way to make a low-cost SDI sys- tem, but its installed cost will probably be about $0.20/m2 as opposed to $0.084/m2 for a surface tube irrigation system (Jack Keller, Keller-Bliesner Engineering LLC, personal communication, April 29, 2008). That might put SDI out of reach for all but small-scale growers of high-value crops. If it could be made more affordable, SDI offers a number of other potential advantages over other types of irrigations systems, including the following: Better nutrient managementâWhen water is applied to the surface of the soil, it carries away nutrients like nitrogen. With SDI, water is applied at the roots, so less nitrate is leached from the soil. If used in conjunction with fertilizer (fertigation), the most active part of the root zone will receive nutrients more directly.
Water Resource Availability 131 Better weed controlâA dry soil surface means that weed seeds may have a more difficult time germinating, and the absence of moisture on the parts of the crop above the soil may reduce conditions for disease. Ability to use waste waterâBecause water is applied below the surface, contamination of the crop with disease-causing microorganisms is greatly reduced. Thus the opportunity to use wastewater will result in even greater efficiency. However, this would not be appropriate for root crops (e.g., cas- sava, yam, radish, turnip, carrot, potatoes). Longer system lifeâBecause they are placed underground, drip lines are protected from damage due to cultivation and other farm operations. Furthermore, buried tubes will last longer than those above ground, due to the exposure to heat and ultraviolet sunlight. The robustness and longer life of SDI systems makes their comparatively higher costs economical in the long term, relative to above ground systems. Water Storage The storage of water in large aboveground tanks during the mon- soon season and its use during the dry season for irrigation and human and livestock needs have been practiced in SA for millennia. Tanks have been a traditional common-property resource, especially in southern India (Anbumozhi et al., 2001). The efficiency of the ancient technology is low because of the large losses during conveyance and through evaporation and percolation. In the last 4 decades, systems have evolved to store water underground in aquifers, and a recent study by the National Research Council found that, given the âgenerally successful track record of managed underground storage [MUS] in a variety of forms and environments, MUS should be seri- ously considered as a tool in a water managerâs arsenalâ (NRC, 2008). In MUS systems, surface water, groundwater, treated effluent, and occasionally storm water are stored in different types of underground aquifersâfrom unconsolidated alluvial deposits to limestone and fractured volcanic rocks. Water to be stored is directed into an aquifer through recharge basins or recharge wells and recovered for use with extraction wells or dual-purpose recharge-extraction wells. The recovered water is used for drinking, irriga- tion, industrial cooling, and environmental and other purposes. The science and technology of MUS are still emerging. Scientists are learning through experience that the matrix, hydrogeological, and geochem- ical characteristics of some types of aquifers are better suited than others for storing water and that different recharge, storage, and recovery methods are needed for different aquifers. Research is needed to assess the suitability of recharge sites and the hydrogeological characteristics of candidate aquifers.
132 Emerging Technologies to Benefit Farmers The potential interactions of the stored water with other surface-water and groundwater supplies must also be carefully investigated. It is well documented that underground storage has âthe capacity to attenuate many chemical constituents and pathogens via physical (e.g., fil- tration and sorption), chemical, and biological processes. In places where the groundwater quality is saline or otherwise poor, the implementation of MUS will likely improve overall groundwater quality and provide a benefit to the aquiferâ; and âa monitoring program is needed to document the water quality behavior and establish the reliability of the MUS system. This will involve installation of monitoring points to track the behavior of the water and the constituents in the water as the source water is introduced, stored, and eventually extractedâ (NRC, 2008). In general, there appears to be a rich body of research that supports the use of MUS in arid parts of the United States, and this may be a subject for exploration in SSA and SA. Such research could yield substantial benefits to farmers, assuming that efficient systems for bringing stored water back to the surface and distributing it for use in agriculture can be devised. Wastewater Reclamation Because of its ability to improve water quality, MUS might be used in conjunction with treatment of urban storm-water runoff and munici- pal wastewater. Wastewater is already used in periurban agriculture but often without substantial treatment standards (Van Rooijen et al., 2005). Modern wastewater-treatment plants typically treat wastewater biologi- cally and then pass it through a final sand filtration step before it is used for irrigation (NRC, 2004). Wastewater reclamation has benefited from the recent development of membrane bioreactorsâbioreactors coupled to filtration unitsâthat enable biomass to be concentrated without impeding the flow of water through the filter (Daubert et al., 2003). The treatment of wastewater might also be accomplished by nanofiltration devices that are rapidly emerging for small applications, such as household use. Although the committee was not able to undertake a thorough investigation of all these devices, several may have applications on the scale that could make wastewater a source of both irrigation and drinking water (see Box 4-1). Desalination One way to create additional water supply is to remove salt from sea- water or inland brackish aquifers. Desalination technology is evolving and, in addition to removing salt, has been proposed as a method for treating wastewater. The committee was not able to conduct an in-depth exploration of desalination technologies, but one expert who addressed the committee
Water Resource Availability 133 BOX 4-1 Nanomaterials for Water Purification Several nanomaterial-based products that are both economical and effective in purifying water have been developed, including â¢ he NanoCeram filter that uses a positive charge to attract negatively charged T viruses and bacteria (20 to 100 nm). â¢ fused carbon nanotube mesh that can filter out waterborne pathogens, lead, A uranium, and arsenic. â¢ arbon nanotube filters for water filtration. C â¢ inc oxide nanoparticles to remove arsenic from water with an âat-the-tapâ Z purification device. â¢ anoparticle filters to remove pesticides (such as DDT, endosulfan, malathion, N and chlorpyrifos) and other organic particles from water. suggested that for cost and energy considerations, desalinated water should be produced for human consumption and municipal wastewater used for agriculture (Donald Slack, University of Arizona, presentation to the com- mittee, October 16, 2007). However, another expert suggested that small- scale desalination was possible and economical for specialized applications, such as the production of high-value greenhouse crops, and that integrated systems could be engineered for this purpose (David Furukawa, presenta- tion to the committee, October 16, 2007). A review of the U.S. desalination and water purification roadmap pro- vided a cogent summary of the state of the technology, its potential, and research directions (NRC, 2004). That review is a useful vantage point for considering what developments in desalination technologies might benefit small-scale farmers in SSA and SA. Because profit margins on clean water production by desalination are small, commercial interest in the technology is weak. Most desalination projects are heavily subsidized with public funding, and research focuses on reducing costs by expanding economies of scale and optimizing operational efficiency. Nevertheless, there have been major advances in the performance and cost of membranes used in reverse osmosis, one of the two most com- mon methods of removing salt from water; the other major method is thermal distillation, which is used throughout the Middle East because of the lower costs of fuel in the region. The major cost of either type of operation is power (which accounts
134 Emerging Technologies to Benefit Farmers for 44 percent of the cost of reverse osmosis and 59 percent of the cost of thermal distillation). The other major expense is capital costs. Although the use of alternative sources of energy was discussed in the National Research Council report, it focused on reducing energy costs in reverse-osmosis plants by improving the water-pretreatment processes and the precision of the membranes in removing specific contaminants or salts and in thermal- distillation plants by cogenerating the heat needed for desalination with electrical power generation. In either of the two dominant techniques, the current cost of desalination is about $2 to $3 per 1,000 gallons of seawater and $1 to $1.50 per 1,000 gallons of brackish water (Hinkebein, 2004). Two other thermal techniquesâsolar distillation and membrane dis- tillationâhave remained somewhat undeveloped (Buros, 2000). In solar distillation, salt water in a shallow basin is evaporated by the sun and condensed on a sloped glass roof. In membrane distillation, the vapor from heated salt water passes through a membrane (which allows vapor but not water to pass) and then condensed. Those methods require more space (and more energy per unit of clean water produced), but their simplicity and the need for only small temperature differentials to operate make them viable technologies where inexpensive thermal energy, such as that from solar col- lectors, is available (Cooley et al., 2006). Two other aspects of desalination that are worthy of further explora- tion for SSA and SA are related to the disposal of the salty concentrate that remains after desalination. In some applications, the concentrate is disposed of at sea, where dilution theoretically minimizes adverse effects. In an inland situation, an alternative is to deposit the salty brine in a solar-energy pond, where the lower dense layers of salty water reach high temperatures. A heat exchanger can be used to extract the stored energy from the bottom layer of the pond (NRC, 2004). The National Research Council report (2004) also speculated on whether the leftover salt concentrate might have com- mercial value. Because membrane systems can be designed to be selective in their recovery of chemical compounds, the production of commercially valuable salt solidsâsuch as gypsum, sodium chloride, and magnesium sulfateâmay be possible. Weather Modification: Cloud Seeding Cloud seeding involves the introduction of agents into a cloud to in- crease the efficiency of its precipitation. Commonly used agents include silver iodide, dry ice (granulated solid carbon dioxide), and salt. The agents are introduced by ground flares or deployed from an airplane (Hunter, 2007) and act as nuclei around which water vapor coalesces into ice crystals that are released from clouds as snow or rain. The practice of seeding clouds began in the 1940s and remains some-
Water Resource Availability 135 what controversial, generally because of the difficulty of measuring the ef- fect of seeding on the amount of precipitation. A 2003 National Research Council report concluded that the overall efficacy of intentional cloud- seeding efforts was still unproved, not on the scientific basis of the weather modification concepts, but on the âabsence of adequate understanding of critical atmospheric processesâ that reduce its predictability (NRC, 2003). In spite of its uncertain results, cloud-seeding programs have been implemented in at least 24 countries, including India, Zimbabwe, Burkina Faso, South Africa, Honduras, Mexico, Cuba, Australia, Thailand, Egypt, Israel, Japan, the United Arab Emirates, China, and the United States (Salleh, 2007); the largest number of individual projects take place annually in China and the United States (Chalon, 2007). In recent years, weather- modification projects in the United States have been implemented in 11 western states with funding by state agencies and private (mostly hydro- electric power) agencies. In California, cloud seeding has been conducted to increase the snowpack in the Sierra Nevada Mountains since the 1950s (CDWR, 2005). States and countries pursue the programs because of the need to aug- ment local water resources. Most of the projects are not research-oriented; that is, they are implemented with little scientific guidance that might improve their chances of success. And the programs do not methodically document their circumstances or quantify their results. However a few cloud seeding experiments (see Box 4-2) have been repeatedly scrutinized by the scientific community, and some successes have been confirmed, even if many questions remain about what is taking place in the clouds (Silverman, 2003). The individuality of clouds and the weather systems in which they op- erate confound the transferability of seeding techniques because scientists lack a reliable record of the conditions and outcomes of cloud-seeding trials and need better models of the physical and chemical interactions in clouds. A National Research Council report (NRC, 2003) described a framework of basic research needs on weather modification and recommended steps to validate cloud-seeding operations that include statistical evaluations of their effects, tracking of introduced seeding agents and their effects in cloud cells, and physical measurement of rainfall. Cloud seeding is not a solution for breaking a drought or making it rain where no clouds exist. Its potential for SSA and SA might lie in increasing overall rainfall that could be captured and distributed to farmers and in avoiding some of the otherwise inevitable effects of climate change. For example, the Himalayan glaciers have been receding rapidly (WWF, 2005), and enhancing snowfall in these regions could increase the availability of water to the Indus River Basin that relies on the spring thaws. Typically, only about 30 percent of the atmospheric water vapor that enters southern
136 Emerging Technologies to Benefit Farmers BOX 4-2 Cloud-Seeding Experiments Silverman (2003) analyzed four datasets from controlled experiments in which clouds were randomly seeded or not seeded from airplanes with different kinds of nucleating materials. Two experiments were conducted in cold convective clouds (in South Africa from 1992 to 1997 and in Mexico from 1997 to 1998), and two were conducted in warm convective clouds (in Thailand from 1995 to 1998 and in India from 1973 to 1986). In all cases, statistically significant differences were observed between the experimental and control clouds on the basis of radar- based estimates of rain mass that was produced. However, the hypothesis that the seeding material would act to increase the efficiency of the rain-forming process by accelerating the coalescence of particles in a cloud to a size at which precipita- tion would occur was not supported by physical measurements of particle sizes in the clouds. In summary, although the experiments appeared to have worked, there was no independent confirmation of the cause-effect aspects of seeding. Africa reaches the ground as precipitation (Mather et al., 1997). Increas- ing the efficiency of rainfall by 10 percentâa figure that the American Meteorological Society considers to be the conservative potential of the technologyâcould significantly affect water availability. Because clouds are large systems, precipitation gains from cloud seeding in one location are thought to be incremental and additive; that is, seeding does not cre- ate rain in one location at the expense of another along a cloudâs path of travel. However, in reality, the models and measurements that can be more definitive on this point are only now evolving. The cost-to-benefit ratio of cloud-seeding projects in the western United States has been estimated at $1-20/acre-feet of water produced. In the case of Wyoming, where an estimated 10 percent increase in snowpack in the projectâs targeted areas would provide 130,000 to 260,000 acre-feet of water in additional runoff each spring, conservative estimates value the extra water at $2.4 million to $4.9 million. Of course, the scarcity of water determines the monetary value of additional water supplies, but the amount of water that would be needed to support farmers on a large scale across a large landscape requires further evaluation (UCAR, 2006). Weather-modification technology goes hand in hand with weather pre- diction, and efforts to improve one will assist in improving the other. Be- cause of the major impact of weather and water availability on farmers, research on both should be pursued with the involvement of local institu-
Water Resource Availability 137 tions and with adequate training, funding, and equipment to increase their ability to participate. Weather and Climate Forecasting For four reasons, SSA and SA would benefit from climate and weather forecasting: early warnings of severe weather could be provided to rural populations; farmers and planners could be informed of the likelihood and intensity of drought; forecasters could help farmers to anticipate the onset of the monsoon or rain in general; and long-term climate change that would affect the lives of the rural poor in these regions could be predicted. The effects of regional human activity on local weather might also be discerned. For example, it has been observed in Indonesia and the Amazon that par- ticles of soot (from burning of forests) may have inhibited rainfall despite the presence of clouds (Rosenfeld and Woodley, 2003). Physical evidence of cloud changes induced by air pollution downwind of urban areas has been documented with satellite measurements (Rosenfeld, 2000). Climate scientists and meteorologists worldwide use observational data provided by numerous remotely positioned satellites, which they access by subscription through Earth-based receiving stations or the Internet. The data, often represented as an image or a map, are assimilated into a climate model by using algorithms (rules for incorporating the data); as datasets are collected and integrated, the models are used to predict climate trends and weather events. Until recently, however, predictive weather and climate models have relied on global averages of climate data and have focused on upper atmospheric processes even though it is known that such landscape features as soil moisture, terrain, and type of vegetation affect regional and local climate and weather. That information was not consistently avail- able for a given swath of land, and the algorithms to integrate it into cur- rent models did not exist. As a result, the predictive power of the models has been relatively weak with respect to regional outcomes (Pielke et al., 2007). In the last decade, remote devices that can measure those characteristics at increasingly high resolution have been placed on satellites, and they offer a much better opportunity to understand how local landscapes, anthropo- genic activity, and geophysical processes influence mesoscale convections of wind, clouds, and rainfall. The new datasets form the basis of an emerg- ing technology that could provide important tools for farmers and others in SSA and SA. If algorithms for new models can be developed for those regions with specific applications for agriculture, for severe weather alerts, and prediction of such long-term weather events as the coming of the mon- soon, farmersâ decision-making capability could be greatly improved. Many remote measurement tools are in development or have been de-
138 Emerging Technologies to Benefit Farmers ployed recently, but it was beyond the expertise of the present committee to understand the potential utility of each unique source of data fully. Four examples of such devices are described below. Moderate-Resolution Imaging Spectroradiometer Two moderate-resolution imaging spectrometer (MODIS) instruments have been placed on satellitesâone launched in late 1999, the second in 2002. MODIS generates global maps of several land surface characteristics: surface reflectance, land surface temperature, and vegetation indexes, such as the density of vegetation. MODIS can distinguish urban areas from non- urban areas and distinguish among 11 categories of vegetation (deciduous forests, coniferous forest, crops, grassland, and so on) and between bare soil and water. It can be used to monitor water quality by sensing the tur- bidity and dissolved oxygen in surface water (rivers, lakes, and estuaries). Because the maps are generated daily, they provide data on changes in land cover and land uses that can be fed into models of climate and weather. MODIS provides data all day and every day, so it is considered complemen- tary to LandSat, an older remote observation system that provides data at a higher resolution but only once every 16 days. Gravity Recovery and Climate Experiment Launched in 2002, the Gravity Recovery and Climate Experiment (GRACE) consists of twin satellites positioned bout 220 km apart in space. They track changes in Earthâs gravity field by sensing tiny changes in the distance between the satellites with a microwave ranging system and a global positioning system. The motion of water and air on time scales ranging from hours to decades contributes to variations in Earthâs gravity field that affect the relative positions of the two satellites. Every 30 days, GRACE generates a model and a global map of changes in Earthâs gravity field that correspond to mass changes caused in part by the movement of water. By combining information from GRACE with soil-moisture and other data, hydrologists can monitor groundwater storage in aquifers on a monthly basis. GRACE was used to measure the rapid melting of the Greenland ice sheet essentially by detecting changes in its mass. It has a wide variety of applications for estimating changes in water quantity in snowpack, lakes, river basins, aquifers, and soils and for evaluating and modeling such pro- cesses as river runoff and evapotranspiration. The sensitivity of GRACE is such that it detects changes in Earthâs mass in response to weather patterns and climate change in periods of at least a month (Adam, 2002).
Water Resource Availability 139 The expansion of irrigation, especially in SSA, requires credible assess- ment of groundwater and surface-water resources. In that regard, remote sensing systems and geographic information systems can be extremely useful (Chowdary et al., 2003). The same technology can also be used to monitor the risks of non-point-source pollution of groundwater (Chowdary et al., 2005). Tropical Rainfall Measuring Mission The Tropical Rainfall Measuring Mission (TRMM) was launched in 1997 by the U.S. and Japanese space agencies. It consists of a satellite- based precipitation radar (the only one of its type in space) that can provide three-dimensional images of clouds; a microwave instrument that provides quantitative estimates of rainfall, water vapor, cloud water content, and sea surface temperature; a visible and infrared spectrum scanner; and a lightning detector. The data that TRMM has provided have been of great use to the scientific community in learning about tropical weather systems and the structure of hurricanes and typhoons. For a number of reasons, the data have not yet been incorporated into weather-prediction models. How- ever, they do reveal interesting information, for example, that in monsoon regions urban areas have more precipitation (Lei et al., 2008). The program has numerous partner countries involved in validating data collected by satellite, although they do not include partners in Africa or SA. There is a proposal to launch a global precipitation measurement program, which would expand the number of satellites with TRMM-type equipment so that more sensitive, real-time data can be made available (NRC, 2006). Advanced Microwave Scanning Radiometer for the Earth Observing System The Advanced Microwave Scanning Radiometer for the Earth Observ- ing System (AMSR-E) is a 12-channel, 6-frequency radiometer system for passively detecting Earth-emitted microwave radiation. It was launched in 2002, and its main value is in measuring cloud water, water vapor, sea sur- face winds, sea surface temperature, ice, snow, and soil moisture. AMSR-E is considered a valuable tool in the study of the movement of water from the oceans to the atmosphere and back again as precipitation, and it pro- vides critically important data on global climate change. In addition, it can be used to provide accurate information on moisture in soil even if it is densely covered with crops (Bindlish et al., 2006).
140 Emerging Technologies to Benefit Farmers Model Development for Climate and Weather Prediction NOAH Land Surface Model To be understood in the context of what is occurring on local and regional scales, information derived from remotely based satellite tools, such as the ones described above, must be integrated into models. That requires the development of novel algorithms. The NOAH Land Surface Model (LSM) was developed in 1999 by a partnership of the National Centers for Environmental Prediction, Oregon State University, the U.S. Air Force, and the former Hydrologic Research Lab of the National Weather Service; the first letter of each partnerâs name gave the project its acronym. The model attempts to describe the effects of land surface on climatic and atmospheric chemistry on micro-scale and meso-scale levels (Figure 4-4). The model continues to be enhanced (Gochis and Chen, 2003). The NOAH Unified Noah/OSU Land Surface Model FIGURE 4-4â Schematic of NOAH Land Surface Model. 4-4.eps SOURCE: UCAR, 2008. 2008 Â© University Corporation for Atmospheric Research. Used with permission. bitmap image (material in top box masked and re-set per hard copy)
Water Resource Availability 141 LSM measures 33 characteristics: 10 related to vegetation (such as minimal stomatal resistance, leaf area index, and canopy water evaporation) and 23 to soil properties (such as slope, porosity, and soil moisture). The model can be run for nine soil types and 13 types of vegetation cover (Hogue et al., 2005). NOAH LSM was initially developed in the context of a temper- ate region, and its utility in other environments is being tested, including environments in Burkina Faso, West Africa, where investigators concluded that adjustments to the model were needed to take seasonal dynamics into account (Bagayoko et al., 2006). Land-Data Assimilation System Weather prediction relies on remote observations of land surface condi- tions (for example, soil moisture and temperature) combined with models, such as NOAH LSM. However, both sources of information are subject to small errors that accumulate and reduce the accuracy of predictions. A land-data assimilation system is a method to correct the modeled land- surface fields in an ad hoc fashion by using real-time output from multiple observational data streams to improve the realism of weather-prediction models (Houser, 2006). One of the challenges is to ensure that atmospheric states and land states (which affect each other) are fully consistent with each other in the prediction system. Efforts are under way to create global land-data assimilation systems in addition to those developed regionally. Such systems currently do not exist for Africa or SA. Predicting the onset of a rainy season and of the occurrence and fre- quency of 1- to 2-week drought periods during the growing season could be important to farmers. An early-warning system for severe weather would also save lives. And better regional predictions could improve the ability of financial institutions to take risks with investments to sustain and lead regional growth. References Adam, D. 2002. Gravity measurement: Amazing grace. Nature 416:10-11. Anbumozhi, V., K. Matsumoto, and E. Yamaji. 2001. Towards improved performance of ir- rigation tanks in semi-arid regions of India: Modernization opportunities and challenges. Irrigation and Drainage Systems 15(4):293-309. Aujla, M. S., H. S. Thind, and G. S. Buttar. 2005. Cotton yield and water use efficiency at various levels of water and N through drip irrigation under two methods of planting. Agric. Water Manag. 7:167-179. Bagayoko, F., S. Yonkeu, and N. C. van de Giesen. 2006. Effect of seasonal dynamics of vegeta- tion cover on land surface models: A case study of NOAH LSM over a savanna farm land in eastern Burkina Faso, West Africa. Hydrol. Earth Syst. Sci. Discuss 3:2757-2788.
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