52, November/December 2002
Special issue: Selected papers from the IALC Conference:
Assessing Capabilities of Soil and Water Resources in Drylands:
The Role of Information Retrieval and Dissemination Technologies
by Kenneth N. Brooks and M'Hammed Tayaa
"Water scarcity is a global problem that is particularly acute in the drylands of the world. As such, global attention on water management issues and solutions is needed. . . .A multifaceted approach is suggested here to cope with water scarcity on one hand and extremes of too much water on the other."
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Freshwater scarcity is a global problem (Postel 1989; Falkenmark 1989; Gleick 1993; Kundzewicz 1997; Vorosmarty et al. 2000) that is particularly acute in drylands, and that threatens our ability to achieve food security, alleviate poverty, and improve human health. The occurrence of droughts compounds water scarcity and often leads to excessive use of natural resources. Extended droughts result in famine in the poorest of countries. In contrast, many drylands can experience periodic excessive rainfall that can cause flash floods and consequently, loss of life and property. With few exceptions, water scarcity and the extremes of droughts and floods impact rich and poor countries alike.
Land scarcity compounds the problems of water scarcity, making people more vulnerable to the extremes of droughts and floods and leading to widespread exploitation of natural resources. Accelerated soil erosion can accompany intensive human development and natural resource exploitation, whether it is urbanization or grazing and cropping on hillslopes. Surface and gully erosion exacerbate problems of low productivity, further diminishing soil resources, which threatens the future productivity of the land. Water quality can become impaired, which when coupled with high sediment levels, constrains our ability to develop sustainable water resource management. Clearly, actions must be taken on many fronts to develop sustainable solutions and improved management of land and water in dryland environments.
Land use and water are inextricably linked together, but are not often managed in concert with one another. Watershed management offers the framework for achieving this integrated management approach to increase or sustain food and natural resource production while protecting the soil and water resources upon which this productive capacity depends (Brooks et al. 1997; Gregersen et al. 1987). Implicit in this approach is the recognition that land use in uplands affects the flow and quality of water reaching downstream areas; in contrast, water resource development (e.g., reservoirs, canals, and others) can affect the type and intensity of land use throughout a watershed. Transforming this recognition into effective solutions, however, is hampered by inadequate policies and an absence of institutions and organizational arrangements that are needed to achieve the integration and intersectoral cooperation upon which implementation depends (Kundzewicz 1997; Rosegrant 1997; Scherr and Yadav 1996). This paper considers the major issues facing natural resource management of drylands and discusses the opportunities to achieve more sustainable solutions through comprehensive watershed management. Although the focus will be more on the poorer areas of the world, where these issues are more pronounced, the principles and applications apply in all areas, regardless of economic status.
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Hydrologic Characteristics of Drylands
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Precipitation alone does not define the climatic character of drylands. Aridity describes the normal conditions of drylands that are the result of factors that create low levels of moisture availability (Heathcote 1983), which includes not only low annual precipitation amounts, but also high evaporation demands. Ratios of evapotranspiration to precipitation are generally large in drylands, exceeding 95% in some areas (Branson 1976). As a result, soil moisture is normally low during much of the year and is usually the controlling factor affecting vegetative growth. The exception is in oases or other riparian areas, such as in the southwestern United States, where evapotranspiration can exceed precipitation amounts because of locally available soil moisture (DeBano et al. 1996; Baker et al. 1998).
A distinction needs to be made between aridity, described above, and droughts. Whereas aridity is the normal condition of low available moisture of drylands, droughts result from periods of below-normal precipitation that persist. Heathcote (1983) defines a drought as " . . . an unexpected shortage of available moisture sufficient to cause severe hardship human resource use . . . ," and points out that a drought can occur in any climate. Coping with droughts is particularly challenging in drylands, where water is normally of short supply.
As a result of low and erratic precipitation in drylands, ephemeral or intermittent streams are the norm rather than perennial streams. It is not uncommon for perennial streams to originate in higher elevation zones, or from springs or seeps at the toe of slopes in mountainous areas, and then become intermittent downstream from the source area. Transmission losses of water (i.e., water that infiltrates into the channel bottom) can, however, result in a mounding of shallow groundwater below and near stream channels and in adjacent floodplains. Dense vegetative communities in these riparian zones, often dominated by phreatophyte vegetation (water-loving plants, often deep rooted trees), are indicators of such conditions. Saltcedar (Tamarix spp.), Mesquite (Prosopis spp.), and cottonwood (Populus spp.) are examples of phreatophytes which have been shown to transpire 1,000 mm to more than 2,000 mm of water per year from shallow water tables in the southwestern United States (Horton and Campbell 1974; Van Hylckama 1970). During periods of prolonged rain, or snowmelt runoff from high elevation zones, the water table can rise to the surface channel, only to retreat below stream channel bottoms during dry periods as a result of excessive evapotranspiration by phreatophytes. As a result, perennial streams in downstream valleys are rare as rainfall or snowmelt cannot be relied upon to sustain streamflow throughout the year.
Floods are not uncommon in drylands and can occur during the rare periods when there is prolonged and excessive rainfall or snowmelt runoff, or more frequently, from convective rainfall that brings localized but intense rainfall. Flash floods from high-intensity rainfall are highly variable across the landscape and are common in the southwestern United States (Marti et al. 2000). It is not uncommon for dry stream channels to become torrents within hours of convective storms that may have occurred several kilometers upstream. Furthermore, one stream channel can be flooding while another channel a few kilometers away can be completely dry. Such floods are the result of surface runoff or overland flow that occurs from watersheds with sparse vegetative cover and shallow, poorly developed soils with low infiltration capacities. Much, if not all, of the stormflow from flash floods that results from runoff into dry channels is lost to transmission losses and bank storage. As a result, the volume of streamflow often diminishes rapidly as the stormflow event moves farther downstream. Some of the transmission losses may recharge groundwater aquifers, but much of the bank storage and water that recharges shallow groundwater may be eventually lost back to the atmosphere via transpiration by phreatophytes.
Soil Erosion and Sediment Delivery
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Intensive grazing and cultivation of marginal farmland occurs in many drylands of the world and have caused excessive soil loss from surface and gully erosion. Gully erosion is an indicator of advanced watershed degradation and can reduce the production potential of the land dramatically. When accelerated erosion occurs immediately upstream of a reservoir, the resulting sediment delivered to a reservoir can impact the function and life of the reservoir. Two examples are provided in Sidebar 1.
Several lessons can be learned from the examples in Sidebar 1. In the Lesotho case, once gully erosion begins, and if land use and structural changes are not implemented early on, land degradation proceeds rapidly with the potential for serious economic impact both on-site and downstream. The implications for watershed management would suggest that erosion from overgrazing led to both a loss of land production potential and shortened the economic life of the reservoir. Costly rehabilitation efforts involving both structural and vegetative measures would be needed to reverse these processes. From a watershed management perspective, preventing such losses in the first place through improved grazing practices can be (and should be) considered as an economic benefit, and thus a basis for justifying grazing management programs.
In the Moroccan example, land use and rainfall interactions dictate soil erosion and sedimentation rates. The highest rates of sedimentation correspond to the watershed with the most intense land use (and least forest cover), but with the lowest rainfall, averaging 350 mm/yr. Herein lies a dilemma for watershed management. With such low rainfall and severely overgrazed watersheds, opportunities to reduce erosion (estimated to be >250 t/ha/yr) and sedimentation through revegetation are limited. Costly structures would be required to accompany any vegetation program which takes a long time to materialize; furthermore, the ability to reduce grazing pressures during the rehabilitation period dramatically constrains rehabilitation in such areas where the rural poor are directly dependent on their livestock. As a result, reservoirs fill with sediment at an alarming rate. Where rainfall is greater, the potential is also greater for restoring the watershed. A watershed restoration program that controlled grazing and increased forest and fruit tree production on the Loukos watershed was shown to be economically justified with an internal rate of return of 15.9% (Brooks et al. 1982).
There is another aspect to sediment delivery at reservoir sites that needs to be addressed when considering watershed restoration measures. Sediment that is entrapped in streamflow is the product of upland erosion processes, streambank erosion, and channel scour. Streams are formed over geologic time in response to dynamic and complex fluvial processes and hydraulic factors. When watershed conditions change sufficiently to alter either the streamflow response or the amount and type of sediment that enters the channel, the stream will adjust and can become unstable, in some instances accelerating sediment delivery downstream (Rosgen 1994). Similarly, changes in the riparian plant community that alter streambank stability and that affect the velocity of flow through floodplains during flood events, all affect downstream flows and sediment regimes. Therefore, floods and sediment are linked together and are further linked to the condition of upland watersheds and their riparian plant communities. Flash floods are natural, episodic events that transport large quantities of sediment and debris. The extent to which land use on upland watersheds affects the magnitude and frequency of flash flooding, and the sediment and debris load that these floods carry, is not well understood in most drylands. We do recognize, however, the importance of riparian vegetation in helping to mitigate the effects of such events. Where riparian vegetation becomes patchy and less dense, it is less effective in dissipating streamflow energy and stabilizing stream banks (Medina 1996; Medina et al. 1996; DeBano and Baker 1999). Excessive livestock grazing, improper cultivation, improper tree cutting, roads, urban development, and so forth, all can accelerate runoff and the erosion processes that contribute to both watershed degradation and stream channel instability.
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In all drylands receiving intensive use, some common issues and problems arise, which need to be addressed through management. Foremost, drylands have commonly been viewed in the past as wastelands, not worthy of economic concern or political attention. Global concerns about desertification over the past few decades have focused more attention to dryland issues and the need for land use reform. Desertification is often used to describe areas that have become desert-like in their appearance as a result of human-induced degradation. To some extent, emerging programs to combat desertification have helped generate the political and economic support that is needed to reverse land degradation. However, there are certain inherent characteristics of drylands that place limits on the potential for agricultural, natural resource and urban development. There are also conditions that make watersheds vulnerable to degradation, and that constrain our ability to restore or rehabilitate the land.
Water scarcity is a reality that must be dealt with before any type of development can be sustained. Opportunities for enhancing water supplies are discussed later, but there are other factors that need to be addressed in the planning and management of land and water resources so that the productivity of the land is sustained, and the soils and vegetation communities of dryland watersheds are not degraded. Any type of development of drylands should recognize that vegetative communities and soils are sensitive to intensive use and once degraded, many decades are needed to restore the production and hydrologic function of these ecosystems. Low and erratic precipitation prevents the rapid reestablishment of vegetation, leaving a degraded landscape exposed to water and wind erosion for long periods of time.
Although aridity typifies the general condition of drylands, it should also be noted that most dryland regions have "islands of abundant moisture" that can be found in higher elevation mountains (as in the southwestern United States), in oases, and in other riparian areas. Because of the presence of water, high productivity, and ecological values, riparian areas receive an inordinate concentration of human and animal use, many times leading to overuse and degradation. As a result, riparian areas have become a focal point of natural resource managers and developers in drylands (DeBano et al. 1996; Medina 1996; Medina et al. 1996).
Because of the favorable climate, many drylands are targeted for urban and agricultural development, all of which hinges upon the ability to develop and enhance water supplies. Many large cities and retirement communities have emerged in the driest areas of the southwestern United States. In contrast, large-scale irrigation projects likewise have emerged in the drylands of China, India and Pakistan. The sustainability of such efforts is becoming problematic as populations grow, demands for water increase, and conflicts over water use arise. In the past, engineering technologies have been relied upon that capture water in water abundant areas (or during periods of high precipitation) and then store and transfer water to areas (or during time periods) where or when it is needed (Cech 2003; Brooks et al. 1997). The following considers how land use and watershed management can complement efforts to enhance water supplies and cope with water scarcity.
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Manipulating vegetation on watersheds to increase water yields has been given considerable attention for several decades (Bosh and Hewlett 1982; Whitehead and Robinson 1993; Ffolliott et al. 2000). Studies indicate that water yield can be increased when:
Hibbert (1983) considered opportunities for increasing water yield from drylands in the western United States and concluded that the above manipulations of vegetation increase water yield only when annual precipitation exceeds 400 mm/yr, but increases are marginal unless annual precipitation exceeds 500 mm/yr. Before vegetation manipulation for water yield enhancement is implemented, several factors must be taken into consideration. First, there must be some means of capturing the water yield increase; reservoir storage should be both adequate to accommodate any increases in water yields and close to the watershed outlet. If water yield is increased during periods when reservoirs are full, there is little value in carrying out such management. As discussed earlier, stream channels in drylands characteristically have high transmission losses and ET rates of riparian vegetation can further diminish water flows from upland watersheds. Brown and Fogel (1987) determined that less than half of any water yield increases in the Verde River Basin of Central Arizona would actually reach the Phoenix area (about 150 km downstream) because of transmission and evaporative losses. Therefore, the best opportunities for this approach are in uplands where annual precipitation exceeds 500 mm/yr, and where watersheds can be dedicated for water supply purposes immediately above reservoirs. In marginal rainfall zones of between 400-500 mm/yr, some limited opportunities may exist to increase water yield by converting shrub lands to grasses, which improves grazing production as well. This concept was considered nearly 50 years ago in central Arizona (Barr 1956).
In addition to the above factors, implementing water yield enhancement strategies hinges on other considerations. Changing vegetative cover on a watershed can affect wildlife habitat as well as other uses of the land, such as outdoor recreation. Changes in erosion, sediment transport, and water quality must all be understood prior to implementation.
Vegetation manipulations for purposes of conserving groundwater were the focus of early work that considered the removal of phreatophytes in floodplains, or otherwise reducing their transpiration (Brooks and Thorud 1971). Because of the high transpiration rates of phreatophytes discussed earlier, there is considerable potential to salvage groundwater in areas where groundwater supplies are critical. Such possible benefits must be weighed against the value of such vegetation in terms of riparian values for flood control, channel stability, wildlife habitat, and so forth.
Clearly there are limitations in many dryland areas to enhancing water yield through vegetation manipulations. Other technologies can be used to increase water yield or to complement other water conservation measures. Some are tailored more for local household or farm use, while others can be components of more comprehensive water management systems.
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Several different designs are possible, but generally at least 80-100 mm of annual rainfall is required (National Academy of Sciences 1974). Apron type water harvesting systems have an impervious catchment area, a storage facility and a water distribution system and have been widely used for providing water for livestock. Catchment areas can be a few square meters up to 1,000 m2, and consist of natural rock outcrops, paved roads, or soils that have been treated to make them impervious. For livestock watering, storage tanks should be covered or situated where evaporation losses are minimal.
Water harvesting has been widely practiced in sub-Saharan Africa. Such systems have been effective in augmenting rain-fed agriculture in areas with unreliable and erratic rainfall that normally occurs over short periods of 70-120 days per year (Rockstrom et al. 1999). For example, in Burkina Faso and Kenya, catchments of 1-1.2 ha have been used to harvest rainfall to irrigate crops where annual rainfall averages 600-700 mm/yr, but potential evapotranspiration exceeds 1,500 mm/yr. Such applications can be crucial to subsistence farmers and pastoralists in many parts of the world. Economics, social factors, and careful consideration of alternatives should be taken into account before investing in water harvesting systems.
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Even more so than with water harvesting, water spreading systems have limited applications. Economic considerations and the need for coordination and cooperation among upstream and downstream users of water are all critical to the implementation of water spreading.
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Technologies abound that have been designed to help people cope with the extremes of too much or too little water; some of them have been discussed earlier Cech 2003; Brooks et al. 1997). The weak links in actually coping with droughts and floods, however, are usually related to poor planning, the absence of a comprehensive, integrated approach to land and water management, and the means to coordinate and implement comprehensive programs. To begin with, planners and managers must recognize that droughts and floods will occur, and that people can do little to affect their occurrence. However, we can affect their impact on humans.
Land and water scarcity affects human behavior and to some extent constrains our ability to cope with extreme hydrologic events. In most drylands, people become concentrated near stream channels, and in the adjacent riparian or floodplain areas. Such areas are, of course, the most prone to flooding. Even though floods may not occur every year in ephemeral streams, we know that they will eventually occur and the nature of flash floods gives little time for humans to escape their destructive power. In mountainous terrain, steep stream channels can become charged with sediment over time, which can become debris torrents when high intensity rains occur. Such debris torrents are extremely destructive to the downstream areas that lie in their path. One or more of the following approaches can be taken to cope with floods and debris torrents:
Given that the first option is costly and can provide only limited protection, and flood warning systems in areas subjected to flash flooding is unrealistic, zoning of hazardous areas (e.g., flood plain management) is considered to be the most viable option in most cases. This approach recognizes that where people and their dwellings become concentrated in areas that are prone to hazards of flooding or debris flows, disasters will occur.
Policies and institutions must be in place that provides incentives for people to avoid hazardous areas. Delineating flood-prone areas can be facilitated with GIS-based terrain analysis (Gupta and Joshi 1990, Sidle 2000). Similarly, methods of delineating flood plains and zoning are well known (Bedient and Huber 1988). However, incentives are needed to change people's behavior in such hazardous areas; an example is the Federal Flood Insurance program (USA) which links insurance rates to the degree of hazard.
Coping with droughts is more problematic and requires that many contingencies be considered. Two perspectives are needed, one to prepare for droughts and the other to deal with droughts after they occur. Efforts to increase water supplies in drylands and that can carry over supplies for several years have been the goal of many water resource projects. Reservoirs, water transfer systems, etc. all have some merit in developing dependable water supplies for periods of water shortages. Once droughts occur, water conservation programs can become accelerated by means of reducing consumption. However, communities that already have strict water conservation programs in place may find that once the drought occurs, there may be no buffer in the system; that is, there are no additional water conservation actions that can be implemented without incurring severe hardship. In many instances, programs can be established that incrementally reduce water use on an established priority basis at the outset of droughts. Restrictions on water use can become prioritized based on established criteria for a region. Those uses which have the least adverse impact on people become restricted early on and progress into uses that have greater impacts. In any event, coping with droughts requires the necessary policies and institutions that can implement a combination of water conservation measures and drought contingencies. Options, unfortunately, seem to be more restricted in the poorer countries, where the effects of droughts are more dramatic.
A watershed management approach can play an effective role in coping with the types of hydro-meteorological extremes discussed above. In the process of planning and implementing agricultural, water resource, and other natural resource projects, a watershed approach brings into the process the explicit recognition of the linkages that exist between land use and water and between upstream and downstream areas. Land use that leads to loss of vegetative cover, soil disturbance and overall watershed degradation can exacerbate flooding and debris flows and can constrain the ability of local inhabitants to cope with droughts. The benefits of soil and water conservation through watershed management are expanded upon below.
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Freshwater benefits to downstream areas naturally accompany sound management of upland and riparian areas. Preventing watershed degradation should, therefore, be realized as beneficial; avoiding losses that would occur from exploitation of resources on the land is equally as beneficial as increasing the productivity of an already degraded land. In contrast, management can be targeted to specific watershed objectives, such as enhancing the quantity and quality of water yield above a reservoir. In either case, benefits can be masked by:
Changes on the land can have incremental effects, which individually may not be apparent, but when viewed over the watershed and over time, can have more serious cumulative effects. This complexity has blurred the vision of decision-makers in many parts of the world, has constrained economic assessments, and overal, has weakened commitments to watershed management. To overcome this problem, the economic costs and benefits of comprehensive watershed management must become better known.
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Water is heavily subsidized in many parts of the world, including drylands, and is often treated as a free good. Water scarcity is now causing people to become more realistic in determining the true value of freshwater, as discussed below.
A new perspective on the global water economy is now emerging in which freshwater is viewed more as an economic commodity rather than a publicly managed resource (Anderson 2002). For example, in southern California (USA), farmers pay $10/acre-ft ($8 per 1,000 cubic meters) for irrigation water that has been supplied from reservoirs located in the northern, water-rich parts of the state. In contrast, the City of Santa Barbara pays $2,000/acre-ft ($1,600 per 1,000 cubic meters) for its drinking water through the process of desalinization of ocean water. Here, water has more value than many of the crops being irrigated, resulting in some farmers willing to sell their water to municipalities. In such instances, there may be sound economic justification for managing watersheds strictly for water supply purposes. Such economic reality faces hurdles in developing countries, where water has often been treated as a free good because of long-standing practices and religious beliefs (Rosegrant and Cline 2002). In all cases, however, more efficiency-oriented water allocation and innovative pricing policies can provide the incentives needed to support watershed management for water supply purposes. In contrast, policies that continue to treat water as a free good, or that heavily subsidize water, will continue to promote waste in developing and developed countries alike.
Institutional and policy considerations
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Watershed and political boundaries rarely coincide; as a result, the necessary coordination of land and water management depends upon functional organizations that can resolve transboundary issues and water use disputes. The lack of effective watershed-level organizations led to the formation of more than 1,000 watershed districts in the United States during the 1990s to deal with upstream-downstream issues (Lant 1999). Internationally, the Nile Basin Initiative established a basin-wide partnership of nine riparian countries to help resolve transboundary issues and to deal with inequities of water distribution in the basin (Baecher et al. 2000). More than 80% of the flow in the Nile River originates in mountainous Ethiopia, a river upon which downstream Sudan and Egypt are heavily dependent. Without cooperation and coordination of both land use on watersheds and water development projects within the basin, disputes and conflicts could erupt. In all situations, a strategy is needed that promotes coordination and cooperation among the stakeholders in a watershed. Policies are needed that provide the incentives to these stakeholders to achieve comprehensive and sustainable soil and water management of dryland watersheds.
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Kenneth N. Brooks, Professor, Department of Forest Resources, 115 Green Hall, 1530 Cleveland Ave. North, University of Minnesota, St. Paul, MN 55108, USA; Tel: +1 (612) 624-2774; FAX: +1 (612) 625-5212; Email: firstname.lastname@example.org
M'Hammed Tayaa, Department of Soil Science, Institute of Agronomy and Veterinary Science Hassan II, Rabat, Morocco.
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