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Article

Center-pivot irrigation systems started in the United States in the mid-20th century as an irrigation method which surpassed the traditional surface irrigation methods. At that time, they had the potential to bring about higher irrigation efficiencies with less water consumption although their requirements in energy were higher too. Among their benefits, it is highlighted the feasibility to control water management as well as the application of agro-chemicals dissolved in the irrigation water and thus, center-pivot irrigation systems have spread worldwide. Nevertheless, since the last decade of the 20th century, they are facing actual concerns regarding ecosystem sustainability and water and energy efficiencies. Likewise, the 21st century has brought about the cutting edge issue “precision irrigation” which has made feasible the application of water, fertilizers, and chemicals as the plant demands taking into account variables such as: sprinkler´s pressure, terrain topography, soil variability, and climatic conditions. Likewise, it could be adopted to deal with the current key issues regarding the sustainability and efficiency of the center-pivot irrigation to maintain the agro-ecosystems but still, other issues such as the organic matter incorporation are far to be understood and they will need further studies.

Article

Flowing through the North China Plain, one of China’s major agricultural regions, the Yellow River has long represented a challenge to Chinese governments to manage. Preventing floods has been an overriding concern for these states in order to maintain a semblance of ecological equilibrium on the North China Plain. This region’s environment is heavily influenced by seasonal fluctuations in precipitation, leading to a long history of famine, particularly in the late 19th and early 20th centuries when water management structures disintegrated with the deterioration of the imperial system. In the 20th century, new civil and hydraulic engineering techniques and technologies held the promise for enhanced management of the region’s waterways. After 1949, the new government of the People’s Republic used a hybrid approach consisting of the tenets of multipurpose water management combined with the tools of mass mobilization that were hallmarks of the Chinese Communist Party. The wide-ranging exploitation of surface and groundwater resources during the Maoist period left a long shadow for the post-Mao period that witnessed rapid consumption of water to fuel agricultural, industrial, and urban reforms. The challenge for the contemporary state in China is creating a system of water allocation through increased supply and demand management that can sustain the economic and social transformations of the era.

Article

Luis S. Pereira and José M. Gonçalves

Surface irrigation is the oldest and most widely used irrigation method, more than 83% of the world’s irrigated area. It comprises traditional systems, developed over millennia, and modern systems with mechanized and often automated water application and adopting precise land-leveling. It adapts well to non-sloping conditions, low to medium soil infiltration characteristics, most crops, and crop mechanization as well as environmental conditions. Modern methods provide for water and energy saving, control of environmental impacts, labor saving, and cropping economic success, thus for competing with pressurized irrigation methods. Surface irrigation refers to a variety of gravity application of the irrigation water, which infiltrates into the soil while flowing over the field surface. The ways and timings of how water flows over the field and infiltrates the soil determine the irrigation phases—advance, maintenance or ponding, depletion, and recession—which vary with the irrigation method, namely paddy basin, leveled basin, border and furrow irrigation, generally used for field crops, and wild flooding and water spreading from contour ditches, used for pasture lands. System performance is commonly assessed using the distribution uniformity indicator, while management performance is assessed with the application efficiency or the beneficial water use fraction. The factors influencing system performance are multiple and interacting—inflow rate, field length and shape, soil hydraulics roughness, field slope, soil infiltration rate, and cutoff time—while management performance, in addition to these factors, depends upon the soil water deficit at time of irrigation, thus on the way farmers are able to manage irrigation. The process of surface irrigation is complex to describe because it combines surface flow with infiltration into the soil profile. Numerous mathematical computer models have therefore been developed for its simulation, aimed at both design adopting a target performance and field evaluation of actual performance. The use of models in design allows taking into consideration the factors referred to before and, when adopting any type of decision support system or multicriteria analysis, also taking into consideration economic and environmental constraints and issues. There are various aspects favoring and limiting the adoption of surface irrigation. Favorable aspects include the simplicity of its adoption at farm in flat lands with low infiltration rates, namely when water conveyance and distribution are performed with canal and/or low-pressure pipe systems, low capital investment, and low energy consumption. Most significant limitations include high soil infiltration and high variability of infiltration throughout the field, land leveling requirements, need for control of a constant inflow rate, difficulties in matching irrigation time duration with soil water deficit at time of irrigation, and difficult access to equipment for mechanized and automated water application and distribution. The modernization of surface irrigation systems and design models, as well as models and tools usable to support surface irrigation management, have significantly impacted water use and productivity, and thus competitiveness of surface irrigation.

Article

E. Ann McDougall

The Sahara: bridge or barrier? Today, most would answer that the desert was more a historical facilitator than hindrance in moving commodities, ideas, and people between North and sub-Saharan Africa. A recent publication even coined a new name for the region: “trans-Saharan Africa.” However, the Sahara is also a place where people live. Complex societies, sophisticated polities, extensive economies—all flourished at various times, waxing and waning in response to much the same factors as societies elsewhere. It is just that in the Sahara the vagaries of climate and the availability of water always established the parameters of development. A long-term drying era led to the dispersal of the Late Stone Age Dhar-Tichitt agro-pastoral settlements in eastern Mauritania, but in the east, Lake “Mega-Chad” shrank, leaving rich, sandy soils that attracted new cultivators. The Garamantes people of the Libyan Fezzan overcame their lack of water by developing a sophisticated underground irrigation system that supported an urbanized, cosmopolitan civilization that outlasted the Roman Empire. The introduction of the camel in the 4th century and the gradual growth of Islam from at least the 9th century added new possibilities for economic, cultural, and religious life. The Sahara benefited from the sequence of medieval empires emerging across its southern desert edge. Camel pastoralism, salt mining, oasis agriculture, and expansive trade networks shaped the region’s economy; those same networks facilitated cultural and scholarly exchanges. As Islam took root, growing its own understandings of North African and Middle Eastern schools of thought, a prodigious body of Saharan scholarship was created. It underpinned much of the jihad-led political upheaval and state-building in the 18th and 19th Sahel. Saharan clerics also directed their religious fervor against the invasion of French imperialists; “pacification” took the colonialists decades to achieve. But the impact of this violence exacerbated traditional clan conflict and disrupted economic life. So too did policies aimed at sedentarizing pastoralists and reshaping their social relations in the interests of the colonial economy. Much talked-about but largely ineffective efforts to abolish slavery had far less real impact than taxation policies; these both suppressed traditional exactions such as those levied by “warriors” and introduced new ones, including those to be paid in forced labor. Life in the Sahara became increasingly untenable. The arrival of Independence did nothing to address colonial legacies; the years of drought that devastated herds and crops in the desert and along its edge less than a decade later further fueled both political instability and economic crisis. That today the region nurtures radicalized Islamic movements promising to return “true meaning” (not to mention material benefits) to that life is not surprising.

Article

Matthew V. Bender

East Africa is among the most environmentally diverse regions of the continent, and this diversity is reflected in its hydrology. The steppe plains, home to much of the region’s great wildlife, are defined by scarcity of rainfall and surface water resources. Within this sea of aridity, mountain peaks such as Kilimanjaro, Kenya, and Meru induce large amounts of rainfall and give rise to rivers that reach out into the grasslands. To the west, the forest–savannah mosaic and the shorelines of the Great Lakes likewise feature plentiful precipitation and surface water, giving rise to abundant vegetation and marine life. The Indian Ocean coast falls between in terms of rain, but its fate has been shaped by oceanic trade. In short, East Africa is a hydrological mosaic that has long influenced the social, cultural, and economic diversity of its human populations. The peoples of East Africa have long depended on the region’s water resources for their livelihoods. They have made sense of the region’s waterscapes, and developed strategies to manage them, in ways that reflected their own needs. Water management consisted not just of hydrological and technological expertise, but also cultural, spiritual, and political expertise. These in turn shaped economic as well as social relationships and hierarchies. With the onset of European colonization in the 19th and 20th centuries, water management became a focal point of struggles between local communities and various colonial actors—government officers, scientists, missionaries, and settlers—who developed very different impressions of the region’s waterscapes. These struggles involved not only conflict over the physical control of water resources, but also debates over what constituted useful and relevant water-management knowledge. Colonial actors described their water management in terms of science and modernity, while existing knowledge and practice were framed as primitive, wasteful, and destructive. Over the 20th century, conflicts intensified as users, African as well as European, demanded larger shares of increasingly scarce water resources. The post-colonial period did not spell an end to these struggles. Since the late 20th century, water management has emerged as a key aspect of national strategies for economic and social development. Yet decades of emphasis and millions of dollars spent have not led to sufficient progress in providing water to everyday people. Today, millions of East Africans lack access to clean, reliable water, a problem that is likely to worsen in the future.

Article

The Basin of Mexico is a key world region for understanding agricultural intensification and the development of ancient and historic cities and states. Archaeologists working in the region have had a long-standing interest in understanding the dynamics of interactions between society and environment and their research has been at the forefront of advances in both method and theory. The Basin of Mexico was the geopolitical core of the Aztec empire, the largest state in the history of Mesoamerica. Its growth was sustained by a complex economy that has been the subject of much research. Two themes underlie a broad interest in the pre-Hispanic agriculture of the Basin of Mexico. First, how with a Neolithic technology did the Aztecs and their predecessors sustain the growth of large cites, dense rural populations, and the largest state system in the history of pre-Hispanic Mesoamerica? Second, what is the relationship of agricultural intensification and urbanization and state formation? Mesoamerica is the only world region where primary civilizations developed that lacked domestic herbivores for either food or transportation. Their farming depended entirely on human labor and hand tools but sustained large cities, dense populations, and complex social institutions. Intensive agriculture began early and was promoted by risk, ecological diversity, and social differentiation, and included irrigation, terracing, and drained fields (chinampas). Most farming was managed by smallholder households and local communities, which encouraged corporate forms of governance and collective action. Environmental impacts included erosion and deposition, but were limited compared with the degradation that took place in the colonial period.

Article

Soil salinity has been causing problems for agriculturists for millennia, primarily in irrigated lands. The importance of salinity issues is increasing, since large areas are affected by irrigation-induced salt accumulation. A wide knowledge base has been collected to better understand the major processes of salt accumulation and choose the right method of mitigation. There are two major types of soil salinity that are distinguished because of different properties and mitigation requirements. The first is caused mostly by the large salt concentration and is called saline soil, typically corresponding to Solonchak soils. The second is caused mainly by the dominance of sodium in the soil solution or on the soil exchange complex. This latter type is called “sodic” soil, corresponding to Solonetz soils. Saline soils have homogeneous soil profiles with relatively good soil structure, and their appropriate mitigation measure is leaching. Naturally sodic soils have markedly different horizons and unfavorable physical properties, such as low permeability, swelling, plasticity when wet, and hardness when dry, and their limitation for agriculture is mitigated typically by applying gypsum. Salinity and sodicity need to be chemically quantified before deciding on the proper management strategy. The most complex management and mitigation of salinized irrigated lands involves modern engineering including calculations of irrigation water rates and reclamation materials, provisions for drainage, and drainage disposal. Mapping-oriented soil classification was developed for naturally saline and sodic soils and inherited the first soil categories introduced more than a century ago, such as Solonchak and Solonetz in most of the total of 24 soil classification systems used currently. USDA Soil Taxonomy is one exception, which uses names composed of formative elements.

Article

The vast region known as “Soviet Central Asia” encompassed the territory of five Soviet republics, Kazakhstan, Kirgizia, Uzbekistan, Tajikistan, and Turkmenistan. Because of the region’s environmental features, particularly its aridity, historically there had been a close linkage between people and the environment in this region. But the Soviet regime set out to radically reshape this relationship, focusing on the fields of agriculture and animal husbandry, large-scale water engineering, nuclear and biological weapons testing, and medicine and public health. By focusing on the environmental impact of these policies, scholars can see how Moscow’s efforts brought many benefits to the region. Cotton production boomed, and Moscow declared the eradication of malaria. But they also left horrific scars. Josef Stalin’s program of agricultural collectivization devastated Kazakhstan, resulting in the death of more than 1.5 million people. The Aral Sea, once one of the world’s largest bodies of water, began to shrink dramatically during the Soviet era, a development due in large part to Moscow’s efforts to divert the waters that fed the sea to cotton production.

Article

Over the last seven thousand years, humans have gradually domesticated the environment of South China. Transitioning from a reliance on wild environments, humans tamed plants and animals and transformed the landscapes and waterscapes to better fit their needs. Rice paddies, orchards, and artificial ponds and forests replaced naturally seeded woodlands and seasonal wetlands. Even the Yangzi River, and many of the other rivers, lakes, and seashores, were transformed by polders, dikes, and seawalls to better support human activities, especially rice agriculture. In the last thousand years, farmers intensified their control of the cultivated landscape through terracing, irrigation, flood prevention, and new crop rotations. They planted commercial crops like cotton, fruits, oilseeds, tea, and sugar cane in growing concentrations. Migrants and merchants spread logging, mining, and intensive agriculture to thinly settled parts of the south and west. Since the 17th century, New World crops like sweet potatoes, chilis, maize, and tobacco enabled a further intensification of land use, especially in the mountains. Since the early 1800s, land clearance and river diking reached extremes and precipitated catastrophic flooding, social unrest, and a century of warfare. Since 1950, the People’s Republic has overseen three further waves of degradation accompanying the mass campaigns of the Mao era and the market reforms under Deng Xiaoping. Following catastrophic flooding in 1998, the government has increasingly worked to reverse these trends. Nonetheless, South China remains one of the most intensively cultivated environments in the world and continues to feel the effects of new attempts to tame and expropriate the forces of nature.

Article

Assessing the environmental footprints of modern agriculture requires a balanced approach that sets the obviously negative effects (e.g., incidents with excessive use of inputs) against benefits stemming from increased resource use efficiencies. In the case of rice production, the regular flooding of fields comprises a distinctive feature, as compared to other crops, which directly or indirectly affects diverse impacts on the environment. In the regional context of Southeast Asia, rice production is characterized by dynamic changes in terms of crop management practices, so that environmental footprints can only be assessed from time-dependent developments rather than from a static view. The key for the Green Revolution in rice was the introduction of high-yielding varieties in combination with a sufficient water and nutrient supply as well as pest management. More recently, mechanization has evolved as a major trend in modern rice production. Mechanization has diverse environmental impacts and may also be instrumental in tackling the most drastic pollution source from rice production, namely, open field burning of straw. As modernization of rice production is imperative for future food supplies, there is scope for developing sustainable and high-yielding rice production systems by capitalizing on the positive aspects of modernization from a local to a global scale.

Article

Noa Kekuewa Lincoln and Peter Vitousek

Agriculture in Hawaiʻi was developed in response to the high spatial heterogeneity of climate and landscape of the archipelago, resulting in a broad range of agricultural strategies. Over time, highly intensive irrigated and rainfed systems emerged, supplemented by extensive use of more marginal lands that supported considerable populations. Due to the late colonization of the islands, the pathways of development are fairly well reconstructed in Hawaiʻi. The earliest agricultural developments took advantage of highly fertile areas with abundant freshwater, utilizing relatively simple techniques such as gardening and shifting cultivation. Over time, investments into land-based infrastructure led to the emergence of irrigated pondfield agriculture found elsewhere in Polynesia. This agricultural form was confined by climatic and geomorphological parameters, and typically occurred in wetter, older landscapes that had developed deep river valleys and alluvial plains. Once initiated, these wetland systems saw regular, continuous development and redevelopment. As populations expanded into areas unable to support irrigated agriculture, highly diverse rainfed agricultural systems emerged that were adapted to local environmental and climatic variables. Development of simple infrastructure over vast areas created intensive rainfed agricultural systems that were unique in Polynesia. Intensification of rainfed agriculture was confined to areas of naturally occurring soil fertility that typically occurred in drier and younger landscapes in the southern end of the archipelago. Both irrigated and rainfed agricultural areas applied supplementary agricultural strategies in surrounding areas such as agroforestry, home gardens, and built soils. Differences in yield, labor, surplus, and resilience of agricultural forms helped shape differentiated political economies, hierarchies, and motivations that played a key role in the development of sociopolitical complexity in the islands.

Article

The increased pressure on groundwater has resulted in a major deterioration of the overall status of this resource. Despite efforts to control the degradation of underground water bodies, most aquifers worldwide experience serious quality and quantity problems. New emerging issues around groundwater resources have become relevant and pose additional protection and management challenges. Climate change, with predictable impacts on temperature and precipitation, will cause considerable fluctuations in aquifer recharge levels and subsequent problems in the status of these water bodies. Expected reductions in water availability will increase groundwater withdrawals not just for irrigation but also for urban and industrial water use. Declines in stored water will have an impact on many freshwater ecosystems whose survival depends on the status of groundwater bodies. Furthermore, land subsidence, as a side effect of aquifer overexploitation, involves land collapse and deformation that are especially harmful for urban areas and deteriorate physical and hydrological water systems. All these new challenges require integrated planning strategies and multisectorial solutions to curtail the deterioration of these resources. Although these issues have been studied, in-depth analyses of the economic, social, and policy implications of groundwater management strategies are still necessary.

Article

Henry Darcy was an engineer who built the drinking water supply system of the French city of Dijon in the mid-19th century. In doing so, he developed an interest in the flow of water through sands, and, together with Charles Ritter, he experimented (in a hospital, for unclear reasons) with water flow in a vertical cylinder filled with different sands to determine the laws of flow of water through sand. The results were published in an appendix to Darcy’s report on his work on Dijon’s water supply. Darcy and Ritter installed mercury manometers at the bottom and near the top of the cylinder, and they observed that the water flux density through the sand was proportional to the difference between the mercury levels. After mercury levels are converted to equivalent water levels and recast in differential form, this relationship is known as Darcy’s Law, and until this day it is the cornerstone of the theory of water flow in porous media. The development of groundwater hydrology and soil water hydrology that originated with Darcy’s Law is tracked through seminal contributions over the past 160 years. Darcy’s Law was quickly adopted for calculating groundwater flow, which blossomed after the introduction of a few very useful simplifying assumptions that permitted a host of analytical solutions to groundwater problems, including flows toward pumped drinking water wells and toward drain tubes. Computers have made possible ever more advanced numerical solutions based on Darcy’s Law, which have allowed tailor-made computations for specific areas. In soil hydrology, Darcy’s Law itself required modification to facilitate its application for different soil water contents. The understanding of the relationship between the potential energy of soil water and the soil water content emerged early in the 20th century. The mathematical formalization of the consequences for the flow rate and storage change of soil water was established in the 1930s, but only after the 1970s did computers become powerful enough to tackle unsaturated flows head-on. In combination with crop growth models, this allowed Darcy-based models to aid in the setup of irrigation practices and to optimize drainage designs. In the past decades, spatial variation of the hydraulic properties of aquifers and soils has been shown to affect the transfer of solutes from soils to groundwater and from groundwater to surface water. More recently, regional and continental-scale hydrology have been required to quantify the role of the terrestrial hydrological cycle in relation to climate change. Both developments may pose new areas of application, or show the limits of applicability, of a law derived from a few experiments on a cylinder filled with sand in the 1850s.

Article

floods  

Gregory S. Aldrete

In classical history and mythology, floods frequently appear in both negative and positive contexts, serving as a force for destruction and retribution, but also for growth and renewal. Floods are inextricably linked with foundational aspects of civilization and urbanization, most notably in connection with irrigation and agriculture, but they also constitute a leading form of natural disaster that can result in widespread devastation and loss of life.The role of cataclysmic floods in the mythologies, legends, and religions of numerous ancient civilizations is well known, as are the many similarities among these narratives, such as floods being sent as divine punishment, an inundation being used to delineate the end of an era, a chosen figure who receives warning of the impending disaster and constructs a watercraft, and the subsequent repopulation of the world by a small group of survivors. Prominent examples include Mesopotamian versions such as the flood narrative in the Epic of Gilgamesh, the biblical account of Noah, and, in the Classical era, the legends of Deucalion, Ogyges, and Dardanus.

Article

Aridity, a significant characteristic of the U.S.–Mexico borderlands, has affected water use patterns for different groups of people in this region for thousands of years. From indigenous groups to European invaders and colonizers to 20th- and 21st-century farmers, ranchers, and policy-makers in Mexico and the United States, controlling the area’s scarce water resources has been a vital concern for survival and economic success. Given that an international border divides the region, national-era relations between the United States and Mexico often have been marked by water issues and the development of water projects and policies. And on both sides of the border these projects and policies have caused environmental changes that merit attention. Much of that history revolves around agricultural development with the need to ensure steady sources of water for irrigation. But industry and urban areas have also been enormous consumers of scarce water resources in the region, issues that are discussed here.

Article

Agrarian societies in Latin America and the Caribbean have accomplished some of the most important and influential innovations in agricultural knowledge and practice in world history—both ancient and modern. These enabled indigenous civilizations in Mesoamerica and the Andes to attain some of the highest population densities and levels of cultural accomplishment of the premodern world. During the colonial era, produce from the region’s haciendas, plantations, and smallholdings provided an essential ecological underpinning for the development of the world’s first truly global networks of trade. From the 18th to the early 20th century, the transnational activities of agricultural improvers helped turn the region into one of the world’s primary exporters of agricultural commodities. This was one of the most tangible outcomes of the Enlightenment and early state-building efforts in the hemisphere. During the second half of the 20th century, the region provided a prime testing ground for input-intensive farming practices associated with the Green Revolution, which developed in close relation with import-substituting industrialization and technocratic forms of governance. The ability of farmers and ranchers to intensify production from the land using new cultivars, technologies, and techniques was critical to all of these accomplishments, but often occurred at the cost of irreversible environmental transformation and violent social conflict. Manure was often central to these histories of intensification because of its importance to the cycling of nutrients. The history of the extraction and use of guano as a fertilizer profoundly shaped the globalization of input-intensive agricultural practices around the globe, and exemplifies often-overlooked connectivities reaching across regional boundaries and between terrestrial and aquatic environments.

Article

Satellite reconnaissance of the Earth’s surface provides critical information about the state of human interaction with the natural environment. The strongest impact is agricultural, reflecting land-use approaches to food production extending back to the dawn of civilization. To variable degrees, depending upon location, regional field patterns result from traditional farming practices, surveying methods, regional histories, policies, political agendas, environmental circumstances, and economic welfare. Satellite imaging in photographic true or false color is an important means of evaluating the nature and implications of agricultural practices and their impacts on the surrounding world. Important platforms with publicly accessible links to satellite image sets include those of the European Space Agency, U.S. National Aeronautics and Space Administration, the Centre D’etudes Spatiales, Airbus, and various other governmental programs. Reprocessing of data worldwide in scope by commercial concerns including Digital Globe, Terrametrics, and GoogleEarth in the 21st century enable ready examination of most of the Earth’s surface in great detail and natural colors. The potential for monitoring and improving understanding of agriculture and its role in the Earth system is considerable thanks to these new ways of viewing the planet. Space reconnaissance starkly reveals the consequences of unique land surveys for the rapid development of agriculture and political control in wilderness areas, including the U.S. Public Land Survey and Tierras Bajas systems. Traditional approaches toward agriculture are clearly shown in ribbon farms, English enclosures and medieval field systems, and terracing in many parts of the world. Irrigation works, some thousands of years old, may be seen in floodplains and dryland areas, notably the Maghreb and the deep Sahara, where center-pivot fields have recently appeared in areas once considered too dry to cultivate. Approaches for controlling erosion, including buffer zones, shelter belts, strip and contour farming, can be easily identified. Also evident are features related to field erosion and soil alteration that have advanced to crisis stage, such as badland development and widespread salinization. Pollution related to farm runoff, and the piecemeal (if not rapid) loss of farmlands due to urbanization can be examined in ways favoring more comprehensive evaluation of human impacts on the planetary surface. Developments in space technologies and observational platforms will continue indefinitely, promising ever-increasing capacity to understand how humans relate to the environment.

Article

The semi-arid interior of Brazil’s northeast region, known as the sertão, has long been subject to droughts. These can devastate the agricultural and ranching economy and cause serious hardship for the area’s inhabitants, particularly those who labor on farms and ranches belonging to the landowning elite. A prolonged drought in the late 1870s led the Brazilian government to begin soliciting advice from engineers about how to redress the periodic crisis. In 1909 the federal government established a permanent federal agency, the Inspectorate for Works to Combat Drought, to undertake reservoir construction throughout the sertão along with other measures that would alleviate future droughts. In subsequent decades the activities of the drought agency expanded to include constructing irrigation networks around reservoirs and establishing agricultural experiment stations to teach sertanejo farmers improved methods of farming in semi-arid conditions. Although powerful landowners lobbied for federal aid to construct reservoirs, which helped to sustain their own cattle herds through drought years, they were often opposed to initiatives like the establishment of irrigated smallholder colonies around reservoirs, which threatened to alter the social order in the sertão. Support for the federal drought agency’s work waxed and waned during the 20th century under different presidential administrations. Often it would rise in response to a period of damaging drought, then diminish once the crisis abated. Droughts have affected the sertão at irregular intervals since at least the colonial era. They vary in temporal duration and geographic expanse. Their impact on human populations depends on how the area of reduced rainfall overlaps with human settlement patterns and land use. Over the 20th century the years in which drought most severely impacted human communities (including crops and livestock) in the sertão included 1915, 1919–1920, 1931–1932, 1942, 1951–1953, 1958, 1970, 1979–1983, and 1998–1999. These are the periods when local, state, and federal governments received the most persistent pleas for assistance from affected populations. The precise cause of droughts in the region is debated, but they are thought to be triggered by changes in major wind patterns, particularly the El Niño Southern Oscillation (ENSO), that prevent Atlantic Ocean precipitation from reaching the sertão.

Article

Luis Jaime Castillo Butters and Karla Paola Patroni Castillo

The Moche developed in the north coastal valleys of Peru between 200 and 850 ad. These societies evolved from earlier regional civilizations like Cupisnique and Gallinazo thanks, in part, to their advances in irrigation agriculture and the extension of fields into the deserts, which permitted population increases never seen before in the Andean region of South America. The Moche were never organized as a single, centralized polity but rather constituted multiple interacting medium- and small-scale regional societies, possibly complex chiefdoms and early forms of archaeological states, with two large regional divisions in the northern and southern valleys. Due to their fragmentary nature, there were more aspects that were differences between these societies than those aspects that were common. They seem to have spoken two different languages, Muchik in the north and Quignam in the south. Religions and ritual practices; a shared pantheon of divinities; and mythical narratives expressed in their iconography and performed in monumental structures, locally called huacas, were shared among Moche polities. It is hypothesized that Moche elites were also moving between polities, due to marriage and political alliance. The Moche excelled in multiple crafts, particularly metallurgy and ceramics, and were responsible for the development of multiple technological innovations. During most of their history, the Moche were isolated from other Andean societies, interacting only between themselves. This isolation was permitted by a specialization in the agriculture of the coastal valleys and in the exploitation of marine resources. Between 800 and 850, and due to external and internal causes, the Moche polities experienced different processes of rapid decline that led to the formation of a new generation of civilizations, the Lambayeque in the northern region, and the Chimú in the southern.