At the ends, where there is less sand to move, material may be blown forward more easily and a dune may become a crescent-shaped barchan. Arid-land territories need to be maintained at their present low density population 192 levels. Paleoclimatic evidence presented in Chapter 8 shows that the Sahara was not always a desert, and that natural changes can transform its climate to a much wetter one. In some ways that are discussed in this article, drylands may be seen as possessing some attributes that may in fact make them somewhat resilient to climate change. Category G was divided into four subcategories (G1, G2, G3, and G4) according to the value of NDVIymx (or AI). Arid lands comprise the most widespread terrestrial biome on Earth and are home to over 20% of the world's people. These include four categories according to their aridity index: hyperarid, arid, semiarid, and dry subhumid regions. For geomorphic studies, quantifying the origin, transport, and deposition of regolith/sediment is crucial for understanding and predicting the origin of the CZ, particularly in desert environments. Our delimitation methodology appears to be applicable to other NDVI data sets, e.g. In this article, we focus on the biodiversity and consequences of human socioecology on oases in the Sonoran Desert in southwestern North America (Fig. The distinctive natural environment in arid lands, coupled with the growing human populations in drylands, is one of the primary reasons behind the recent upsurge in the global study of deserts. The radiative heating results from enhanced daytime shortwave absorption due to a substantial aerosol or dust loading. Deserts (arid lands/drylands) constitute about 35% of the land areas of the world and are typically characterized by rainfall scarcity, higher temperatures and evapotranspiration, lower humidity, and a general paucity of vegetation cover. When the rains fail and the surface vegetation is removed, the dominant surface covering may become windblown sand, which has a higher albedo than a vegetated surface. According to Leprieur, Kerr, and Pichon (1996), NDVI and MSAVI are proportionally related in arid regions. According to the linear regression method, the constants of the linear regression line (c and d are constants) become (c = 0.93, d = 0.08) and (c = 1.041, d = 0.07) in the case of AI < 0.5 and AI < 0.4, respectively, under the common condition −0.10 < NDVIymx < 0.55. Ss, Sm, and Sw: severity of soil degradation (strong, moderate, and weak, respectively). Category S is divided into three subcategories (severe–moderate–light) according to NDVIymxAI−1. People also read lists articles that other readers of this article have read. However, owing to the development of sea breezes during the daytime, the vertical motion in the lowest levels can become upward during the day. G1–G4: vegetation condition (sparse ∼ dense). Irrigated areas along the Am Darya have shrunk due to drainage, and the Aral Sea is also continuing to shrink. Almost all arid regions associated with hot or warm climates are included within the selected region. Histograms of original 125 NDVIymx for each year from 1982 to 2001. Using the results of problems 6 and 7 and the tropical humidity distribution in Fig. Concern over air pollution has been well documented (Brimblecombe, 1978). Standard deviation of NDVIymx (SDNDVI) for 20 years (1982–2001). SDNDVI for Category S is smaller and narrower than for Category G, because soil degradation, unlike drought events, prevents the rapid recovery of vegetation following precipitation. However, the vegetation characteristics and landscape for this group are determined during the wet summer season, when the biomass is the greatest. 1994), etc. This outflow in turn reduces the surface pressure and assists in the inflow of air into the surface low. Although the linear relationship between NDVIymx and AI is ascertained, it is difficult to determine appropriate constants of the linear relationship (c and d) due to the large scatter in the data. Adjusted values, i.e. Above 800 hPa there is sinking motion during both day and nighttime hours, although it is weaker during the day owing to short-wave radiative heating of the surface and atmosphere. But having being invalided out of the army due to an illness he was appointed to Imperial College, London, where he conducted wind-tunnel experiments on sand and developed models for aeolian transport. As an area of research and development, arid-zone agriculture, or desert agriculture, includes studies of how to increase the agricultural productivity of lands dominated by lack of freshwater, an abundance of heat and sunlight, and usually one or more of: Extreme winter cold, short rainy season, saline soil or water, strong dry winds, poor soil structure, over-grazing, limited technological development, poverty, … There appears to be a linear relationship between AI and NDVIymx in arid regions, where AI is less than about 0.5, although there is a large scatter in the data. For the CZ in deserts, we will only focus on the regolith (see Fig. Badland, pediments, bajadas and playas are special types of landforms of arid and semi-arid re­gions caused by mechanical disintegration of rocks and water action. That is, Category S seems to represent areas affected by desertification. Degrees of soil degradation in GLASOD are strong, moderate, and light. The distribution of Category S is comparable to the soil degradation areas mapped by Global Assessment of Human-Induced Soil Degradation (GLASOD) data. Category G was also divided into four classes, according to degree of vegetation (or aridity). Ss, Sm, and Sw indicate the severity of soil degradation (strong, moderate, and weak, respectively). D.R. Rainfall scarcity, higher temperatures and evapotranspiration, lower humidity, and a general paucity of vegetation cover characterize deserts. A detailed world irrigation map was produced by Thenkabail et al. At night, radiational cooling acts to decrease θ in the lowest kilometer, creating a nocturnal inversion. Conventionally, arid regions are described by the AI. Bagnold's overall treatment of aeolian landforms was mathematical, physical, and experimental, but became more qualitative when considering macroforms such as dunes (rather than ripples), which could not be investigated experimentally. Geographical distribution of the long-term mean of NDVIymx for 1982–2001 (Pathfinder). We attempted to classify arid areas by the relationship between vegetation index and AI. SDNDVI for Category S is smaller and narrower than that for Category G. In the Sahara and the Sahel, Category G areas are clearly delineated by the difference in standard deviation values, whereas SDNDVI in the Sahel is affected by soil degradation (belonging to Category S) and not so large. True deserts, where the standard deviation of NDVIymx is very small, were selected from the ‘severe desert’ group. One can classify drylands into four sub-types: The degree of severity of soil degradation (a kind of desertification risk index) was defined on a trial basis by the value of NDVIymxAI−1, as shown in Figure 5. Tucker, Dregne, and Newcomb 1991). In Asia, regions from the Anatolian Peninsula to the Zagros Mountains, southern hillside of the Hindu-Kush Mountains, Rajasthan east of Thar D, west part of Deccan Plateau, river-mouth of the Volga, east part of Kazakh Steppe (which extends south, surrounding Balkhash Lake), west and central Mongolian Steppe, Ordos D (including Kubuqi D and Mu Us D), areas between east Mongolian Steppe (Category G) and Ordos D, Horqin D (or Horqin Sandy Land, which is the nearest desert to Beijing) are included in this category. Category G represents semi-arid regions where NDVIymx increases as AI increases. If the density of human and animal populations is such that the land is denuded of vegetation, then the desert may advance into regions that were previously semiarid. The GLASOD database was prepared by more than 250 soil scientists from every continent. The deserts of the Middle East, the alluvial valleys of the Central Asia, the Imperial Valley of California, and the Columbia basin are examples of areas reclaimed from a near-desert state to a highly productive intensive agriculture. Compared to humid lands, the relative importance of desert atmospheric and geomorphic processes and the magnitude and frequency of their operation is rather distinctive. Part I: Development of a 1961–90 Mean Monthly Terrestrial Climatology, International Soil Reference and Information Centre, Natural Evaporation from Open Water, Bare Soil and Grass, Global Analysis of Empirical Relations between Annual Climate and Seasonality of NDVI, A Modified Soil Adjusted Vegetation Index, The NOAA/NASA Pathfinder AVHRR 8-km Land Data Set, Global Analyses of Satellite-Derived Vegetation Index Related to Climatological Wetness and Warmth, Global Irrigated Area Map (GIAM), Derived from Remote Sensing, for the End of the Last Millennium, An Approach Towards a Rational Classification of Climate, Expansion and Contraction of the Sahara Desert between 1980 and 1990, AVHRR Data Sets for Determination of Desert Spatial Extent, Satellite Remote Sensing of Total Herbaceous Biomass Production in the Senegalese Sahel: 1980–1984, Assessing the Effects of Human-Induced Degradation in the Former Homelands of Northern South Africa with a 1 km AVHRR NDVI Time-Series, Characteristics of Desertification and its Rehabilitation in China. aridity index (AI), and vegetation data, i.e. Category G seems to include tropical savanna in low latitudes and steppe in middle latitudes. Tchakerian, in Encyclopedia of Atmospheric Sciences (Second Edition), 2015. Severe deserts are divided into ‘Aa’ (true deserts), ‘Ab’ (very severe deserts), and ‘Ac’ (severe deserts), according to the rate of the superdry region for an individual severe desert (see Table 1). Thus, AI = PE p −1, as estimated by the Thornthwaite method, is overestimated in arid regions and underestimated for moist climates. From the surface to about 5 km or 550 hPa, θ is approximately constant and q decreases with height. Land Types of the Arid Zone. The eastern half of the Iberian Peninsula (the soil is highly salinized) in Europe is also included in this category. In this method, the quantity is determined empirically as a function of the mean monthly temperature only, except that the average number of daylight hours for the month is also taken into consideration. For the time being, a standardized assessment of desertification remains unresolved (Veron, Paruelo, and Oesterheld 2006). To investigate these phenomena experimentally, he constructed a wind tunnel with a fan at one end to drive sand grains down the tunnel, the fan being able to create varied wind speeds. The deserts in Asia included in this category are the Arabian Peninsula occupied by Arabian Desert comprising Syria D, Nahud D, Dahna D, Rub Al Khali D, etc. It also extends over large areas of subtropical ocean. We attempted to delimit arid regions in the simplest possible form, wherein vegetation was expressed as the yearly maximum NDVI (NDVIymx) and climate was expressed as the annual AI, determined by the ratio of annual precipitation to annual potential evapotranspiration, based on Thornthwaite method (1948). Severe deserts (Category A) divided into superdry region (SDNDVI) ≤ 0.03) and ordinary dry region (SDNDVI > 0.03). Conversion of a barchan to a seif. 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( 1135–1204 ) wrote ( Goodhill, 1971 ) drought is a lateral transport of energy out of sea!

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