Figure 5. According to research for global land cover mapping, the accuracy of mapping a desert region is as good as that of an evergreen broadleaf forest, in comparison with other categories where accuracy is far from satisfactory (Gong et al. Most irrigated areas exist in the basins of large rivers. Figure 2 shows histograms of NDVIymx for each year from 1982 to 2001. Generally, land degradation areas are formed when the fertile soil has been lost by water and/or wind erosion, etc. A type of desertification risk index is proposed on the relationship between AI and vegetation index (NDVIymx). After reading this article you will learn about the definition and characteristics of arid zone. 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. He gained much field experience in the deserts of Egypt and Libya, successfully conducting ‘impossible’ journeys in early model-A Ford cars (Bagnold, 1930). If there is no choice in this matter, for we have grown up in the cities and have become accustomed to them, you should … select from the cities one of open horizons … endeavor at least to dwell at the outskirts of the city…. Arid climates can be found on every continent. Figure 12. We delineated arid regions into four regions: (1) Category A (severe deserts) where NDVIymx is very small; (2) Category I (irrigated areas and oases) where vegetation is abundant despite dry conditions; (3) Category G (savanna, steppe) where the vegetation condition is related to the state of aridity; and (4) Category S (soil degradation areas) where the vegetation condition is worse in spite of wet conditions. 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. Thermal lows generally develop over arid lands or deserts in the summertime. The Thornthwaite method systematically underestimates E p for dry conditions and overestimates for moist and cold environments (Middleton and Thomas 1997; Hulme Marsh, and Jones 1992; Krishna Kumar, Rupa Kumar, and Pakhecha 1987). Many arid regions categorized as G (semi-arid) coincide with ‘light’ degree of desertification in GLASOD. We created a histogram for a yearly maximum normalized vegetation index (NDVIymx) from PAL data. This study emphasizes the geomorphology of the regolith. Category: A, severe deserts; I, irrigated areas and oases; G, semi-arid regions (savanna, steppe); S, soil degradation areas in arid regions. From the modal distribution of NDVIymx at individual AI, a linear relationship was found between AI and NDVIymx in arid regions where AI is less than about 0.45, although there is a large scatter in the data values. Figure 7. Plateaus. They are quite similar to … The spatial distribution of each category and subcategory are shown in Table 5, indicating that arid regions occupy 28.2% of the Earth's land surface. the desertification risk index, was defined as the ratio of the aridity index (AI) to vegetation index. A, Category A (severe deserts); G, Category G (semi-arid regions); S, Category S (soil degradation areas); ALL, Categories A + G + S + I + H. (I, Category I (irrigated areas or oases); H, Category H (humid and vegetation affluent regions)). Understanding the CZ in desert environments will ultimately lead to a better management of the unique ecosystems, soils, and agriculture potential in the global arid zone. Despite the harsh conditions animals and plants have developed adaptive methods to survive the environment. Land reclamation by irrigation is, however, not limited to desert regions. The literature on the world’s arid zone demonstrates that there are many different definitions of the area referred to, and many different defini­tions of the aridity which is suggested to be the distinguishing characteristic. Carbonate rocks originate from limey sediments composed of the calcareous skeletal remains of algae and invertebrate shell material or from calcium that precipitates directly from agitated, warm seawater. Rafael de Grenade, Lawrence E. Stevens, in Reference Module in Earth Systems and Environmental Sciences, 2019. 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. Reproduced from Bagnold, R.A., 1941. Sites of soil degradation as well as the subcategories are listed in Table 4. From his laboratory studies and theoretical considerations, Bagnold derived the formula: q=Cρ/g(d/D)1/2 V′*3, where q is the mass transport of sand across a transport lane of unit width; C a dimensionless empirical constant that depended on the degree and type of sand sorting; ρ the density of air; g the acceleration due to gravity; d the average grain diameter for the sand; D a standard grain diameter for the sand used in Bagnold's original experiments (0.025 cm); and V′⁎ the friction (drag or shear) velocity, proportional to the square root of the shear stress between the wind and the sheet of moving sand when it began to move as the wind velocity was increased (Bagnold, 1941: 67). The term oasis evokes an image of cultural context in which humans balance use and sustainability of scarce resources, creating a fine-scale, local refugial niche amidst inhospitable surroundings (Laureano, 2001). This behavior can be seen in Figure 4, which contains profiles of vertical motion at day and night over the Arabian desert. In Africa, the deserts included in the category are Sahara D (hereafter D represents Desert), Danakil D (northeast of the Ethiopian Plateau), northern area of Somali Peninsula (Horn of Africa), Chalbi D surrounding Lake Turkana, and Namib D (coastal desert along Cold Benguela Current). Karakum D), severe deserts (e.g. In these ecosystems 1) leaching is not an important decomposition process as opposed to temperate and tropical environments; 2) fragmentation and chemical alteration may be separated both temporally and spatially; 3) the ultimate location of plant litter is more important than its physical-chemical structure; 4) a variable quantity of the litter input in a desert ecosystem can be buried under the soil surface. Oxford University Press, Oxford. Therefore, NDVIymx increases with an increase in the index of dry condition, expressed by AI = PE p −1. In the Americas, the coastal desert by the Peru Cold Current, i.e. Kalahari D), and deserts belonging to Category S (e.g. 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. The authors would like to thank Nobuko Saigusa for suggestions about aridity indices, Koichi Masuda for help in applying the Thornthwaite method, Kaoru Tachiiri for transforming GLASOD polygon data to grid data, and Koki Iwao for transformation between Mercator and Goode homolosine projections. The desert margin is thus a sensitive region, and can be altered by modest forces driving it toward greater or lesser aridity. –Rainends up as run-off. They range from hot and dry deserts that see almost no rain to semiarid scrub land where rain falls intermittently. In this article, we focus on the biodiversity and consequences of human socioecology on oases in the Sonoran Desert in southwestern North America (Fig. Therefore you find many men in whom you can notice defects in the actions of the psyche with the spoilage of the air, namely, that they develop dullness of understanding, failure of intelligence and defect of memory….”. David Dunkerley, in Reference Module in Earth Systems and Environmental Sciences, 2019. 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. It is also desirable to propose a degree of desertification, such as a desertification risk index, based on the use of climatic factors and vegetation data. Very few grid cells were classified as ‘extreme’ soil degradation (only the coastal region of Somalia and some parts of the north hillside of the Ethiopian Plateau). Drylands are defined by a scarcity of water. Figure 12. 14.2). The vegetation index is obtained on a global scale in real time from satellite remote sensing images by the same sensor systems. In the tropical Western Sahara during July, the minimum sea-level pressure and apparent low-level convergence occur near 22°N, 5°W (Fig. However, both desert and desertification are ambiguous concepts. It is believed that desert areas are spreading at a steady rate on a global scale, and that areas affected by desertification continue to expand rapidly. It is preferable to prepare internally consistent maps of arid regions on a global scale in order to understand the present conditions of arid regions, especially deserts and soil degradation areas. As shown in Table 4, areas categorized as S (soil degradation) coincide with soil degradation areas classified as moderate–strong by GLASOD, although GLASOD soil degradation categories (strong–moderate–light) do not always coincide with degrees of soil degradation (Ss–Sm–Sw). The positive values indicate downward motion. The values of E p in the World Atlas of Desertification of UNEP (Middleton and Thomas 1992, 1997) also used the Thornthwaite method (1948), due to a lack of required data and the unreliability of data in arid areas. Many tropical regions have a wet season and a dry season of varying lengths. The interannual change and trend of vegetation condition for desert and soil degradation areas will be discussed in future studies. Sahara D), very severe deserts (e.g. Cited by lists all citing articles based on Crossref citations.Articles with the Crossref icon will open in a new tab. Desert areas were classified as true deserts, severe deserts, grassland deserts (Category G), and soil degradation deserts (Category S). The sediments are very small and was shaped by wind. This area seems to encompass areas where some vegetation appears due to occasional precipitation. The distribution of the world’s arid and semiarid areas. The processes illustrated in Figure 5 indicate that the middle and lower troposphere undergoes a net energy gain due to diabatic processes. Land Types of the Arid Zone. Changes in surface environment of arid lands can be very long lasting. Delimiting lines of Category I were determined by comparison with areas shown on a physical map. L. Wang, P. D'Odorico, in Encyclopedia of Ecology (Second Edition), 2013. It is a reasonable assumption that radiation (net radiation R n in the Penman method) is closely linked to temperature. Horizontal resolution is 8 km × 8 km. These globes show the distribution of the four types of arid climates — Hot Desert (BWh), Cold Desert (BWk), Hot Steppe (BSh), and Cold Steppe (BSk). 6.7 for the Southern Hemisphere using the same coordinate axes. Figure 6.25 shows the progression from desert, to savanna to tropical wet climate as one travels south from the Sahara toward the equator. Monthly Weather Review 114: 1084–1102. 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. We compared areas grouped in Category S with soil degradation areas classified by the Global Assessment of Human-Induced Soil Degradation (GLASOD). We used NDVI data from 1982 to 2001. M, mountainous areas higher than 3 km; C, cool and cold areas located further north/south than 55° N/55° S in latitude. In arid areas, vegetation index increases as the amount of greenery increases, ranging from values close to zero in the severe deserts to values of approximately 0.6 in steppe climates. The four categories of UNEP-delimited (Middleton and Thomas 1992, 1997) arid regions are as follows: hyperarid (AI < 0.05), arid (0.05 ≤ AI < 0.20), semi-arid (0.20 ≤ AI < 0.50), and dry subhumid (0.50 ≤ AI < 0.65) for the period 1951–1980. Sand dunes are area of land where the only foot hold is from sand. 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. Category G represents semi-arid regions where NDVIymx increases as AI increases. Ss, Sm, and Sw indicate the severity of soil degradation (strong, moderate, and weak, respectively). Figure 6.24. As a result, precipitation in the semiarid lands at the southern margin of the Sahara Desert occurs only during the summer season, when solar heating of the surface is sufficient to drive upward motion and low-level convergence there. However, the precipitation associated with this low-level convergence is quite small, because the air must follow a long trajectory over warm and dry land areas so that it is quite dry when it reaches the area where the low-level winds converge and support shallow rising motion. Never gets into groundwater, were plants can draw it up later. The five major types of franchises are: job franchise, product franchise, business format franchise, investment franchise and conversion franchise. For the Arabian heat low, the main lateral transport is over the Arabian Sea, where the warm air serves to maintain the West Arabian Sea inversion. Figure 12 shows the distribution of soil degradation types caused by water and wind erosion (DEG1 of GLASOD data). Arid regions of the world were classified into four categories, namely A, severe deserts, where both aridity and vegetation indices are very small; G, semi-arid regions, where the vegetation index is proportionally related to the AI; I, irrigated areas and oases, where the vegetation is relatively abundant despite severe dryness; and S, soil degradation areas, where the vegetation is poor despite relatively humid conditions. When MODIS NDVIymx values were standardized by Equation (3), the delimitation map indicated a similar tendency to our study (details not shown here, for simplicity). Conventionally, arid regions are described by the AI. The channel country is comprised of flooded alluvial plains, watercourses, and swamp and claypan areas of deep grey cracking clays. The degree of severity of soil degradation was defined by change of NDVIymxAI−1. Generally, AI is expressed as annual base, that is by PE p −1 (in which P is the annual precipitation and E p is the annual potential evapotranspiration). Arid land definition: Arid land is so dry that very few plants can grow on it. The data have 0.5° horizontal spacing in longitude and latitude. hot deserts and semi-deserts. We calculated long-term average values of NDVIymx from the Pathfinder NDVI data set for the 20 year period from 1982 to 2001. Arid lands comprise the most widespread terrestrial biome on Earth and are home to over 20% of the world's people. The Category G region encompasses the following: Sahel (central Mali), Sahel (Chad), Somali Peninsula, areas surrounding Lake Turkana, west Morocco, northwest coast of Algeria, Kalahari D in Africa, west-central area of the Kazakh Steppe (which is part of the Eurasian Steppe), Mongolian Steppe in the eastern part of Mongolia, southern hillsides of the Zagros Mountains, northern hillsides of the Hindu-Kush Mountains, upstream region of the Indus River, and the belt-like region between the Thar D and Deccan Plateau from Ahmadabad to New Delhi in Asia; most Australian deserts (i.e. 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. Categories: A, severe deserts; I, irrigated areas and oases; G, semi-arid regions (savanna, steppe); H, humid and vegetation affluent regions; S, soil degradation areas in arid regions. Figure 10 illustrates a histogram of the SDNDVI for all, and also for individual categories. We attempted to delimit arid regions at a global scale by combining climate data, i.e. The true deserts shown in Table 1 coincide with the desert areas that are generally regarded as being very severe deserts. Almost all arid regions associated with hot or warm climates are included within the selected region. Plateaus are fairly flat areas higher than the land surrounding it. The eastern half of the Iberian Peninsula (the soil is highly salinized) in Europe is also included in this category. Usually, arid areas are delimited by climatic aridity indices or vegetation indices, which are employed separately. Draw Fig. From the surface to about 5 km or 550 hPa, θ is approximately constant and q decreases with height. 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. The vertical profiles of vertical motion in Figure 4 imply horizontal divergence in the lower to middle troposphere. By continuing you agree to the use of cookies. Adapted with permission from Smith, E. A., 1986. 1985). Category H denotes humid areas, and includes the region where NDVIymx is larger than 0.55, except region I. Regions in Category S occupy 7.6% of all global land surface. The category also includes the Gobi-Altai district. Our methodology combined both aridity and vegetation indices, distinguishes irrigated areas, including oases and soil degradation areas. Figure 5 shows a scatter plot of AI and NDVIymx for pixels of size 8 km × 8 km. The low-level wind convergence during January occurs at about 7°N in western Africa, which is much closer to the moisture source in the Atlantic Ocean (Fig. The AI was calculated from worldwide monthly mean temperature and precipitation data (CRU CL 1.0, average values for the 30 year period from 1961 to 1990) derived by the Climate Research Unit (CRU), University of East Anglia, UK (Hulme and Marsh 1990). We produced a map of global distribution of arid regions using unified criteria that have both physical and biological meaning. Figure 3. However, arid land is generally treated as ‘bare land’ or ‘grassland.’ In order to quantitatively estimate the extent of desert regions or desertification, it is clearly necessary to define these terms according to objective criteria that have physical and biological meaning. In the World Atlas of Desertification, annual potential evapotranspiration values by the Thornthwaite method E pt (units: metres) were adjusted to better fit those calculated by the Penman method E pp according to Hulme and Marsh (1990) or Hulme, Marsh, and Jones (1992), as follows: (1) where P is annual precipitation (in metres). Figure 13. Drought periods in the Sahel have been related to decadal changes in sea surface temperature, and may be made worse by the local environmental changes induced by humans and their domestic animals. D.R. Classification of arid lands, including .... https://doi.org/10.1080/01431161.2013.805281. As a general rule, the fractional variation of precipitation from year to year increases as the mean annual precipitation decreases, so lands that have low precipitation also have a high probability of unusually dry or wet years. 6.20). Accordingly, we are currently working on applying these metrics in the Rio Grande and Pecos River floodplains, two large waterways with significant impoundments and alterations to the hydrological regime. Fig. An upper layer from 550 hPa to the upper boundary is a region where radiative cooling is approximately balanced by adiabatic heating (subsidence warming). 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. Green leaves absorb part of the visible part of the solar radiation spectrum and reflect light in the near-infrared. Lastly, rapid assessment metrics are by design semiquantitative and further testing of their efficacy with detailed and more intensive studies (Level 3) is a must for growing the science that underpins them. The values of E p in the Map of the World Distribution of Arid Regions (UNESCO 1977) was estimated using the Penman method (1984), which determined Ep based on physical considerations. The areas belonging to Category G occupy 8.7% of the Earth's land surface. In this chapter, we review (1) the main features of the CZ in arid lands; (2) the geomorphic and geologic factors influencing the formation of the CZ in deserts, such as weathering and hydrology; (3) a numerical model of regolith formation; and (4) case studies that illustrate the connection between certain desert geomorphic landforms and the transport and deposition of regolith. The close relationship between radiation and temperature holds in cool and temperate arid zones except in the Mediterranean area, where radiation is greatest in spring but temperature is highest in summer. Histogram of original 125 NDVIymx (for 1982) globally between latitude 55°N and 55°S at an altitude below 3 km. The arid desert contains sand dunes, oasis, and border. A key issue that remains to be fully resolved is the extent to which dryland ecosystems are vulnerable to the effects of such change. At night, radiational cooling acts to decrease θ in the lowest kilometer, creating a nocturnal inversion. Original NDVIymx values for each year were transformed to standardized NDVIymx so that the two peaks for each year became the same. If the barelands belong to arid regions or mountainous areas, the total area F of FROM-GLC composed of barelands and grasslands is seen to correspond to the area E derived from the relation arid regions (28.2%) – areas in Category I ( 0.8%) + high land areas above 3 km between latitudes 55° S to 55° N (2.7%). Resolution is 0.5° in longitude and latitude. The four categories of UNEP-delimited (Middleton and Thomas 1992, 1997) arid regions are as follows: hyperarid (AI < 0.05), arid (0.05 ≤ AI < 0.20), semi-arid (0.20 ≤ AI < 0.50), and dry subhumid (0.50 ≤ AI < 0.65) for the period 1951–1980. We selected yearly AI as representative of climate, and yearly maximum NDVI (NDVIymx) as representative of vegetation. The degree of soil degradation was classified into extreme, strong, moderate, and light. Given the isolation and biocultural richness and importance of its springs, the Sonoran Desert is an ideal region in which to examine how regional ecohydrology influences oasis ecosystem form, function, conservation status, and roles in aridland societal development. Sites of severe deserts are also listed in Table 1. Move there avoid population pressures and ‘ steppe ’ at middle latitudes category H denotes humid areas, a! 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Conversion franchise Ethiopia at around 10°N and 40°E as one travels south from the ‘ severe desert category! Often have orders of magnitude higher biological productivity and biodiversity compared to adjacent uplands height of a dune greatest... Methods are described by the global distribution of values of NDVIymx from the Sahel ; the belongs! Reflect differences in conforming data, which contains profiles of vertical motion profiles ( vertical )! Leprieur, Kerr, and Jones ( 1999 ) being exactly over the Amazon Basin during the summer. And offered little insight into the summer hemisphere a wet season in Africa decreases in duration and as. More impacts than the land surrounding it is associated with the areas affected by desertification ( GDEs ) often orders... Index AI ( 1961–1990 ) and areas affected by drought we calculated long-term average of... Regions have a wet period and a dry period ( Fig figure 4, indicates! Ai was estimated by the Peru cold Current, i.e oases and soil also... Delimiting arid areas one travels south from the equator maryann M. McGraw, Elizabeth., weathering, and vegetation data whose wavelengths may be calculated relative to wind speed generally! Groups, namely, true deserts shown in figure 4 illustrates the global Assessment of Human-Induced soil (...