Image acquired by the VENµS satellite, 2018, over the Zin Valley, Negev Desert, Israel
Generally speaking, desertification is defined as spatial extension of desert-like conditions of soil and vegetation into marginal areas outside the climatic desert and intensification of such conditions over a period of time. Desertification processes result from various factors, including climate variationsand human activities such as overgrazing, deforestation,drought, and the burning of extensive areas. Once commenced,the desertification process is characterized by progressivedestruction of native vegetation, significant loweringof the water table, a reduced supply of surface water, increasedsalinity in natural waters and soils, encroachment ofsand dunes, and an accelerated rate of erosion. Climatic effectsassociated with this phenomenon include increased albedo,reduced atmospheric humidity, and greater atmosphericdust (aerosol) loading.
According to the United Nations, desertification is one of themost devastating and widespread threats to environmental security faced by many countries in the world. It affects onethird of the world population, and impacts upon people on allcontinents. It is associated with social strife including poverty, hunger, social unrest, high infant mortality and disease. Research into the abatement of desertification therefore contributes to efforts to solve these problems.
Desertification can be difficult to assess from the ground, sinceground observations are limited in space and time. For example, a farmer may realize that the topsoil is blowing away fromhis field or that salinization is occurring, but these observationsdo not indicate the state of degradation a few kilometers away. Each site requires manpower for intensive survey and sampling. Furthermore, since desertification processes fluctuateover time, repeated and ongoing observations are required todetermine whether progressive degradation is actually occurringand to track its progress.
Monitoring desertification processes over vast and distant areastherefore necessitates innovative techniques. Remote sensing from satellites offers a considerable potential meansto do this all over the globe. Spaceborne systems derive informationabout the ground from measurements made at a distanceand without coming into physical contact with it. Such measurements are based on analyzing the electromagnetic radiation that is reflected or emitted from the Earth’s surface orthe atmosphere.
[2] Filaments of biological soil crust consisting of cyanobacteria and fine soil particles.
[1] The sharp contrast across the Israel-Egypt border as seen in satellite images.
The brightness contrast across the Israeli-Egyptian political borderline is a typical exampleof a desertification phenomenon triggered by humanimpacts on a fragile ecosystem [1]. The sand dunes of the Negev (Israel) are almost completely covered by biological soil crusts undisturbed by anthropogenic activity. These crusts consist of microorganisms called cyanobacteria along with fine soil particles [2]. On the other side of the border, int he sand dunes of Sinai (Egypt), such crusts are absent from the top soil due to intensive trampling by humans and animals. Consequently, the Israeli Negev dunes are stable withmore vegetation, while the Sinai dunes are bare andmobile. The two sides of the political borderline, although similar from geological, geomorphological, pedological, and climatic points of view, demonstrate opposing processes of desertification in Egyptand rehabilitation in Israel.
Dry bare soils in arid environments are the majorsource of fine particles (or aerosols), termed mineraldust, that are blown up in dust storms. Huge duststorms originating in the north African and Mediterraneandeserts cross the Atlantic and settle in areas asfar as Europe and the Americas, while dust stormsfrom China and Mongolia may cross the Pacific. Apartfrom the health and environmental consequences of these storms, mineral dust particles can affect climate through the absorption and scattering of solar radiationand, in the case of large particles, by interaction withthermal radiation.
Therefore, it is important to monitor satellite data of such storms to obtain information on dust coverage at climatic scales.
While remote sensing of dust over the ocean or dark targets is possible, since the reflectance received by the satellite is mainly related to aerosol content, this task ismade much more difficult over bright desert surfaces [4].
[4] Saharan dust over the eastern Mediterranean. Note that the dust can be clearly seen over thesea but is hardly detectable over the bright desertsurface (image courtesy of NASA Visible Earth).
Since the desert surface reflectance ishigh, the satellite signal is only slightly affected by the dust-scattering contribution. Therefore, adequate techniques forremote sensing over land still have to be developed. The Aerosol Index is a measure of how much ultraviolet light is absorbed by aerosol particles within the atmosphere, and its ability to sense dust clouds even over desert surfaces is illustrated in [5].
References
- Kaufman, Y.J., Karnieli, A. and Tanre, D. 2000. Detection of dust over deserts using satellite data in the solar wavelengths. IEEE Transactionson Geoscience and Remote Sensing, 20, 525-531.
[5] Aerosol Index indicates a large cloud of dust blowing from northeastern Africa across Egypt, the Sinai Peninsula, over Israel and into the Middle East region on March 19, 2002. Red area sindicate high aerosol index values and correspond to the densest portions of the dust cloud, while yellows and greens are moderately high values (image courtesy of NASA Visible Earth).
Original aerial photograph
Crust Index
Active dune sands
Stabilized crusty inter dune area
Many deserts around the world are aeolian,dominated by winds that are an important erosive force where there is little soil surface protection by organic matter or vegetation cover. Destruction of the vegetative cover or biological soil crusts over sandy areas may accelerate wind erosion and depletion of soil fertility. Areasof blowing sands can be easily identified in satellite images. The role of remote sensing is toidentify zones of active and inactive sand dunesand assess the rate of migration of the dunes towards settled or cultivated areas. A specialspectral Crust Index developed in the Remote Sensing Laboratory enables separation of different lithological/morphological units such as activedune sands, stabilized crusted inter dune areas, and playas in the north-western Negev Desert. The absence, presence, and distributionof soil crusts are important items of information for desertification and climate change studies[6].
Based on change detection techniques applied to satellite images from 1973, 1992, and 2002,we estimated that sand dunes in the Gobi desertof Mongolia moved about 47 m during the 29-year study period [7].
[6] Spectral Crust Index showing the distinction between stabilized surfaces, covered bysoil biological crusts, and mobile sands in the north-western Negev Desert .
[7] Change detection technique reveals that a barhan (arcshaped)dune moved 47 m during 29 years in the Gobi Desert. Photograph credit: Jean Jacques CORDIER.
References
- Karnieli, A. 1997. Development and implementation of spectral crust index over dune sands. International Journal of Remote Sensing, 18, 1207-1220.
- Bayarjargal, Y. and Karnieli, A. 2004. Assessing land-useand land-cover change in Bulgan Soum by remote sensing change detection technique. Arid Ecosystems,10, 127-133.
Drought can be defined as a period of abnormally dry weather that persists long enough to produce a serious ecological, agricultural, or hydrological imbalance (e.g., cropdamage, water shortage, etc.). The severity of the drought depends upon the degree of moisture deficiency, the duration,and the size of the affected area. In this context, meteorological drought refers to a prolonged period of below average precipitation. Drought is one of the causes of desertification and exacerbates already poor situations.
[8] A satellite image of central Israel.Note the location of the Yatir forest onthe desert fringe, visible as the sharpcontrast between bright tones (semi-aridzone) and dark tones (sub-humid zone).
References
- Volcani, A., Karnieli, A. and Svoray, T. The use of remote sensing and GIS for spatio-temporal analysis of the physiological state of asemi-arid forest with respect to drought years. Forest Ecology and Management, 215, 239–250.
[9] NDVI as indicator ofbio-physiological conditionof the Yatir forest.(A) High index values(winter 1995); (B) lowvalues (winter 2000);and (C) detectedchange between thetwo years products (C).
Drought years are a very frequent phenomenonin Israel. Between the years 1994/5 and 2001/2,Israel experienced five drought years. Consequently the Yatir forest, a pine forest locatedon the desert fringe [8], suffered from a notable water shortage. Remote sensingmethods enable us to detect and assess inter-annual changes in the forest treeswith respect to the drought effect. The state of the forest was studied by applying aspectral vegetation index, namely the Normalized Difference Vegetation Index (NDVI),based on spaceborne images. High NDVI values are referred to healthy conditionswhile low values to stress conditions [9 A and B]. The degree of change between thetwo years is estimated by applying a change detection technique on the two NDVIproducts [9 C]. It can be seen that in 2000, after five consecutive drought years, theYatir forest was suffering from severe water shortage.
References
- Karnieli, A., Bayarjargal, Y. and Bayasgalan, M. 2005.Do Vegetation Indices Reliably Assess VegetationDegradation? Proceedings of the InternationalConference on Remote Sensingand Geoinformation Processing , September7th to 9th, 2005, Trier (Germany).
The Mongolian railway, more than 1000 km in length from thenorthern border with Russia to the southern border with China,was established in the 1960s. Since then, it has been protectedalong its length by fences to prevent animals from crossing therailway. As a result, no grazing occurs inside the fences, in contrastto the intensive grazing that characterizes the surroundingarea. Advantage was taken of this unique fencing phenomenonto investigate anthropogenic rangeland degradation in thesteppe biome, through which the railway passes. When thetrack curves the area between the fences can be as wide as 4km, enabling the use of remote sensing methodsusing high resolution imagery [10 A]. Imageprocessing has revealed unpalatable toxicspecies that invaded the grazed areas at theexpense of the native palatable grasses. Thelarge extent of this phenomenon over thesteppe biome of Mongolia indicates severevegetation degradation due to overgrazing [10B and C].
[7] Change detection technique reveals that a barhan (arcshaped)dune moved 47 m during 29 years in the Gobi Desert. Photograph credit: Jean Jacques CORDIER.
Desertification around watering pointshas been well observed by satellite imagesin many drylands around the world. Itcan be identified as radial belts of brightnessfading as a function of distance from wells [11]. The primary goal of our study was tocharacterize spatial and temporal land degradation/rehabilitation in Central Asian drylandsin terms of vegetation and soil patterns,in different time periods, with respectto socio-economic conditions before and afterthe collapse of the Soviet Union. In orderto implement this goal we developed a geostatisticalmodel based on high-resolutionsatellite imagery [12] in three key time periods(mid-late 1970s, late 1980s, and 2000) [13 A and B]. We conducted a change detectionanalysis in order to assess the directionand intensity of changes between thestudy periods and specifically we linked thefindings to the socio-economic conditionsbefore and after the collapse of the SovietUnion that influenced the grazing gradientsand hence the land-use/ land-cover state ofthe study sites.
[12] Landsat-TM imageof the Kyzylkum Desert,Kazhastan. Each smallbright dot indicates awatering point. The agriculturalfields, in red, arelocated along the SyrDarya River.
[11] Watering point in the Sahel. Radialbrightness belts fading as a function of distancefrom the wells can be seen.Photograph credit: Dr. Compton J. Tucker.
A Grazing gradient model 1991
B Grazing gradient model 2000
C Change detection analysis (2000-1991)
[13] Geostatistical modeling of the grazing gradient around watering points in the Kyzalkum Desert of Kazakhstan. Most of the area recoveredafter 1991 due to socio-economic changes resulting in reduced grazing after the collapse of the Soviet Union.
Our results were mixed. On one hand we found that the Kyzalkum Desert of Kazakhstan is characterized by land rehabilitationprocess in rangelands that can be explained by the historicalevents of the last decades [13 C]. Following independenceof the former Soviet states in 1991 and the imposition ofdifficult economic conditions with transition reforms, several majorsocio-economic changes occurred that caused drastic declinesin livestock populations, with a major drop in the number
References
- Karnieli, A. 1997. Development and implementation of spectral crust index over dune sands. International Journal of Remote Sensing, 18, 1207-1220.
- Bayarjargal, Y. and Karnieli, A. 2004. Assessing land-useand land-cover change in Bulgan Soum by remote sensing change detection technique. Arid Ecosystems,10, 127-133.
of sheep and goats, and hence vegetation recovery. However,in the Ust-Urt Plateau of Kazakhstan, degradation of the areacontinues to occur due to recent exploration and exploitation ofthe gas and oil reserves in the region. Consequently, large areasunderwent intensive ‘technological desertification’, i.e. theuse of heavy duty equipment, large-scale plants, and vehiclesthat damage the soil surface.
In semi-arid and arid areas of the world, where surface water is very scarce,water supply is produced from inland water bodies or shallow groundwateraquifers at depths of around 100 m. The spatial extent and depth of these aquifers,along with their characteristics, are important factors in determining theiruse. Drying up of lakes [14] or depletion of the aquifers due to climatic fluctuationsor intensive use, or even lowering of the groundwater level below depthsallowing efficient pumping with simple instruments and/or the root zone, is oneof the main reasons for desert encroachment and the migration of people andherds to other areas.
Since spaceborne imagery detects only the surface of the Earth (and not thesubsurface), identification of shallow aquifers and monitoring their status is amajor challenge for the potential applications of remote sensing techniques tohydrological studies. It can be achieved by using indirect indicators that are correlatedto hydrogeological factors, themselves indirectly related to the presenceof groundwater. One example is the visible vegetation along the Amatzia Faultin the northern Negev [15 A]. The vegetation indicates water seepages fromunderground aquifers. A similar example is observed at the edges of the alluvialfans of the Turfan Depression in China [15 B].
[14] Comparison of two satellite images (2001 vs. 1990) demonstratingthe disappearance of a water lake in the Gobi Desert(Mongolia) and drying up of the adjacent vegetation (seenin red in the 1990 image) due to climate change. Salty soils(in white) appear instead of the lake in the 2001 image.
The recession of the Aral Sea waterlevel is probably the most staggeringenvironmental crisis of the twentieth century.Until the 1960s the Aral Sea was theworld’s fourth-largest fresh water lake.However, since then, Soviet planners divertedwater from the two main riversfeeding the lake for irrigating cotton andrice fields, the most water-demandingcrops. Satellite images document thecontinual shrinking of the lake to less thanhalf its original size [16 A-C]. This mismanagedengineering project led to accelerateddesertification process in the wholeregion. Agricultural areas were irreversiblydestroyed by secondary salinization.The local climate has reportedly shifted,with hotter, drier summers and colder,longer winters. Thirty six thousand squarekilometers of salty soil lake-bed was exposedand blown by the strong windscommon in the region.
A 1973
A 1989
C 2000
References
- Orlovsky L., Orlovsky N. and Durdyev A. 2005. Dust storms in Turkmenistan. Journal of Arid Environments, 60, 83-97.
D Salty dust storm
Dust storms blow up to 75,000 tons ofthis exposed soil annually [16 D], dispersingsalt particles and pesticideresidues. This air pollution hascaused widespread nutritional andrespiratory problems in humans, andcrop yields have been diminished bythe added toxic salt deposits, even insome of the same fields irrigated withthe diverted water.
[16] Drying up of the Aral Sea as observed by satellite images in different years (A – C)with respect to the 1964 shoreline (in yellow).Salty dust storm blowing from the former lake-bed towards the agricultural areas south of the lake (D).
[15] Indirect evidences for existence of groundwater aquifers. Presenceof vegetation (seen in red) indicates seepages along the Amatzia Fault,Negev desert, Israel (A) and at the edges of the alluvial fans of the TurfanDepression, China (B).
References
- Bayarjargal, Y. and Karnieli, A. 2004. Assessing land-use and land-cover change in Bulgan Soum by remote sensing change detectiontechnique. Arid Ecosystems, 10, 127-133.
Soil compaction, soil crusting, infiltration, anderosion are all terms associated with medium-and fine-textured soils e.g. clay. Physicalsoil crusts [17] can be formed by natural processessuch as the impact of rainfall (or sprinkled water) on the topsoil along with physiochemical dispersion of clay minerals; sedimentation of fine material during floods; or by human activities such as intensive agriculture involving more and heavier machinery. Physical crust formation is a common and widespread phenomenon in aridand semi-arid soils. Once the topsoil is encrusted, less rainwater infiltrates to the sub surface and more runoff is created. Intensive runoffon bare desert soils creates sheet and gully(small channels) erosion. Significant correlation was found between encrusted soil and soil reflectance properties, enabling us to map the infiltration rate from a remote distance, using advanced hyperspectral technology [18].
[17] Physical soil crust.
[18] Use of advanced hyperspectral remote sensing data for mapping infiltration rate.
References
- Ben-Dor, E., Goldshalager, N., Braun, O., Kindel, B., Goetz, A.F.H., Bonfil, D., Agassi, M., Margalit, N., Binayminy, Y. and Karnieli, A. 2004. Monitoringof infiltration rate in semiarid soils using airborne hyperspectral technology. International Journal of Remote Sensing, 25, 2607-2624.
[19] Image clarification of different levels of sanitization and water logging in the Dashoguz Oasis, Turkmanistan.
References
- Ben-Dor, E., Patkin, K., Banin, A. and Karnieli, A. 2002. Mapping of several soil properties using DAIS-7915 hyperspectral scanner data: A case studyover clayey soils in Israel. International Journal of Remote Sensing, 23, 1043-1062.
Salinization of groundwater (downward salinization) and of soils (upwards salinization) is one of the main desertification processes in arid and semiari dregions, especially in agricultural areas. Anthropogenic activities such asirrigation, water drainage, regulation of surface flow, modification of hydrographic networks, and construction of water reservoirs affect the stability andrhythm of natural processes and cause significant changes in ecology at localand regional scales. Negative consequences include upward secondary salinization, waterlogging, and increased mineralization of return drainageflow.
Although there is a pressing need to identify salt-affected soils, detection and monitoring of such areas have posed problems to remote sensing applications since most of the salts are featureless in the reflectivity portion of the electromagnetic spectrum. Different indirect methods need to be developed andtested. These could be based on detecting changes in soils (over differentseasons), vegetation status data, integration of thermal data, principal component analysis, and more.
Turkmenistan is one country that suffered severely from secondary salinizationdue to mismanaged irrigation schemes. Irrigation in Turkmenistan ismainly concentrated in oases, where water is diverted from several local riversand from a long system of canals. Water is lost at a considerable rate fromthe unprotected banks of the canals. This has caused massive waterloggingand salinization of the surrounding land. In undrained irrigated fields, dissolved salts are pushed deeper into the soil by the irrigation water, and at thefield margins, these salts spread to vacant lands. Figure [19] demonstrates remote sensing methods we have developed to detect and map different degreesof salinization in the Dashguz oasis in northern Turkmenistan.
[7] Change detection technique reveals that a barhan (arcshaped)dune moved 47 m during 29 years in the Gobi Desert. Photograph credit: Jean Jacques CORDIER.