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Monitoring the impact of desertification in western Rajasthan using remote sensing
Journal of Arid Environments (\992) 22: 293-304
Monitoring the impact of desertification in western Rajasthan using remote sensing Ramachandran Kaushalya * Central Research Institute for DtylandAgriculture, Hyderabad 500659, India (Received 26 November 1990, accepted 5 December 1990) Desertification is a self-fuelling mechanism. The cumulative effect of the processes of desertification on an arid or semi-arid region like that of western Rajasthan, is a desert-like terrain depleted of all regenerative forces unless a consciouseffort is made to halt and reverse the process. The study area is a part of the Thar desert, and with a population growth rate of 2·2% annually, it is the most populated desert in the world. Desertification is continuing at an alarming rate because of the degradation of the fragile ecosystem caused by increasing human and livestock population in the region. With the help of remote sensing this study attempted to assess the impact of desertification around Jodhpur. The analysis of the topographical sheets of 1960-61 and Landsat Thematic Mapper (TM) data of 1986revealed that in the study area only 10% of the open scrub land remains and the area under forest stand has decreased by half. Area under loose sand which was only SOlo of the total area in 1960-61 had doubled in 1986. Similarly, urban expansion had increased four-fold and quarrying by sixfold since 1960-61. All these factors have destabilized the precarious ecological balance and the situation has been made worse by the droughts of the 1960sand 1970sin the region. Considering the magnitude and crucial nature of these processes a study of land-use and land cover of this region was undertaken and a temporal study of the situations prevailing in 1960-61 were compared with those prevailing in 1986-87.
Introduction The arid regions of the world, which comprise 35% of the earth's land surface, are heavily populated (c. 850 million). These populations are directly affected by the decreases in productivity associated with the desertification process. It is estimated that between 50,000 and 70,000 km 2 of useful land is going out of production every year in the world because of desertification (FAO & UNEP, 1984; Karrar & Stiles, 1984; Wijkman & Timberlake, 1984). Desertification can be defined as a comprehensive expression of economic, social, natural and induced processes which destroy the equilibrium of soil, vegetation, air and water in the areas subjected to edaphic or climatic aridity (Bryson & Baerreis, 1964) or both (Grainger, 1982). Continued deterioration of environmental conditions leads to a decrease in or destruction of the biological potential of the land (Kassas, 1977), deterioration of living conditions and an increase in desert landscape (Karrar & Stiles, 1984; Wijkman & Timberlake, 1984). Desertification is a self-accelerating process and its * Formerly at Central Arid Zone Research Institute, Geography Unit,Jodhpur, India. 0140-1963/92/030293 + 12$03-0010
© 1992 Academic Press Limited
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final stage is irreversible (Glantz, 1977). Only through man's action can it be slowed down and stopped (UNEP, 1986). The FAO-UNEP (1984) methodology to study desertification processes has been adopted in this paper. These natural and induced processes include the degradation of vegetative cover, water erosion, wind erosion, salinisation, reduction in soil organic matter, soil crusting and compaction and accumulation of substances toxic to plants and animals (Dregne, 1970; Nechaeva, 1978). The first four are determinant processes which have a major impact on land productivity while the last three are subordinate (UNCOD, 1977a,b). The temporal study was made possible by the use of remote sensing. Methodology and data source Toposheets and Landsat Thematic Mapper (TM) data were used to study the indicators of desertification in western Rajasthan since remote sensing enables the perception of change over a period oftime as data are available at regular intervals instantaneously; this is not the case with conventional geodetic surveys. There are several advantages of using satellite data for delineation and mapping of various land features and dynamic processes. For instance, Landsat imagery provides a synoptic view of a large area, up to 34,000 km 2 (185 x 185 km"), and it has a monitoring capability due to the repetitive coverage (every 16 to 18 days) in a relatively short time. There is now access to data every 8 days with two satellites orbiting regularly. The accuracy of this data is also very high because of relatively low planimetric and image distortions amenable to correction procedures. Remote sensing is relatively fast and economical for gross estimation when compared to any other method of surveying. Satellite data provides reliable, near real-time and unbiased baseline information. However, there are limitations. For example, imagery has limited spatial resolution, i.e. 1·1 acre or 0·45 ha. The scale is small (1: 1,000,000) and images lack stereoscopic view. A mappable unit on a 1: 1million scale imagery is 1 km 2 (1 mm x 1 mm on the map) and any isolated features of less than 1'0 km dimension cannot be mapped. Thematic Mapper (TM) data have several advantages over multi-spectral satellite (MSS) data. For example, TM has a higher spatial resolution which facilitates enlargement on a scale of 1: 50,000. Ground features can be perceived better in TM due to digital clusters, whereas in MSS feature delineation is difficult due to a lower resolution. Radiometric quality ofTM is better than that of MSS. TM thus provides better contrast and better perception of images as a result of its 8-bit quantisation and light segregation. In T M spectral bands are relatively well defined and as a result of which feature contrast is greater. The length of the near-infrared band in the case of MSS is O'8-1'1 mm while it is 0·76-0·90.um in TM and still better in Linear imaging self scanning sensors (LISS) of Indian Remote Sensing Satellite (IRS-IA) (0·77--o·86pm). TM has the advantage of a larger scale over MSS. It is enlarged four times to 1:250,000 scale. For the present study TM data, which were digitally enlarged five times and further optically enlarged four times, were used.
Data used Toposheets of western Rajasthan numbered 45 B, 45 C, 45 F and 45 G of I: 250,000 scale and 45 B/15, 45 B/16, 45 F/3and45 F/40fl: 50,000 were taken to prepare a base map. The toposheets of 1960 were compared with the maps prepared on the basis of photointerpretation of Landsat T M data in the form of standard FCC print generated from band 2 (0·5--o·60.um) band 3 (0·63--o·69.um) and band 4 (0·76-0·9.um) by exposing through blue, green and red guns, respectively. The Landsat-5 TM data (D-149-042) pertained to two dates, i.e. 20 April 1986and 31 October 1986. The technical reports 'Groundwater Reconnaissance Survey of UNDP Project Area' and 'Groundwater Evaluation of the Doli-Jhanwar-Pal Area, Rajasthan, India, May 1971' and 'Census of India Report of 1961 and 1981' were taken as the basis for assessing the
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recent groundwater status, water table, electrical conductivity, fluoride content and salt content in irrigation water (Kaushalya & Balak Ram, 1987). Geographical location, climate and physiography of study area Sixty-one per cent of the total area under Thar desert lies in the state of Rajasthan. The study area is located at the eastern fringe of the Thar between 26°16'N-26°30'N and n050'E-73°1O'E (Fig. 1) covering an area of 6075 km 2 and is mapped in four toposheets 45 B1l5, 45 B1l6, 45F/3 and 45 F/4. The 300-mm and 350-mm isohyets lie to the west and east of this region, respectively. Rainfall is erratic from July to September with a coefficient of variation (CV) ranging from 70 to 80%. Both diurnal and annual temperature variations are very high and the noon temperature in summer reaches 45-47°C and falls to sub-zero levels at night in winter. Mean relative humidity varies from 36 to 50% during summer and from 66 to 70% during monsoons. During the post-meridian period, relative humidity falls perceptibly. Potential evapotranspiration (PE) values in summer (Apri1June) vary from 7 to 9 mm and mean daily wind speed varies from 8 to 20 km h .. \ with a maximum in dry summer months (Krishnan, 1977). The region is part of the stable block of the Deccan Peninsula. Although its geology is very complex it is now covered by a thick mantle of sand. According to Wadia, the aeolian deposits belong to the Pleistocene and Recent times and are essentially sand drifts derived from the weathering of rock outcrops. The sand consists predominantly of quartz with a little feldspar and hornblende and the grains are uniformly rounded by attrition. The dominant landforms are the rhyolite ranges topped by a layer of sandstone on one hand and
Figure 1. The study area.
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the old alluvial plain transformed by wind on the others. The outcrops of Upper Vindhyan are sandstone and limestone of Pre-Cambrian granite, and rhyolite, Aravalli slates and quartzite are skirted by short piedmont and pediment plain. The old alluvial plain has a general slope towards the south-east and south and is drained by the [oiri, Sukhri and Gusainadi rivers which are saline and partly ephemeral. Saline patches or playas and quarries are dotted all over the region. Thus the study area comprises of a variety of landscapes ranging from thickly populated irrigated pockets to fertile dry farming tracts and sand dunes, rugged ranges to tabletop hills and desolate gravelly plains (CAZRI, 1982).
Soils Deep to very deep light-textured soils, Chirai series-normal phase-with a moisture retention capacity of 65-90 mm m- 1 depth are prominent in this region (CAZRI, 1982). Around Pal village, loamy fine sand and slightly heavier sub-soil is found which is less erodible and more suitable for agriculture. Sand movement as a result of wind action is prominent in the western and south-western sectors of the region. Soils with a compact kankar pan at shallow depth and a gravelly rocky pediment are also found in smaller areas. Younger alluvial plain and natural saline depressions are found near the drainage channels in the north-east and south-east sectors of the study area. Land-use capability of the different land units within the region is as follows: dune soils which are highly erodible are best suited for silvipastoral systems (class VI) while deep light-textured soils with hummocky relief are suitable for arable farming. Shallow gravelly soils are suitable for controlled grazing while rocky surface is good for water harvesting.
Vegetation Mixed xeromorphic woodland with non-thorny and evergreen species and spiny vegetation are found in a few sections within the region. The common plant community under this vegetation type is Prosopis cineraria-Capparis decidua-Zizyphys nummularia. Flat plains with heavier soils are dominated by Salvadora oleoides-Prosopis cinereria-Acacia nilotica community. Protected hills of sandstone and limestone with a thin layer of soil support Acaciasenegal and Grewia tenax, while for sand dune stabilisation AcaciasenegalMaytenus emarginatus-Prosopis cineraria-Calligonum polygonides-Salvadora oleoides have been used. Lithophytic scrub-like vegetation is found on eroded rocky surfaces, gravelly plains and exposed pediments around Jodhpur. Psammophytic scrub vegetation is found on sandy undulations, dunes, interdunal plains and buried pediments while unprotected sand dunes have a Calligonum polygonidesPanicum turgidum-Acacia jacquemontic community. Saline depressions have halophytic scrub-type species-Suaeda fruticosa, Salsola baryosma, Haloxylon salicornicum, Spotobolus marginatus. Grass communities found on sand dunes and sandy plains are composed of Lasiurus sindicus, Cymbopogenjwarancusa, Panicum turgidum and Cenchrus bifiorus. The cultivated study tract supports Aristida funiculata and rocky gravelly pediment supports Oropetium thornacum-Dactylocteniam sindicus grasslands (CAZRI, 1982). Results and discussion
Assessment of the impact of desertification is difficult mainly because of a lack ofsufficient data for analysing the amount of land degradation which has occurred over a period of time. Assessment depends heavily on the observations of a few aged inhabitants and on extrapolation of data collected and analysed from smaller areas (Nechaeva, 1978; Kharin, 1986; FAO & UNEP, 1984; UNEP, 1986). Six desertification processes, viz. vegetation degradation, wind erosion resulting in sand drift, quarrying and mine spills, expansion of settlement, pressure of human and animal population on land, depth of ground water
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table, salinity and fluoride levels of groundwater, have been taken as indicators and the study area has been evaluated for possible degradation. The results are as follows.
Degradation of vegetative cover The removal or destruction of vegetative cover by various agents mostly catalysed by human interference (Cloudsley- Thompson, 1978; Grove, 1979) in the natural ecological balance leads to desertification. Tree felling, cultivation practices, bushfires and overgrazing all result in removal of vegetative cover from the ground which has low potential for revegetation. This leaves the soil barren and fully exposed to other processes of desertification (Charney et aZ., 1975). The progressive disappearance of palatable pasture sources is also a desertification process (Satyanarayana, 1960; Shankarnarayan & Singh, 1979). The topographical sheets of 'India and Adjacent Countries Series' numbered 45 B/15, 45 B/16, 45 F/3 and 45 F/4 of 1960-61 on 1: 50,000 scale were studied and the areal extent of forest stand and open scrub were mapped to prepare a base map. The Thematic Mapper (TM) data of 1986 (April and October) were then visually analysed and interpreted and the vegetative stands mapped. April is a dry pre-monsoon month which is highly suitable for mapping the core forest area, while October is a post-monsoon month with lower temperatures and late Kharif crop over the ground; an October image would thus provide information on total cropped area along with the actual extent of permanent vegetative stand. The toposheets of 1960-61 covering the study area depicted an area of 100'25 km 2 as under open scrub and 5'75 km 2 as under forest stands (figures provided by the Forest Department of the State). As a percentage of the total study area of 6075 km", open scrubs accounted for less than 2% of the area while the forest stand was less than 0·1 %. However, since 1960-61 the deterioration has been immense. The TM data of 1986 depict further degradation of the open scrub which has shrunk in areal extent to 11'25 km 2 which is less than 0·2% of the study area. According to the same data source, the forest stand has been reduced to an areal extent of 3·00 km 2 which is only 0·04% of the study area. The Thematic Mapper imagery of October 1986 revealed that 2·25 km 2 area of the study region was under kharif crop which amounts to 0'03% of the study area (Fig. 2). Extent of sand drift and sand encrustation
The combined effect of the erosive force of wind and the lack of ground cover has resulted in sand drift. The loose soils are re-worked by the wind and they encroach on agricultural fields, roads, rails and settlements besides silting and clogging canals and drainage channels. Cultivation of marginal lands causes destabilisation of the soil surface resulting in sand movement activated by wind action (Rapp et al., 1976). Sand encroachment is thus an indicator of desertification; it is both a cause and a result of desertification. Wind action also causes deflation of the soil surface resulting in blowout depression and gravelly pediment, locally called a magra which is totally devoid of vegetation. The toposheets of 1960-61 revealed that an area of 157'75 km 2 (2'6% of the study area) was covered by loose sand. The Thematic Mapper data of 1986 revealed that the areal extent of sand encrustation had greatly increased. Loose sand covered an area of 252'75 km 2 which was 4·2% of the total area in 1986. The TM data of April was found to be suitable for mapping sand cover because the dry season resulted in clear sky and minimum vegetation cover (Fig. 2). Areas covered by loose sands are being cultivated. Hence, wherever tubewells are available for irrigation, early rabi crops are found on the ground and loose sand is not seen as extensively as in October. The maximum combined extent of loose sand based on the interpretation of imagery of both dates (April and October 1986) has been taken and verified on the ground and conclusions of the extent of loose sand cover in the area have been drawn. The figures thus indicate that the areal extent of loose sand has increased tremendously in the study area and has encroached on fertile agricultural fields, roads and rail track and lowered the water table in the study area (Fig. 3).
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Growth of urban agglomeration - increase in theareal extent of built-up areas Expansion of settlements causes desertification in two ways. An increase in the human population increases the pressure on marginal land resulting in its overexploitation. Settlements expand horizontally and this leads to clearing of scrub land, construction of buildings, laying out of roads, rails and mud tracks, etc. Constant trampling by human feet loosens the soil surface and impedes regeneration of vegetative cover. The toposheets of this area surveyed in 1960-61 revealed that the horizontal extent of Jodhpur city was 10·25 km 2 which increased to over 45·25 km 2 in 1986. It is thus evident that in a period of 25 years (1960-61 to 1986)Jodhpur city, as indicated by its built-up area, has expanded by 400% (Fig. 2). This is a tremendous rate of growth for an urban centre based on a fragile ecosystem, and has actually accelerated the process of desertification.
Increase in quarrying activity causing ecological destabilisation Unrestricted urban expansion and proliferation of building construction has given impetus to quarrying activity in the region. This has led to destabilisation of the lithosphere, destruction of the vegetative cover, mine spills and pollution of the atmos-
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ph ere due to quarry dust, etc. Quarrying of sandstone for building purposes is undertaken in the northern and western sectors of the study area. Limestone quarries are located in the eastern sector. In 1960-61 quarrying was confined to an area of2·50 km 2 which increased to 11·5 km 2 in 1986-an increase of over 600% in a period of25 years. The areal extent of quarries in the area is almost equal to that of open scrub land in the area. The TM data of April are ideal for mapping stony pediments, saline patches and quarries as there is least vegetation during this period of the year. Agriculture and crop productivity
In arid regions throughout the world agriculture is essentially rainfed except small pockets which are irrigated carefully and managed properly (Grainger, 1982). The amount of rainfall received in the study area was highly variable (CV 80%) and the mean annual rainfall was less than 30 em. Failure of rainfall and associated drought events are fairly common, as are floods. In a study on droughts in India (Kaushalya & Ramana Rao, 1990)it was noted that western Rajasthan has experienced over 31 severe droughts and 17 floods between 1870 and 1980. The variable nature of rainfall is reflected in the composition of crop cafetaria and the agricultural productivity. The crops sown in this region are
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essentially millets and are hardy varieties, viz. pearl millet (Bajra), sorghum (Jowar), finger millet, maize, barley, kidney bean (moth) and oilseeds (castor and seasmum). Average yields are lower than the State averages, due to low erratic rainfall and sandy degraded soils. However, sound irrigation projects and better farming practices can help in increasing agricultural productivity six-fold as noted by Grainger (1982) and seen in a few irrigated pockets in the neighbourhood at Pal village, Mathaniya village, Iawai, Bilara town and in the Pali district.
Rural land-use in the studyarea Besides the urban agglomeration of Jodhpur there are about 30 villages or rural settlements within the study area. The total geographical area ofthese villages is 36,717 ha, of which 78% is cultivated, 20% uncultivated and 2% pastures or common grazing ground. Of the total cultivated area in the region, less than 5% is irrigated by tube wells, the rest is rainfed. The major crop combinations are (in order of importance) wheat and pearl millet (Bajra), pearl millet and wheat, pearl millet as the sole crop, pearl millet and kidney bean (moth). Ground waterstatus in the region Water has always been a scarce commodity in the arid zones of the world. Due to low and erratic rainfall and high evapotranspiration rates, surface water is negligible and there is very little ground water recharge in the region. In western Rajasthan the annual panevaporation ratio is 0·79 which is higher than the potential evapotranspiration. Rainwater surplus is also minimal. During wet years an average of 49 mm is available for ground water recharge (Srivastava et al., 1979). In dry years no recharge occurs and soils are completely depleted of moisture by May-June and remain below field capacity. Data on well inventories prepared by PHED (Public Health and Water Works Department) and the Groundwater Board, Jodhpur Division, indicate that there were 42 wells in the study area in 1958 and the depth of the water table ranged from 26' 36 ft in Basni to 162'2 ft in Pal village. In 1986 the water table in these wells had fallen from 26·36 ft to 67 ft and from 162'2 ft at Pal to 270 ft. This has also resulted in a fall in their output. For instance, the Rampura tube well which had an output of 1571akh I day-l initially, wasabletoprovideonly911akh day-l in 1987. Thewaterlevelin the wells within the study areas and the geology of the area is depicted in Fig. 4. The total dissolved solids (TDS) in the ground water near Jodhpur city is 2000 p.p.m. which is the maximum allowable concentration as recommended by the Defence Laboratory, Jodhpur (1966-68, 1974). To the east and south east of Jodhpur the TDS in ground water ranged from 2000 to 3000 p.p.m., which is classified as unpotable. In the wells surveyed at Basni, ground water TDS was 268 p.p.m., at Barli 346 p.p.m., at Pal 490696 p.p.m., at Doli 550 p.p.m. and at Borunda and Jhanwar it was 1112and 1164 p.p.m., respectively. To the south of Jodhpur, TDS was 878 p.p.m. and in the north it was 1352 p.p.m., higher than the normal standard of drinking water given by USPHS (1962) and relaxed up to 1500 p.p.m. by WHO (1963). Here ground water also contains a high percentage of chlorine (permissible limit 600 p.p.m.) and fluoride (2 p.p.m. maximum limit) higher than the levels recommended by ICMR (1975). During the study it was noticed that ground water with 1000 p.p.m. chloride concentration and 3000 p.p.m. of TDS was being used by the local population for drinking and other domestic purposes (Fig. 5). The prolonged drought of the 1970sand 1980sand the insufficient ground water recharge have accentuated the situation. The increased pumping and irrigation of fields by this water has led to another problem-salinity in agricultural fields. Since 1986, drinking water has become scarce and water has to be transported for the urban population of Jodhpur (Kaushalya & Balak Ram, 1987).
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Overgrazing due to an increase in livestock population The livestock population in the Jodhpur tahsil has doubled in 25 years since 1960-61, partly due to better veterinary facilities and also for meeting the growing demand for animal products in the region. Within the livestock structure the population of goats, the perpetrator of desertification, has increased by 272%, while that of sheep has increased by 200% and bovines by 110'27% to 176'7%. The tremendous increase in livestock population is due to the protective attitude of the local residents, the bishnois, who are a grazing community. The increase in livestock herd size has not resulted in higher productivity levels; it has only led to overgrazing which causes desertification (Grainger, 1982; Otterman, 1974). Overgrazing causes a decline in the annual production of pasture vegetation, palatable grass species especially the perennials which are good at binding soils. The trampling by stock near waterholes and overgrazing ofwet season pastures cause soil compaction and sealing, inhibiting the growth of grass. Overgrazing also damages the vegetation at the crest of stable sand dunes, thus destabilising them, and causing sand drifts. An increase in livestock population has a damaging influence on the environment. Degraded pasture land with a lower carrying capacity has caused a fall in livestock productivity; and the increasing livestock population is accentuating the desertification process.
Population growth There has been a 202% increase in the human population in Jodhpur tahsil in the 20 years since 1960-61. The population density in the Jodhpur tahsil was 102 individuals km " (Census ofIndia, 1961) in 1961 while in 1981 it was 207 individuals km- 2 while in urban areas it was 6445 individuals km -2. Thus in 20 years the human population has more than doubled in this fragile ecosystem, while agricultural production has not kept pace. This has led to shortages in food and essential commodities. Milk and livestock products are in short supply in rural areas and the levels of nutrition have fallen. Conclusions This study has revealed that the area under forest cover has decreased while the area under loose sand, quarry and urban settlement has increased enormously. The extreme climatic conditions in conjunction with the high rate of increase of the human and livestock populations have accelerated desertification. Loose sand has covered agricultural land and the overexploitation of ground water has caused the fall in the water table. Increasing use of ground water with a high TDS content (5000 p.p.m. and above) for irrigation purposes has increased soil salinity, thus inhibiting crop growth in agricultural fields. The drought of 1985-87 revealed the decreasing productivity of the land. Large quantities of grain and fodder had to be imported to the area to compensate for the scarcity. In 1987 the mortality rate of livestock was very high in Rajasthan and Gujarat. There was a large-scale migration of people from rural to urban areas despite Government efforts to stem the flow by undertaking massive drought relief in the area. These are the economic implications of desertification in western Rajasthan. Desertification is indeed a self-fuelling mechanism, as is evident from the study. The high rate of increase in the human and livestock populations in this marginal region has led to desertification. The increase in quarrying activity, the expansion of built-up areas, the decrease in forest area, the degradation of open scrub, the destabilisation of sand dunes and the cultivation of marginal land have allied to desertification. Only man's intervention can halt the process. Desertification can be checked even though reversing the trend may be difficult. First, there is an urgent need to check overcultivation, overgrazing, excessive deforestation, the increase in livestock population, bad irrigation, shifting cultivation and population explosion. To control sand drift, sand dunes have to be stabilised with the help of
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mechanical and phytochemical methods. Cultivation on stable and unstable sand dunes should be prohibited as they have been identified to be the source of drifting sand. The concept of 'zero-ploughing' is also relevant to these areas. By this, the friable top soil in these areas will not be disturbed, which tends to erode due to high wind speed. Overexploitation of ground water must be checked and ground water with a high TDS content should be treated before being used for irrigation. Agricultural fields which have turned saline as a result of irrigation with brackish water must be treated with gypsum and fly ash. Lack of adequate recharge is also a reason for the reduced ground water potential in these areas. Hence, recharge structures must be built at appropriate sites to encourage ground water replenishment. The commissioning of Indira Gandhi canal (Rajasthan canal) in the inhospitable terrain in western Rajasthan has facilitated the development of command area. Afforestation of large areas, erection of shelter belts, aerial seeding and revegetation of sand dunes have been undertaken. The canal has enabled farmers to grow groundnut and sugar cane in the Bikaner district which would not have otherwise been possible. However, due to a lack of adequate drainage and proper planning, the canal has increased soil salinity and waterlogging above previous levels. Thus good water management is as essential as availability of adequate water for irrigation and other purposes. To implement all these measures, man's active participation and intervention is obligatory; only then can the process of desertification be checked. The author wishes to acknowledge the encouragement and guidance given by the late Dr K. A. Shankarnarayan, Dr R. P. Dhir, Mr A. K. Sen and Dr Balak Ram ofCAZRI, Jodhpur, and MR K. V. G. K. Murthy (CRIDA) for drawing the maps.
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Kaushalya, R. & Ramana Rao, B. V. (1990). Rainfall abnormalities in India. CRIDA, Hyderabad (unpubl.). Kharin, N. G. (1986). Multipurpose desertification map and methodology for compiling them from space images. SovietJournalofRemote Sensing, 5: 69-80. Krishnan, A. (1977). Climatic changes relating to desertification in the arid zone of north-west India. AnnalsofArid Zone, 16: 302-309. Nechaeva, N. T. (1978). Problem of elaboration of desertification indicators. Problemy Osvoeniya Pustyn, 4: 18-24. Otterman, J. (1974). Baring high albedo soils by overgrazing: a hypothesized desertification mechanism. Science, 186: 531-533. Rapp, A., Le Houerou, H. N. & Lundholm, B. (Eds) (1976). Can desert encroachment be stopped? Ecological Bulletin No. 24. Swedish Natural Science Research Council. Satyanarayana, Y. (1964). Habitats and plant communities of the Indian desert. Proceedings of Symposium on Problems ofIndian Arid Zone, CAZRI, Jodhpur. Shankarnarayan, K. A. & Singh, S. (1979). Application of Landsat data for natural resource inventory and monitoring of desertification. Technical Report, (VISP) Remote Sensing Institute, South Dakota State University, Brookings, South Dakota, U.S.A. Shankarnarayan, K. A. & Sen, A. K. (1983). Combating Desertification. Monograph. Jodhpur: CAZRI. Srivastava, K. K., Kapoor, V. K. & Abbi, S. D. S. (1979). Potential evaporation loss and groundwater recharge in Luni Catchment in West Rajasthan. In: Gupta, S. K. & Sharma, P. (Eds), Current Trends in Arid ZoneHydrology. New Delhi: T. T. Publications. pp. 301-314. UNCOD (1977a). Desertification: an overview. U.N. Conference on Desertification, 74/1, Nairobi, Kenya. UNCOD (1977b). Plan of action to combat desertification. U.N. Conference on Desertification, Nairobi, Kenya. UNEP (1986). Aridland development and combat against desertification: an integrated approach USSR. Committee for UNEP, Moscow, U.S.S.R. USPHS (1962). Drinking water standards. Washington D.C.: USPHS, U.S. Department of Health, Education and Welfare. WHO (1963). International standards for drinking water. Geneva: WHO. Wijkman, A. & Timberlake, L. (1984). Natural Disasters-Acts of God orActs ofMan? Earthscan Publications, International Institute for Environment and Development and Swedish Red Cross.
Monitoring the impact of desertification in western Rajasthan using remote sensing Ramachandran Kaushalya * Central Research Institute for DtylandAgriculture, Hyderabad 500659, India (Received 26 November 1990, accepted 5 December 1990) Desertification is a self-fuelling mechanism. The cumulative effect of the processes of desertification on an arid or semi-arid region like that of western Rajasthan, is a desert-like terrain depleted of all regenerative forces unless a consciouseffort is made to halt and reverse the process. The study area is a part of the Thar desert, and with a population growth rate of 2·2% annually, it is the most populated desert in the world. Desertification is continuing at an alarming rate because of the degradation of the fragile ecosystem caused by increasing human and livestock population in the region. With the help of remote sensing this study attempted to assess the impact of desertification around Jodhpur. The analysis of the topographical sheets of 1960-61 and Landsat Thematic Mapper (TM) data of 1986revealed that in the study area only 10% of the open scrub land remains and the area under forest stand has decreased by half. Area under loose sand which was only SOlo of the total area in 1960-61 had doubled in 1986. Similarly, urban expansion had increased four-fold and quarrying by sixfold since 1960-61. All these factors have destabilized the precarious ecological balance and the situation has been made worse by the droughts of the 1960sand 1970sin the region. Considering the magnitude and crucial nature of these processes a study of land-use and land cover of this region was undertaken and a temporal study of the situations prevailing in 1960-61 were compared with those prevailing in 1986-87.
Introduction The arid regions of the world, which comprise 35% of the earth's land surface, are heavily populated (c. 850 million). These populations are directly affected by the decreases in productivity associated with the desertification process. It is estimated that between 50,000 and 70,000 km 2 of useful land is going out of production every year in the world because of desertification (FAO & UNEP, 1984; Karrar & Stiles, 1984; Wijkman & Timberlake, 1984). Desertification can be defined as a comprehensive expression of economic, social, natural and induced processes which destroy the equilibrium of soil, vegetation, air and water in the areas subjected to edaphic or climatic aridity (Bryson & Baerreis, 1964) or both (Grainger, 1982). Continued deterioration of environmental conditions leads to a decrease in or destruction of the biological potential of the land (Kassas, 1977), deterioration of living conditions and an increase in desert landscape (Karrar & Stiles, 1984; Wijkman & Timberlake, 1984). Desertification is a self-accelerating process and its * Formerly at Central Arid Zone Research Institute, Geography Unit,Jodhpur, India. 0140-1963/92/030293 + 12$03-0010
© 1992 Academic Press Limited
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final stage is irreversible (Glantz, 1977). Only through man's action can it be slowed down and stopped (UNEP, 1986). The FAO-UNEP (1984) methodology to study desertification processes has been adopted in this paper. These natural and induced processes include the degradation of vegetative cover, water erosion, wind erosion, salinisation, reduction in soil organic matter, soil crusting and compaction and accumulation of substances toxic to plants and animals (Dregne, 1970; Nechaeva, 1978). The first four are determinant processes which have a major impact on land productivity while the last three are subordinate (UNCOD, 1977a,b). The temporal study was made possible by the use of remote sensing. Methodology and data source Toposheets and Landsat Thematic Mapper (TM) data were used to study the indicators of desertification in western Rajasthan since remote sensing enables the perception of change over a period oftime as data are available at regular intervals instantaneously; this is not the case with conventional geodetic surveys. There are several advantages of using satellite data for delineation and mapping of various land features and dynamic processes. For instance, Landsat imagery provides a synoptic view of a large area, up to 34,000 km 2 (185 x 185 km"), and it has a monitoring capability due to the repetitive coverage (every 16 to 18 days) in a relatively short time. There is now access to data every 8 days with two satellites orbiting regularly. The accuracy of this data is also very high because of relatively low planimetric and image distortions amenable to correction procedures. Remote sensing is relatively fast and economical for gross estimation when compared to any other method of surveying. Satellite data provides reliable, near real-time and unbiased baseline information. However, there are limitations. For example, imagery has limited spatial resolution, i.e. 1·1 acre or 0·45 ha. The scale is small (1: 1,000,000) and images lack stereoscopic view. A mappable unit on a 1: 1million scale imagery is 1 km 2 (1 mm x 1 mm on the map) and any isolated features of less than 1'0 km dimension cannot be mapped. Thematic Mapper (TM) data have several advantages over multi-spectral satellite (MSS) data. For example, TM has a higher spatial resolution which facilitates enlargement on a scale of 1: 50,000. Ground features can be perceived better in TM due to digital clusters, whereas in MSS feature delineation is difficult due to a lower resolution. Radiometric quality ofTM is better than that of MSS. TM thus provides better contrast and better perception of images as a result of its 8-bit quantisation and light segregation. In T M spectral bands are relatively well defined and as a result of which feature contrast is greater. The length of the near-infrared band in the case of MSS is O'8-1'1 mm while it is 0·76-0·90.um in TM and still better in Linear imaging self scanning sensors (LISS) of Indian Remote Sensing Satellite (IRS-IA) (0·77--o·86pm). TM has the advantage of a larger scale over MSS. It is enlarged four times to 1:250,000 scale. For the present study TM data, which were digitally enlarged five times and further optically enlarged four times, were used.
Data used Toposheets of western Rajasthan numbered 45 B, 45 C, 45 F and 45 G of I: 250,000 scale and 45 B/15, 45 B/16, 45 F/3and45 F/40fl: 50,000 were taken to prepare a base map. The toposheets of 1960 were compared with the maps prepared on the basis of photointerpretation of Landsat T M data in the form of standard FCC print generated from band 2 (0·5--o·60.um) band 3 (0·63--o·69.um) and band 4 (0·76-0·9.um) by exposing through blue, green and red guns, respectively. The Landsat-5 TM data (D-149-042) pertained to two dates, i.e. 20 April 1986and 31 October 1986. The technical reports 'Groundwater Reconnaissance Survey of UNDP Project Area' and 'Groundwater Evaluation of the Doli-Jhanwar-Pal Area, Rajasthan, India, May 1971' and 'Census of India Report of 1961 and 1981' were taken as the basis for assessing the
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recent groundwater status, water table, electrical conductivity, fluoride content and salt content in irrigation water (Kaushalya & Balak Ram, 1987). Geographical location, climate and physiography of study area Sixty-one per cent of the total area under Thar desert lies in the state of Rajasthan. The study area is located at the eastern fringe of the Thar between 26°16'N-26°30'N and n050'E-73°1O'E (Fig. 1) covering an area of 6075 km 2 and is mapped in four toposheets 45 B1l5, 45 B1l6, 45F/3 and 45 F/4. The 300-mm and 350-mm isohyets lie to the west and east of this region, respectively. Rainfall is erratic from July to September with a coefficient of variation (CV) ranging from 70 to 80%. Both diurnal and annual temperature variations are very high and the noon temperature in summer reaches 45-47°C and falls to sub-zero levels at night in winter. Mean relative humidity varies from 36 to 50% during summer and from 66 to 70% during monsoons. During the post-meridian period, relative humidity falls perceptibly. Potential evapotranspiration (PE) values in summer (Apri1June) vary from 7 to 9 mm and mean daily wind speed varies from 8 to 20 km h .. \ with a maximum in dry summer months (Krishnan, 1977). The region is part of the stable block of the Deccan Peninsula. Although its geology is very complex it is now covered by a thick mantle of sand. According to Wadia, the aeolian deposits belong to the Pleistocene and Recent times and are essentially sand drifts derived from the weathering of rock outcrops. The sand consists predominantly of quartz with a little feldspar and hornblende and the grains are uniformly rounded by attrition. The dominant landforms are the rhyolite ranges topped by a layer of sandstone on one hand and
Figure 1. The study area.
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the old alluvial plain transformed by wind on the others. The outcrops of Upper Vindhyan are sandstone and limestone of Pre-Cambrian granite, and rhyolite, Aravalli slates and quartzite are skirted by short piedmont and pediment plain. The old alluvial plain has a general slope towards the south-east and south and is drained by the [oiri, Sukhri and Gusainadi rivers which are saline and partly ephemeral. Saline patches or playas and quarries are dotted all over the region. Thus the study area comprises of a variety of landscapes ranging from thickly populated irrigated pockets to fertile dry farming tracts and sand dunes, rugged ranges to tabletop hills and desolate gravelly plains (CAZRI, 1982).
Soils Deep to very deep light-textured soils, Chirai series-normal phase-with a moisture retention capacity of 65-90 mm m- 1 depth are prominent in this region (CAZRI, 1982). Around Pal village, loamy fine sand and slightly heavier sub-soil is found which is less erodible and more suitable for agriculture. Sand movement as a result of wind action is prominent in the western and south-western sectors of the region. Soils with a compact kankar pan at shallow depth and a gravelly rocky pediment are also found in smaller areas. Younger alluvial plain and natural saline depressions are found near the drainage channels in the north-east and south-east sectors of the study area. Land-use capability of the different land units within the region is as follows: dune soils which are highly erodible are best suited for silvipastoral systems (class VI) while deep light-textured soils with hummocky relief are suitable for arable farming. Shallow gravelly soils are suitable for controlled grazing while rocky surface is good for water harvesting.
Vegetation Mixed xeromorphic woodland with non-thorny and evergreen species and spiny vegetation are found in a few sections within the region. The common plant community under this vegetation type is Prosopis cineraria-Capparis decidua-Zizyphys nummularia. Flat plains with heavier soils are dominated by Salvadora oleoides-Prosopis cinereria-Acacia nilotica community. Protected hills of sandstone and limestone with a thin layer of soil support Acaciasenegal and Grewia tenax, while for sand dune stabilisation AcaciasenegalMaytenus emarginatus-Prosopis cineraria-Calligonum polygonides-Salvadora oleoides have been used. Lithophytic scrub-like vegetation is found on eroded rocky surfaces, gravelly plains and exposed pediments around Jodhpur. Psammophytic scrub vegetation is found on sandy undulations, dunes, interdunal plains and buried pediments while unprotected sand dunes have a Calligonum polygonidesPanicum turgidum-Acacia jacquemontic community. Saline depressions have halophytic scrub-type species-Suaeda fruticosa, Salsola baryosma, Haloxylon salicornicum, Spotobolus marginatus. Grass communities found on sand dunes and sandy plains are composed of Lasiurus sindicus, Cymbopogenjwarancusa, Panicum turgidum and Cenchrus bifiorus. The cultivated study tract supports Aristida funiculata and rocky gravelly pediment supports Oropetium thornacum-Dactylocteniam sindicus grasslands (CAZRI, 1982). Results and discussion
Assessment of the impact of desertification is difficult mainly because of a lack ofsufficient data for analysing the amount of land degradation which has occurred over a period of time. Assessment depends heavily on the observations of a few aged inhabitants and on extrapolation of data collected and analysed from smaller areas (Nechaeva, 1978; Kharin, 1986; FAO & UNEP, 1984; UNEP, 1986). Six desertification processes, viz. vegetation degradation, wind erosion resulting in sand drift, quarrying and mine spills, expansion of settlement, pressure of human and animal population on land, depth of ground water
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table, salinity and fluoride levels of groundwater, have been taken as indicators and the study area has been evaluated for possible degradation. The results are as follows.
Degradation of vegetative cover The removal or destruction of vegetative cover by various agents mostly catalysed by human interference (Cloudsley- Thompson, 1978; Grove, 1979) in the natural ecological balance leads to desertification. Tree felling, cultivation practices, bushfires and overgrazing all result in removal of vegetative cover from the ground which has low potential for revegetation. This leaves the soil barren and fully exposed to other processes of desertification (Charney et aZ., 1975). The progressive disappearance of palatable pasture sources is also a desertification process (Satyanarayana, 1960; Shankarnarayan & Singh, 1979). The topographical sheets of 'India and Adjacent Countries Series' numbered 45 B/15, 45 B/16, 45 F/3 and 45 F/4 of 1960-61 on 1: 50,000 scale were studied and the areal extent of forest stand and open scrub were mapped to prepare a base map. The Thematic Mapper (TM) data of 1986 (April and October) were then visually analysed and interpreted and the vegetative stands mapped. April is a dry pre-monsoon month which is highly suitable for mapping the core forest area, while October is a post-monsoon month with lower temperatures and late Kharif crop over the ground; an October image would thus provide information on total cropped area along with the actual extent of permanent vegetative stand. The toposheets of 1960-61 covering the study area depicted an area of 100'25 km 2 as under open scrub and 5'75 km 2 as under forest stands (figures provided by the Forest Department of the State). As a percentage of the total study area of 6075 km", open scrubs accounted for less than 2% of the area while the forest stand was less than 0·1 %. However, since 1960-61 the deterioration has been immense. The TM data of 1986 depict further degradation of the open scrub which has shrunk in areal extent to 11'25 km 2 which is less than 0·2% of the study area. According to the same data source, the forest stand has been reduced to an areal extent of 3·00 km 2 which is only 0·04% of the study area. The Thematic Mapper imagery of October 1986 revealed that 2·25 km 2 area of the study region was under kharif crop which amounts to 0'03% of the study area (Fig. 2). Extent of sand drift and sand encrustation
The combined effect of the erosive force of wind and the lack of ground cover has resulted in sand drift. The loose soils are re-worked by the wind and they encroach on agricultural fields, roads, rails and settlements besides silting and clogging canals and drainage channels. Cultivation of marginal lands causes destabilisation of the soil surface resulting in sand movement activated by wind action (Rapp et al., 1976). Sand encroachment is thus an indicator of desertification; it is both a cause and a result of desertification. Wind action also causes deflation of the soil surface resulting in blowout depression and gravelly pediment, locally called a magra which is totally devoid of vegetation. The toposheets of 1960-61 revealed that an area of 157'75 km 2 (2'6% of the study area) was covered by loose sand. The Thematic Mapper data of 1986 revealed that the areal extent of sand encrustation had greatly increased. Loose sand covered an area of 252'75 km 2 which was 4·2% of the total area in 1986. The TM data of April was found to be suitable for mapping sand cover because the dry season resulted in clear sky and minimum vegetation cover (Fig. 2). Areas covered by loose sands are being cultivated. Hence, wherever tubewells are available for irrigation, early rabi crops are found on the ground and loose sand is not seen as extensively as in October. The maximum combined extent of loose sand based on the interpretation of imagery of both dates (April and October 1986) has been taken and verified on the ground and conclusions of the extent of loose sand cover in the area have been drawn. The figures thus indicate that the areal extent of loose sand has increased tremendously in the study area and has encroached on fertile agricultural fields, roads and rail track and lowered the water table in the study area (Fig. 3).
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Growth of urban agglomeration - increase in theareal extent of built-up areas Expansion of settlements causes desertification in two ways. An increase in the human population increases the pressure on marginal land resulting in its overexploitation. Settlements expand horizontally and this leads to clearing of scrub land, construction of buildings, laying out of roads, rails and mud tracks, etc. Constant trampling by human feet loosens the soil surface and impedes regeneration of vegetative cover. The toposheets of this area surveyed in 1960-61 revealed that the horizontal extent of Jodhpur city was 10·25 km 2 which increased to over 45·25 km 2 in 1986. It is thus evident that in a period of 25 years (1960-61 to 1986)Jodhpur city, as indicated by its built-up area, has expanded by 400% (Fig. 2). This is a tremendous rate of growth for an urban centre based on a fragile ecosystem, and has actually accelerated the process of desertification.
Increase in quarrying activity causing ecological destabilisation Unrestricted urban expansion and proliferation of building construction has given impetus to quarrying activity in the region. This has led to destabilisation of the lithosphere, destruction of the vegetative cover, mine spills and pollution of the atmos-
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ph ere due to quarry dust, etc. Quarrying of sandstone for building purposes is undertaken in the northern and western sectors of the study area. Limestone quarries are located in the eastern sector. In 1960-61 quarrying was confined to an area of2·50 km 2 which increased to 11·5 km 2 in 1986-an increase of over 600% in a period of25 years. The areal extent of quarries in the area is almost equal to that of open scrub land in the area. The TM data of April are ideal for mapping stony pediments, saline patches and quarries as there is least vegetation during this period of the year. Agriculture and crop productivity
In arid regions throughout the world agriculture is essentially rainfed except small pockets which are irrigated carefully and managed properly (Grainger, 1982). The amount of rainfall received in the study area was highly variable (CV 80%) and the mean annual rainfall was less than 30 em. Failure of rainfall and associated drought events are fairly common, as are floods. In a study on droughts in India (Kaushalya & Ramana Rao, 1990)it was noted that western Rajasthan has experienced over 31 severe droughts and 17 floods between 1870 and 1980. The variable nature of rainfall is reflected in the composition of crop cafetaria and the agricultural productivity. The crops sown in this region are
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essentially millets and are hardy varieties, viz. pearl millet (Bajra), sorghum (Jowar), finger millet, maize, barley, kidney bean (moth) and oilseeds (castor and seasmum). Average yields are lower than the State averages, due to low erratic rainfall and sandy degraded soils. However, sound irrigation projects and better farming practices can help in increasing agricultural productivity six-fold as noted by Grainger (1982) and seen in a few irrigated pockets in the neighbourhood at Pal village, Mathaniya village, Iawai, Bilara town and in the Pali district.
Rural land-use in the studyarea Besides the urban agglomeration of Jodhpur there are about 30 villages or rural settlements within the study area. The total geographical area ofthese villages is 36,717 ha, of which 78% is cultivated, 20% uncultivated and 2% pastures or common grazing ground. Of the total cultivated area in the region, less than 5% is irrigated by tube wells, the rest is rainfed. The major crop combinations are (in order of importance) wheat and pearl millet (Bajra), pearl millet and wheat, pearl millet as the sole crop, pearl millet and kidney bean (moth). Ground waterstatus in the region Water has always been a scarce commodity in the arid zones of the world. Due to low and erratic rainfall and high evapotranspiration rates, surface water is negligible and there is very little ground water recharge in the region. In western Rajasthan the annual panevaporation ratio is 0·79 which is higher than the potential evapotranspiration. Rainwater surplus is also minimal. During wet years an average of 49 mm is available for ground water recharge (Srivastava et al., 1979). In dry years no recharge occurs and soils are completely depleted of moisture by May-June and remain below field capacity. Data on well inventories prepared by PHED (Public Health and Water Works Department) and the Groundwater Board, Jodhpur Division, indicate that there were 42 wells in the study area in 1958 and the depth of the water table ranged from 26' 36 ft in Basni to 162'2 ft in Pal village. In 1986 the water table in these wells had fallen from 26·36 ft to 67 ft and from 162'2 ft at Pal to 270 ft. This has also resulted in a fall in their output. For instance, the Rampura tube well which had an output of 1571akh I day-l initially, wasabletoprovideonly911akh day-l in 1987. Thewaterlevelin the wells within the study areas and the geology of the area is depicted in Fig. 4. The total dissolved solids (TDS) in the ground water near Jodhpur city is 2000 p.p.m. which is the maximum allowable concentration as recommended by the Defence Laboratory, Jodhpur (1966-68, 1974). To the east and south east of Jodhpur the TDS in ground water ranged from 2000 to 3000 p.p.m., which is classified as unpotable. In the wells surveyed at Basni, ground water TDS was 268 p.p.m., at Barli 346 p.p.m., at Pal 490696 p.p.m., at Doli 550 p.p.m. and at Borunda and Jhanwar it was 1112and 1164 p.p.m., respectively. To the south of Jodhpur, TDS was 878 p.p.m. and in the north it was 1352 p.p.m., higher than the normal standard of drinking water given by USPHS (1962) and relaxed up to 1500 p.p.m. by WHO (1963). Here ground water also contains a high percentage of chlorine (permissible limit 600 p.p.m.) and fluoride (2 p.p.m. maximum limit) higher than the levels recommended by ICMR (1975). During the study it was noticed that ground water with 1000 p.p.m. chloride concentration and 3000 p.p.m. of TDS was being used by the local population for drinking and other domestic purposes (Fig. 5). The prolonged drought of the 1970sand 1980sand the insufficient ground water recharge have accentuated the situation. The increased pumping and irrigation of fields by this water has led to another problem-salinity in agricultural fields. Since 1986, drinking water has become scarce and water has to be transported for the urban population of Jodhpur (Kaushalya & Balak Ram, 1987).
MONITORING THE IMPACT OF DESERTIFICATION Scale 2 0 I
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Overgrazing due to an increase in livestock population The livestock population in the Jodhpur tahsil has doubled in 25 years since 1960-61, partly due to better veterinary facilities and also for meeting the growing demand for animal products in the region. Within the livestock structure the population of goats, the perpetrator of desertification, has increased by 272%, while that of sheep has increased by 200% and bovines by 110'27% to 176'7%. The tremendous increase in livestock population is due to the protective attitude of the local residents, the bishnois, who are a grazing community. The increase in livestock herd size has not resulted in higher productivity levels; it has only led to overgrazing which causes desertification (Grainger, 1982; Otterman, 1974). Overgrazing causes a decline in the annual production of pasture vegetation, palatable grass species especially the perennials which are good at binding soils. The trampling by stock near waterholes and overgrazing ofwet season pastures cause soil compaction and sealing, inhibiting the growth of grass. Overgrazing also damages the vegetation at the crest of stable sand dunes, thus destabilising them, and causing sand drifts. An increase in livestock population has a damaging influence on the environment. Degraded pasture land with a lower carrying capacity has caused a fall in livestock productivity; and the increasing livestock population is accentuating the desertification process.
Population growth There has been a 202% increase in the human population in Jodhpur tahsil in the 20 years since 1960-61. The population density in the Jodhpur tahsil was 102 individuals km " (Census ofIndia, 1961) in 1961 while in 1981 it was 207 individuals km- 2 while in urban areas it was 6445 individuals km -2. Thus in 20 years the human population has more than doubled in this fragile ecosystem, while agricultural production has not kept pace. This has led to shortages in food and essential commodities. Milk and livestock products are in short supply in rural areas and the levels of nutrition have fallen. Conclusions This study has revealed that the area under forest cover has decreased while the area under loose sand, quarry and urban settlement has increased enormously. The extreme climatic conditions in conjunction with the high rate of increase of the human and livestock populations have accelerated desertification. Loose sand has covered agricultural land and the overexploitation of ground water has caused the fall in the water table. Increasing use of ground water with a high TDS content (5000 p.p.m. and above) for irrigation purposes has increased soil salinity, thus inhibiting crop growth in agricultural fields. The drought of 1985-87 revealed the decreasing productivity of the land. Large quantities of grain and fodder had to be imported to the area to compensate for the scarcity. In 1987 the mortality rate of livestock was very high in Rajasthan and Gujarat. There was a large-scale migration of people from rural to urban areas despite Government efforts to stem the flow by undertaking massive drought relief in the area. These are the economic implications of desertification in western Rajasthan. Desertification is indeed a self-fuelling mechanism, as is evident from the study. The high rate of increase in the human and livestock populations in this marginal region has led to desertification. The increase in quarrying activity, the expansion of built-up areas, the decrease in forest area, the degradation of open scrub, the destabilisation of sand dunes and the cultivation of marginal land have allied to desertification. Only man's intervention can halt the process. Desertification can be checked even though reversing the trend may be difficult. First, there is an urgent need to check overcultivation, overgrazing, excessive deforestation, the increase in livestock population, bad irrigation, shifting cultivation and population explosion. To control sand drift, sand dunes have to be stabilised with the help of
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mechanical and phytochemical methods. Cultivation on stable and unstable sand dunes should be prohibited as they have been identified to be the source of drifting sand. The concept of 'zero-ploughing' is also relevant to these areas. By this, the friable top soil in these areas will not be disturbed, which tends to erode due to high wind speed. Overexploitation of ground water must be checked and ground water with a high TDS content should be treated before being used for irrigation. Agricultural fields which have turned saline as a result of irrigation with brackish water must be treated with gypsum and fly ash. Lack of adequate recharge is also a reason for the reduced ground water potential in these areas. Hence, recharge structures must be built at appropriate sites to encourage ground water replenishment. The commissioning of Indira Gandhi canal (Rajasthan canal) in the inhospitable terrain in western Rajasthan has facilitated the development of command area. Afforestation of large areas, erection of shelter belts, aerial seeding and revegetation of sand dunes have been undertaken. The canal has enabled farmers to grow groundnut and sugar cane in the Bikaner district which would not have otherwise been possible. However, due to a lack of adequate drainage and proper planning, the canal has increased soil salinity and waterlogging above previous levels. Thus good water management is as essential as availability of adequate water for irrigation and other purposes. To implement all these measures, man's active participation and intervention is obligatory; only then can the process of desertification be checked. The author wishes to acknowledge the encouragement and guidance given by the late Dr K. A. Shankarnarayan, Dr R. P. Dhir, Mr A. K. Sen and Dr Balak Ram ofCAZRI, Jodhpur, and MR K. V. G. K. Murthy (CRIDA) for drawing the maps.
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