Desertification occurs in various parts of the world including humid region as well as arid and semi-arid regions. It is reported that more than one-fourth of land area on the earth is involved in this phenomenon. In some areas, land and vegetation degradation has caused such a serious problem as abandonment of farmlands. This phenomenon has been a universal issue since it was discussed at UNCOD (United Nations Conference on Desertification) in 1977. In Japan a research group of arid lands was organized in 1983 as one of the commissions of the Association of Japanese Geographers to proceed to research activities on regional problems of those areas and on desertification in the world. Then in 1985 the group was replaced by a working group. In the period of 1984 to 1986 the regular meeting for research was held 8 times, and the following studies were presented
OHMORI, H. (The Univ. of Tokyo): Natural Environment of Semi-Arid Regions in the Southern Part of Australia
TAKEUCHI, K. (The Univ. of Tokyo): Vegetation of Australia-Zonal Structure and its Anthropogenic Modification
NISHIZAWA, T. (Univ. of Tsukuba): Natural Environment of the Brazilian Northeast and Symptom of Desertification
KADOMURA, H. (Hokkaido Univ.): Savannization in Africa
TAMURA, T. (Tohoku Univ.): Grassfields-Forest Disappearance due to Human Activities in the Western Highlands of Cameroon
WASSON, R. J. (Australia Research Organization of Science and Technology): Geomorphology and Quaternary History of the Australian Desert Dunefields
HAGIWARA, H. (Graduate Student of Rissho Univ.): Problems of Sewerage System in Mexico
OJANY, F. F. (Univ. of Nairobi): Desertification in East Africa
KADOMURA, H. (Hokkaido Univ.): Drought in the Sahel-Sudan Zone (1)
TAKAMURA, H. (Rissho Univ.): Fire-burned fields and Symptom of Desertification in Zambia
MATSUMOTO, S. (.The Univ. of Tokyo): Some Problems Caused by the Development of Agriculture in Arid Zone of Middle and Near East
TAKAMURA, H. (Rissho Univ.): Salt Injury Caused by Groundwater Rising in the Costal Area of Persian Gulf
Moreover the symposium on “Geographical Aspects of ‘Desertification’” was held in September, 1986, in which the following subjects were set up, and 16 studies were presented:
1. Climatic variation and desertification
2. Water use and salinization
3. Agro-pastoral land use and degradation of land
4. Natural environment and change in land use
5. Control of desertification and suggestions for environmental management
Abstructs of paper submitted at the symposium were shown in the Geographical Review of Japan (Ser. A), Vol. 60, No. 2.
The present edition was planned to get those activities, especially at the symposium, into shape. If desertification is brought about not only by natural agency but also by human activities, then it must be examined from both physical and human points of view. Accordingly this edition aims at analyzing the present situation of desertification in the various fields of geography and studying out proper measures against this phenomenon.

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Attention to the problems of desertification by many governments and the scientific communities in many countries has increasingly been paid in the last decade. The drought in the Sahel, which extended from 1968 to 1973, especially focused public attention on the problem of desertification. In response, United Nations Conference on Desertification (UNCOD) was held in Nairobi, Kenya, from August 29 to September 9, 1977.
According to United Nations' definition(Biswas, M. R. and Biswas, A. K., 1980), “desertification is the diminution or destruction of the biological potential of land and can lead ultimately to desert-like conditions: grazing land ceases to produce pasture, dryland agriculture fails, irrigation fields are abandoned owing to salinization, waterlogging or some other form of soil deterioration”.
The purpose of this paper is to make clear the present situation of desertification and its research in the world.
The author explains the present situation and the problems of desertification in the world using distribution map showing the degree of desertification hazards prepared by Hopkins and Jones (1983). Aridlands divided into four types based on the degree of aridity such as hyper arid, arid, semiarid and subhumid regions, and three classes of desertification risk such as very high, high and moderate are shown on the map presented to UNCOD by UNESCO (1977).
Then, the author has described the results obtained from the case study in South-East Spain, and conducted the comparative study on desertification taking place in South-East Spain and in semiarid region of northeastern Brazil.
The author overviewed also the previous researches on desertification from the viewpoint of its definition, causes, reversibility or irreversibility, and degradation of tropical forests.
Lastly, the author has emphasized that the desertification is the subject of interdisciplinary researches which should be faced in collaboration with various fields of science such as geography, politics, economics, cultural anthropology, hydrology, climatology, geomorphology, plant ecology, forestry, agriculture and so on. So, it is of importance to organize an interdisciplinary project team on desertification and to make clear scientifically the causes of desertification.

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In the tropical and subtropical regions, extensive semi-arid regions exist. Recent researches on the atmospheric circulation and climatic change in West Africa and Northeast Brazil are reviewed. In West Africa, the recent decrease in the rainfall since the rainy decade of 1950s accelerated the desertification caused by the rapid increase in population. An investigation on the atmospheric circulation shows that the 850 mb African Monsoon winds are weak (strong) in the dry (rainy) August.
In Northeast Brazil, the recurrent drought in the 20th century were found to occur in ENSO year or one year after ENSO year. In the dry years, the South Atlantic high pressure is strong and extends its influence over Northeast Brazil compared to the average year. Consequently, the ITCZ is located in the Northern Hemisphere. In the rainy years, the ITCZ is displaced south toward Northeast Brazil in the March-April rainy season.
In both of these semi-arid regions, the rainfall were found to be controled by the large-scale atmospheric circulation. Hence the local desertification has little influence on the climatic change. On the other hand, the climatic change toward increased aridity in West Africa accelerates the process of desertification.

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The purpose of this paper is to illustrate briefly the characteristics of the traditional agricultural methods applied by the local peasants in West Asia, where climatical condition is characterized to be arid or semi-arid. The agricultural methods in this region have been developed differentlly from those of Western Europe and Monsoon Asia due to its specific natural conditions.
Basic agricultural technics have been developed generally in order to maintain fertility and to weed the land. However, in case of dry farming in the arid region, the prevention of evaporation of water from field soil and keeping the soil wet should also be added in object. In case of irrigation farming of the arid region, the techniques to prevent the adverse influence of salination of the soil, which is inevitably caused by irrigation, have been developed.
The agricultural methods taken by West Iranian peasants are as follows;
(1) In the course of the crop rotation, a certain portion of field is always laid fallow. The fallow land is used as pastures for rearing livestocks. In this region fertilizer had been not used. Fallow and dung of livestocks on the fallow land effectively maintain the fertility of land.
(2) During the fallow season, the fallow land is repeatedly ploughed. The ploughing has two aims. One is to eradicate weeds, and the other is to prevent soil from drying. The ploughing season starts from May and ends in October. Ploughing in the early part of the season is effective for the eradication of weeds. Especially shallow ploughing is effective for curtailing the evaporation of waters by preventing capillary action. Therefore the ploughing tool in West Asia is designed suitable for shallow ploughing.
(3) In irrigation, the border irrigation system is applied by peasants. This is the method to supply large quantity of water to the soil periodically, and has a effect to bring the salts down to the lower layer of the soil and restrain the adverse effect of salination on crops.
The traditional agricultural methods have not brought about high productivity, but enable the land to gain the long and stable production in this region. Contemporary agricultural entrepreneurs have been attempting new cultivation methods. However, they are not always applicable to the specific natural conditions in the West Asia. Although the farming based on the new methods have achieved high productivity, it also caused the deterioration of agricultural conditions such as salination and erosion of the soil.

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In the Australian Continent, in those areas occupied by Eurpoeans during the past 200 years, the natural environment has been modified; the change are most severe in the semi-arid regions. Prior to Eurpoean settlement, for some tens of thouthands years of Aboriginal occupation, there were slow changes mainly due to occasional burning of vegetation so that the Australian flora was gradually moved towards fire-resisted species. With the introduction of agriculture and grazing drastic changes in the density, floristics, and structure of native vegetation occurred. Disturbance or removal of vegetation has also affected surface and sub-surface hydrology. These changes have resulted in water or wind erosion and salinization. Where vegetation has been regenerated the species established are often those which are unpalatable to grazing stock or can withstand frequent burning. This leads to “desertification”.
The concept of “desertification” is ecologically defined as follows: The natural vegetation in equilibrium with the potentiality of regional climate (climatic climax) is replaced by the vegetation which grows naturally and originally in more arid region due to environmental stress directly induced by man and the accompanying land degradation.
The Murray-Mallee Region in the southern part of Australia, where the late Pleistocene and Holocene sand dunes are widely distributed, is a semi-arid region with annual rainfall of 250-400 mm; it is wholly covered with mallee (Eucalyptus spp.) scrub. The sand dunes which were stable covered totally with such vegetation during the pre-European days have become unstable at many sites due to vegetation clearing, farming and grazing associated with European settlement since the late 19th Century. The remobilization of sand dunes is one, of the typical events of “desertification” and, in extreme cases, the surface remains unvegetated. Various stages in vegetation change can be observed showing “desertification” or regeneration process. The authors describe the processes involved in land and vegetation degradation, and discuss the measures to prevent “desertification” based on the examination of the relationships between activity of sand drifting, thickness of loose sand overlying the stable substrata, floristic composition, community height, and activity of rabbits (Olyctolagus cuniculus).
The natural mallee scrub in the conservation park is dominated by Eucalyptus spp. accompanied by rich lower shrub layer species such as Casuarina stricta, Callitris verrucosa and Hakea leucoptera. The mallee scrub remaining on farmland, however, is very poor in understory resulting from grazing. Triodia spp. and Acacia spp. invade in the understory of the mallee scrub after bushfires. Acacia spp. regenerate easily and are dominant on the land where the surface is disturbed by vegetation clearance and/or wind erosion. Natural mallee scrub will regenerate through sprouting from so-called mallee roots (lignotuber) persisting after surface disturbance. When the lignotuber is exposed by wind erosion or covered by sand accumulation, the recovery to natural mallee scrub becomes very difficult. On such sites, Triodia colonies and/or Acacia scrub stand as disclimax communities with a life cycle of 30 to 50 years (Figs. 6 and 7).
The thicker the loose sand is, the more active and continuous is the sand drifting (Fig. 8). In particular, on many dunes with loose sand thicker than 80 cm, serious drifting has continued for recent tens of years exacerbated by the large amount of mobile sands and the difficulty of vegetation invasion. At the sites with thick loose sands, the population of rabbit introduced from Europe is larger, suggesting that the reactivation and acceleration of sand drifting is induced by rabbit infestation (Fig. 9).

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In India the “Drought Prone Areas” occupy nearly 20 percent of the total land with more than 60 millions of population at present. It should be noted that the Indian government has paid special attention to the welfare of the people and introduced various policies and programmes. In recent years, the government has introduced two types of rural development schemes in these areas: (1) large-scale scheme (construction of canal irrigation system with a big dam), and (2) small-scale scheme (installation of irrigatioe wells, soil conservation and afforestation work etc.).
This study is fundamentally concerned with the rural development and its impact on the changes of land use in the semi-arid region of the southern Deccan plateau. As our intensively investigated area, we selected two villages, Yerdona and Pidarakere, each of which is located respectively within the large-scale or the small-scale scheme areas stated above.
The land use of Yerdona has been drastically changed since the irrigation canals were connected to the fields of this village in 1957. Due to receiving a large quantity of water, the irrigated land gradually increased from only 1.2 percent in 1957 to 54.5 percent of all arable land by 1980. In these irrigated land, the dry crops such as sorghum (“jowal”), pearl millet (“bajra”) were replaced mainly by rice, groundnuts, cotton, etc. which are much more profitable than dry crops.
On the other hand, the land use of Pidarakere has been changed with the adoption of a variety of development schemes based on the five-year plans of the government. For example, as the contour bunding works financed by DPAP (Drought Prone Area Programme) were completed in 79 percent of all arable land between 1959 and 1980, it became possible to have a stable and good harvest of dry crops such as African millet (“ragi”), sorghum, onion, chilli, etc. And as many irrigative wells were installed particularly in the 1970s with the financial aid of IADP (Intensive Agricultural District Programme) and DPAP, the commercial crops such as sugarcane, coconut, banana, etc. were also cultivated in the irrigated fields.
It may be said that the rural development schemes applied to the investigated areas contributed to a great extent to the increased land productivity and changes of land use of the areas. However, at the same time the area affected by water-logging and salinization is rapidly increasing around Yerdona because an excessive amount of irrigation water is supplied to the land. In Pidarakere also the contour bunding works have resulted in the severe gully erosion in the black soil area and the groundwater of the village is almost exhausted due to the installation of many wells. It must be pointed out that these serious problems have caused by the lack of careful considerations about the ecological system of the land.

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More than 30% of the land surface of the earth consists of arid and semi-arid soils, which are generally too dry to produce a good yield. If enough fresh water is avairable and the soil condition is suitable, these soils can be irrigated and used for agricultural land. These regions therefore are expected to be the new promissing land for food production in future. From this background, recently much attention to arid and semi-arid soils has been paid, and more agricultural development projects are being planned and carried out. However, many projects fail in the past and even now, because some years after irrigation the salinity or alkali hazard increases. In this paper, the soil profile investigations of salts accumulation related to the depth of ground water level, some field experiments using lysimeter and the model experiments to clearfy the mechanisms of salt accumulation are illustrated.
From the field investigations and lysimeter experiments of irrigated lands in Khuzistan State, Iran, the following cases have been pointed out as the essential processes of soil salinization.
Case 1; Shallow groundwater formation due to seepage from earth canal and intensive irrigation such as basin irrigation.
Case 2; Logging of irrigation water at the extremely hard pan under plough layer caused by the compaction of tractor loading.
Case 3; Salts addition to the soil irrigated by water having high electric conductivity.
Case 4; Low leaching resulted from the extremely high evapotranspiration under the cultivation during summer dry season.
Case 1 is most dominant process to develop the soil salinization in this regions. Salt contents at surface and subsoil of the field with shallow groundwater table are 131.4 and 20.64mS/cm in EC, respectively. These salts are mixtures of sodium and magnesium salts which are mobile cations through soil profile. Because the critical value of EC for plant growth is 4mS/cm, these findings show that amelioration for these soils may be practically impossible.
Case 2 is repeatedly found in desalinized fields with clayey texture. The continual cultivation in desalinized field causes an extremely hard pan to form under plough layer due to the compaction of tractor loading. This hard pan is apt to become a impermeable layer logged with irrigation water, and provides the “secondary site” which functions to supply sodium salts to surface horizon. In the cultivated land with drainage system, salts exclusion effects by irrigation water is surely promoted, however, one should pay the attention that there occurs unfavorable changes in soil chemical and physical conditions by the participation of artificial effects.
Case 3 comes from quality of irrigation water. Chemical analyses of the irrigation water applied show that about 1.8 kg of total dissolved salts are contained in 1m³ of irrigation water, and that pHc of the irrigation water is far less than 8.4. Low value of pHc means that calcium carbonate dissolved in the irrigation water has a tendency to precipitate in soil when it contacts with soil. Lysimeter experiments reveal the fact that about 65% of the dissolved salts supplied by the irrigation water are drained away from the soil and 35% remain. Considerable differences in the ease of leaching tendency among ions are found. Bicarbonate, chlorine, calcium and magnesium remain in the soil column with comparative ease.
Case 4 occurs in the cultivation under a limited water supply. Leaching fraction is the ratio between the amount of water drained below the root zone and the amount applied in irrigation. Soil salinization has progressed quickly during about two months under the condition that leaching fraction has controlled to only 1% even in applying 760mm of the irrigation water. The yield decrease of alfalfa resulting from the soil salinization is 25% of the yield expected from the case without salinity hazard.

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Paucity of forest is remarkable on the landscape of the West Cameroon Highlands and the Adamaoua Plateau although the annual rainfall and its seasonal distribution of the both areas are considered adequate for the existence of forest. Moreover the difference in landscape between the two areas appears too wide in comparison with the climatic differences between them. In contrast to the densely-populated and highly-domesticated agricultural landscape of the West Cameroon Highlands, the Adamaoua Plateau is dominated by the variously-wooded savanna landscape of rather devastated appearance.
In order to elucidate the above discrepancyy between landscape and present climate in the two areas, history of human impact on land is investigated on the background of late Quaternary environmental history reconstructed by means of stratigraphic interpretation of geomorphic, pedogenic, palynologic and archeologic evidences as well as radiocarbon dates. It became clear that the both areas had experienced similar environmental succession as follows: late Pleistocene climatic desiccation which induced the frequent occurrence of surface wash probably due to forest retrogression, the early Holocene humidification which made the forest readvance possible, and the man-induced forest deterioration since, at latest, ca. 2, 000 y. B. P. in the case of the West Cameroon Highlands. The Adamaoua Plateau suffered the cattle raisers' invasion in the beginning of the 19th century. It was followed by intense grazing as well as rapid decreasing of farming population. The above impacts induced severe erosion which was resulted in the exposure of both gravelly wash deposits provided under late Pleistocene semiarid condition and indurated zones of older age. By contrast, intensive and more careful cultivation has been continued in the West Cameroon Highlands.
The present landscape with very limited forest of the areas is without doubt the product of Holocene human action on the land. The contrast between the West Cameroon Highlands and the Adamaoua Plateau in their landscape seems to have been provided by the difference in the landsurface treatment of the two areas during recent 200 years. Although the development of the landscape apparently indicating more arid environment than actual climatic condition is inevitable in intensely cultivated and/or grazed lands, the productivity of the land does not reduced so much where topsoil is effectively conserved.

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The north-western part of China exists in the midst of Eurasian Continent and there are many big sand deserts which were formed by originally natural conditions. On the contrary in the northern part of this country, there extended very wide and fertile grasslands in old times, which provided good pastures for the nomadic people. But such grasslands were gradually devastated and desertified by their artificial activities. Moreover, we can find some other desertified areas in the west and the east of this country. The former areas are found around the oases and at the downstreams of inland drainage, the latter areas are found in the alluvial plains of the North China Plain and the North East Plain. But most of desertified areas are located on the former grasslands in the northern, semi-arid region. Why such a wide desertified region appeared here? It was brought mainly from the over-development of farmlands, over grazing of sheeps and over-felling of bushes for fuels.
Since about 2, 000 years ago the desertification in China started, and about two-third of such areas were formed in historical ages, but remaining one-third were transformed from the grasslands in recent 50 years, which include the days after the liberation, and it is called the modern desertification. The distribution of desertified areas are shown in Fig. 2, and the acreage is shown in Table 1.
After the liberation Chinese government maintained their agricultural policy to give priority for grain production, and attached importance to the development of the farmland, neglecting the possibilities of desertification. However, after the ending of Cultural Revolution, Chinese government began to notice the terrible influences of desertification, and changed their policy to develop the pastoral economy in the semi-arid region. The improvement of grasslands in the desertified areas is going on, and the reforestation also started to protect the fields and the villages from the hazard of moving sands.
The National Institute of Desert Research in Lanzhou, Academia Sinica, clarified the counterplan for the recovery of the desertified areas, according to their regional characteristics. I will introduce some materials about this counterplan for the discussion in future.

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The United States of America has suffered from the land degradation or desertification in and around semi-arid regions. This paper illustrates the background of desertification, soil erosion and land degradation due to overcultivation and overgrazing, in the U. S. A., mainly in the Great Plains. Measures to soil erosion and overgrazing and environmental management are also discussed. Table 1 summarizes the events related to desertification and environmental management in the United States chronologically.
In protecting non-federal lands and land resources, Soil Conservation Service of U. S. D. A. and the local conservation districts are playing important roles. Soil erosion from croplands is very serious (Fig. 1 and Table 2). The land management in private farms depends much more on the national and international economy. Wars and exports of grains usually brought big profits to farmers, which was an incentive to expand the cropland, and then severe erosion occurred.
The Food Security Act of 1986 includes the soil conservation provisions, which for the first time deny the subsidies to farmers who do nothing to control the soil erosion.
Before Federal Land Policy and Management Act of 1976 was enacted, the federal land had not necessarily been managed well, though the Taylor Grazing Act of 1934 played the significant roles in improving the rangeland conditions, which had been degraded in the late 19th century. After 1976, the improvement of rangeland conditions in federal land goes toward (Table 3) and the management policy is still developing.
Compared with the desertification in developing countries, that in the United States would be caused more by the social and/or economical situations of the nation and the world, under the relatively fragile natural conditions.

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This paper reviews geographical studies of desertification, one of major global environmental issues in this century, in a historical perspective. Mention is made of the present status and future tasks of Japanese studies on this topic.
Studies of desertification can date back to the early 20th century when a debate on the question of “progressive desiccation” and “desert encroachment” on the southern margin of the Sahara was of major concern among French and English geographers. Among others, following two scientists must be noted as the founders of desertification.studies: E. P. Stebbing (1935), English forestry professor who first stressed the spreading of desert conditions and the role of man in environmental deterioration, and A. Aubreville (1949), French ecologist and plant geographer who first used the term “desertification” in his book and persisted in his opinion of the creation of desert-like conditions due to human activities.
Since the early 1970s, when the “Drought in Africa” reached its first culmination, studies on desertification issue, including those by geographers, have been accelerated. The U. N. Conference on Desertification (UNCOD) held in Nairobi in 1977, with the “Drought in Africa” in the backdrop, had drawn the widespread attention of the public and scientists. The activities of world geographers, in particular those organized in the IGU Working Group on Desertification in and around Arid Lands (1972-80), had served great deal in the preparation of the UNCOD and the Plan of the Action to Combat Desertification (PACD), the major product of the UNCOD, by presenting background documents and case studies. Since 1980, international cooperative research on arid lands within the IGU has been succeeded by the Working Group on Resources Management in Drylands.
Recent activities of geographers in and outside of the Working Group have contributed to the implementation of and the assessment of the progress of the PACD. One of the recent trends in the desertification studies is the prevalent attention to the geopolitical approach to the problems of poverty and famine, and the transfer of strategies to combat desertification applied in one region to other regions.
In Japan, overseas research in and and semi-arid lands began as early as the mid-1960s, but the attention to the desertification issue by geographers did not grow until the early 1980s. However, a number of studies in the and to humid regions of the world by Japanese geographers have been more or less related to the desertification phenomenon in a broad sense, i, e. soil erosion, vegetation degradation, water logging and salinization of irrigated lands, etc.
With the “Crisis of Africa” resulting from the second culmination of persistent drought and desertification in the early 1980s in the background, the above studies were brought together into the Symposium on the Geography of Desertification held in September 1986. The papers presented at the symposium and published in this special issue have revealed rapid progress in the desertification studies in Japan in the last years. However, Japanese studies are still young, and following should be reinforced for further development of desertification studies.
1) Clima.tological and meteorological studies on the causes and effects of drought at various scales.
2) Comparative studies between regions under different climatic conditions as well as under different political and socioeconomic conditions.
3) Studies of human aspects in relation to the problems of poverty, population growth, famine, energy supply, etc.

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