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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オーストラリア半乾燥地域,とくに南部半乾燥地域で,植生に重点をおいて人為的な環境変化過程を分析した.丘陵地帯では,開墾によって林地が極度に減少し,顕著な土壌侵食塩類集積・過湿化が進行していた.海岸砂丘地帯では,放牧などによる大規模な砂丘の再活動がみられた.化石砂丘が卓越するマレー・マリー地域でも,砂層の厚いところで,入植・開墾による植生破壊を契機とした砂丘の再活動が活発であった.ここでは,ユーカリのリグノチューバーが残されていれば植生の復元は可能であるが,それが除去された地域では自然植生の再生は困難である.また,植生が回復する場合も本来内陸の乾燥地域に出現するはずの乾生草原やアカシアの妨害極相が出現する.そうした植生が半乾燥地域に出現することは,植生からみた「砂漠化」がこの地域で進行していることを意味する.こうした「砂漠化」現象を緩和させるためには,土地自然の特性を踏まえた適正な植生管理計画の策定が必要である.

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インド半乾燥地域における農村の土地利用が,政府の農村開発計画の適用を受けてどのような変化を生じたかについて,デカン高原南部の2つの村を事例として実証的な検討を行なった.その1つイエルドナ村は, 1957年に大規模水路灌漑の恩恵を受けて以来,それまでの自給的な乾燥地農業から稲,ラッカセイなどの商品作物生産を中心とする近代的な灌漑農業へと著しい変貌を遂げ,村の人口は急増した.一方,独立以来,井戸掘削事業や等高線土堤事業など政府の諸計画を積極的に受け入れてきたビダラケレ村は,天水耕地においても安定した農業生産を確保するようになるとともに,タマネギなどの商品作物を組み合わせて,顕著な農業発展をみた.その結果,近年,人口は大幅に増加しつつある.
しかしながら,それぞれの村では,灌漑用水管理や農地管理の不合理から,湿性地や塩性地の拡大,ガリー侵食の加速化,地下水の枯渇といった土地生態系の破壊を生じつつある.また,商品作経営への急激な転換のために農民の階層分化が激化しつつあること,さらには急速な人口増加に伴い,村落自治能力が低下しつつあることなど,今後の農村開発を阻害しかねない多くの深刻な問題を抱え込んでいる.

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地球の陸上の約30%という広大な面積は乾燥地または半乾燥地土壌で被われ,そのままでは農業生産性はきわめて低い1近年,これらの土地に莫大な資金を投入し,灌漑農業開発が活発に行なわれるようになった結果,飛躍的に農業生産性が向上した。しかし,反面,大量の水の持ち込みは土壌の塩性化を顕在化し,甚大な土壌塩性化のため,農業開発の試みが失敗し,砂漢化を助長するケースも多くなっている。土壌の塩性化の過程を中近東乾燥地域を中心に調査した結果,4つの主たる原因が考えられた.とくに,本地域の灌漑水の土壌への施用は灌漑水中の石灰を土壌に新たに沈積させるような水質であること,高温乾燥期での作物栽培には,作物の根圏域から塩を取り除くためのリーチング用水が多量に必要となり,この水量を確保できない場合には,土壌の塩性化が急激に進むことを明らかにし,これらの原因が土壌塩性化に大きく影響していることを指摘した.

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現在の気候条件からみれば森林が成立していても良いと考えられるカメルーン中部および西部の高地では,更新世末の乾燥化(森林の大幅な後退),完新世初頭の湿潤化(森林の回復)を経て, 2,000y. B. P. 頃には人為的な森林破壊が進み,遅くとも1,000y. B. P.頃にはかなり強度の農耕が行なわれていたと思われる.その後, 150y. B. P. 頃から牛牧畜民の侵入をうけて農耕民が急減し,その結果定期的に火入れされる牛放牧地の鉢大をみた地域と,それを免れかなり集約的な農耕が継続された地域とで,表土層の維持・更新状況に明白な差異が生じるようになった.現在両地域には,森林がきわめて少なく自然状態より著しく乾燥したみかけを呈するという景観上の共通点と,かたや低木と高茎草本がさまざまな割合で混在した景観,他方では樹木の極端に少ない草地・農耕地景観が,それぞれ卓越するという相違点とがみられるが,これらはともに,人為の作用の時代的・地域的な変化と良く対応する.さらに表土の人為的削剥の強弱は,更新世の古気候の産物である鉄・アルミナ硬化層や礫質堆積物の露出状況に差異をもたらしているが,それも現在の景観の地域的な差異の形成に関与しているようにみえる.

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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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