СЭГ Мира / 1 семестр / ПРАКТИКУМ СЭГ мира,часть 1 / ХРЕСТОМАТИЯ электронная / 08 CХ+УДОБРЕНИЯ / wsr

2104

31823

23

40

19

27

31

24

6743

8460

3972

5117

Sub-Saharan Africa

North Africa and Near East Asia and Pacific

North Asia, east of Urals South and Central America North America

Europe

World

4346

1081

%

18

9

(‘000 km2)

TABLE 3

Area of soils without major constraints by region

Land resource potential and constraints at regional and country levels

13

There is a large body of research, published studies and technical manuals on control of soil erosion, including many produced by FAO (e.g. FAO 1977, 1987, 1991). The former focus on earth structures (bunds, terraces, etc.) has given way to the present approach (often known as better land husbandry) which places great emphasis on acceptance and adaptation of conservation practices by local farming communities, as well as on the use of biological methods (e.g. agroforestry) (Shaxson et al., 1989; Douglas, 1994). There is much to be learnt from indigenous soil conservation methods (FAO et al., 1998c). It is clearly recognized that the physical circumstances giving rise to a high potential erosion hazard, as identified here, are not the sole cause of erosion; equally important are the social and economic conditions of land users which may force them to cultivate (or graze upon) unsuitable land, or which fail to provide them with incentives for conservation. Since the circumstances of erosion, and the perceived benefits from adoption of conservation measures, vary so widely with site-specific physical, social and economic conditions, there is much scope for closer dialogue between concerned institutions and land users.

SOILS WITHOUT MAJOR CONSTRAINTS

A range of soils exists which are not affected by any of the eight major constraints covered above. Some fall into the dryland zone, discussed below, and others possess constraints within the soil fertility capability classification which are not included above, because they are either less severe or of relatively low extent.1 These certainly cannot be called ‘soils without problems’, as any farmer would testify! Based on the method by which these have been identified, they may be referred to simply as soils without major constraints.

In terms of the classes shown on the Soil Map of the World, the soils without major constraints fall particularly, but not entirely, into the major soil groups of Chernozems, Kastanozems, Nitisols, Luvisols, Phaeozems and Cambisols.

Just under a quarter of the world’s land area has soils which lack the eight major constraints of the fertility capability classification (Table 3). North Asia (east of the Urals) has the largest total and relative extent, caused particularly by the broad belt of Chernozems and Kastanozems which stretches across the region. The Asia-Pacific region and North America have the next highest total

extents of such soils, and six of the eight regions have between 18 and 31%. The exception is North Africa and the Near East with only 9%, to which must be added the constraint of dryness which affects most of the region.

A notable feature of the country-level results is that 22 of the 36 countries with over 40% soils without major constraints lie in Europe. This includes the extension of the ChernozemKastanozem belt into the Ukraine and adjacent countries. Among non-European countries, the

1These additional constraints are: acidity; dominance of allophane in the clay fraction (Andosols); potassium-deficiency; free calcium carbonate; acid sulphate soils (cat clays). Dryness (an ustic or xeric soil moisture regime) is also recognized by the fertility capability classification as a soil limitation.

14	Soil constraints

FIGURE 9

Countries with >40% of soils without major constraints: (a) Europe; (b) non-Europe

(a) Europe	0		20	40	60	80	100

Malta

Moldova

Ukraine

Azerbaijan

Armenia

France

Belgium

Belarus

Denmark

Bulgaria

Bosnia Herzegovina

Turkey

Croatia

Czech Republic

Georgia

Romania

Slovenia

Greece

Albania

Cyprus

Lithuania

Latvia

(b) Non-Europe

0

10

20

30

4 0

50

60

70

Gambia

Japan

Philippines

Korean Dem. Rep.

Lesotho

Jamaica

Dominican Republic

Haiti

El Salvador

Korean Rep.

Russian Federation

Mozambique

India

Benin

Land resource potential and constraints at regional and country levels

15

greatest total extent by far is found in the Russian Federation (7.7 M km2). Next in order of absolute area are the United States, Canada, China, Australia, India and Argentina. There is a striking coincidence between countries with high total areas of soils without major constraints and the world’s leading grain-producing and exporting countries. Some of the smaller nonEuropean countries listed suggest, to those with personal knowledge of these countries, anomalies in the data.

16	Soil constraints

Land resource potential and constraints at regional and country levels

17

Chapter 3

Drylands and steeplands

In Agenda 21, the programme of action arising out of the 1992 UN Conference on Environment and Development (UNCED, 1992), two sets of environmental conditions are identified as fragile ecosystems, namely drylands and mountain regions1. Drylands are described as such primarily because of their desertification hazard. Mountain regions are identified as fragile through a combination of problems, combining erosion hazard with forest clearance and other forms of land degradation. The special problems of these two types of environment, brought about respectively by climate and landforms, are additional to their soil constraints.

DRYLANDS

All the major continents face problems of land degradation in dryland areas, commonly known as desertification2. Dryland areas are ‘fragile’ in that they are extremely vulnerable to land degradation resulting from over-grazing and other forms of inappropriate land use. There have been a number of previous attempts to map desertification hazard (UNEP, 1984) and actual desertification (Middleton and Thomas, 1997). Here, an attempt is made to identify dryland conditions and desertification hazard in a similar manner to the soil constraints, estimating their world and regional extents, together with the countries most affected. Also identified are countries in which significant concentrations of population are found within drylands, extending previous work at the regional scale (Nachtergaele et al., 1996; UNSO-UNDP, 1997).

Methods

In the earlier FAO work, directed primarily at the tropics, agro-ecological zones were defined on the basis of length of growing period for annual crops, taken as the period during which the soil profile remained humid and temperature permitted crop growth (FAO, 1978). Since in temperate latitudes it is primarily temperature that restricts the growing period, it was necessary to redefine the length of growing period, taking the temperature factor into account in more detail (e.g. various temperature limits are used to define periods of dormancy and of killing frosts). This study makes use of the climatic database developed by Leemans and Cramer (1991), and identifies the length of growing period as limited both by temperature and soil moisture, based on the method of Fischer et al. (1995).

The results have been grouped into four classes of length of growing period (LGP):

1Agenda 21 also describes wetlands, small islands, and coastal areas as fragile ecosystems.

2Following the first UN Conference on Desertification in 1977, the term ‘desertification’ came to be loosely applied, sometimes to all forms of land degradation, and was subject to exaggerated claims. It is now defined as land degradation in arid, semi-arid and dry subhumid areas (UNEP, 1997)

Land resource potential and constraints at regional and country levels

19

•	Hyperarid:	LGP zero days
•	Arid:	LGP 1-59 days
•	Semi-arid:	LGP 60-119 days
•	Dry subhumid:	LGP 120-179 days

The concept of drylands continues to be debated (Eswaran, 1998). In this study, drylands are taken as areas with a potential hazard of desertification. The hyperarid zone is not subject to desertification and is therefore excluded. Hence drylands are defined as the arid, semi-arid and dry subhumid zones, or areas with lengths of growing periods of 1-179 days.

Results

The hyperarid, or extreme desert, environment covers 25.6 M km2, or 19% of the global land surface.

Drylands vulnerable to desertification cover 45% of the global land surface, with 7, 20 and 18% respectively in the arid, semi-arid and dry subhumid zones. The North Asia (east of Urals) region is dominated by dryland conditions (95%), but all regions have substantial parts of their land resources in the arid to semi-arid zones (Figure 10).

FIGURE 10

Percentage of total land area taken up by desert and dryland areas per region

100
80
60
40
20
0
Sub-	North	North Asia,	Asia and	South and	North	Europe	World
Saharan	Africa and	east of	Pacific	Central	America
Africa	Near East	Urals		America

dry subhumid

semi-arid

arid

hyperarid

TABLE 4

Area of drylands by length of growing period zone and region

	Hyperarid	Arid	Semi-arid	Dry	Drylands
				subhumid	(total)
	% of total	% of total	% of total area	% of total	% of total area
	area	area		area
Sub-Saharan Africa	24	6	13	19	38
North Africa & Near East	78	4	11	5	20
North Asia, east of Urals	1	11	51	33	95
Asia and Pacific	24	6	15	17	38
South & Central America	9	11	6	10	45
North America	7	12	28	23	63
Europe	0	<0.5	13	16	29
World	19	7	20	18	45

20	Drylands and steeplands

TABLE 5

Population density by length of growing period zone and region

	Hyperarid	Arid	Semi-arid	Dry	Drylands
				subhumid	(total)
Sub-Saharan Africa	4	7	19	30	22
North Africa & Near East	13	36	45	81	52
North Asia, east of Urals	12	7	5	14	8
Asia and Pacific	12	17	39	147	84
South & Central America	12	13	15	32	24
North America	6	2	3	6	4
Europe	0	21	56	65	61
World	6	2	10	26	28

FIGURE 11

Population density in desert and dryland areas

150

Populationdensity

50

100

Sub-Saharan Africa

North Africa &Near East

North Asia, east of Urals

Asia and Pacific

South and Central America

North America

Europe

World

0
hyperarid	arid	semi-arid	dry subhumid

It is not always appreciated how substantial a proportion of the world’s population lives in drylands. Approximately 1.7 thousand million people, or 38% of the world’s population, live in the arid, and the dry and moist semi-arid zones, and a further 270 million (6%) in the hyperarid zone. In the first comprehensive attempt to estimate the population carrying capacity, completed in the early 1980s, a notable feature of the maps and country results was how frequently the semi-arid zone was assessed as ‘critical’, that is, having current or future populations in excess of their capacity for food production (FAO/UNFP/IIASA, 1983; FAO, 1984).

Two approximate indicators of desertification risk, based on area and population respectively, have been produced by the Office to Combat Desertification and Drought (UNSO/UNDP, 1997). These are:

•area of drylands as a percentage of agriculturally productive land;

•population on drylands as a percentage of population on agriculturally productive land;

where:	drylands = arid + dry semi-arid +moist semi-arid zones;
	agriculturally productive land = total land area – hyperarid zone.

These indicators are shown for countries in Appendix 3, Tables A3a-g. A summary at regional level is given in Table 6.

Land resource potential and constraints at regional and country levels

21

TABLE 6

Desertification risk and population level by region

Desertification risk (UNSO, 1997)

	Population in	Based on area of	Based on population on
	drylands (%)	drylands (%)	drylands (%)
Sub-Saharan Africa	36	50	37
North Africa & Near East	44	91	79
North Asia, east of Urals	89	96	89
Asia and Pacific	44	50	46
South & Central America	24	19	25
North America	19	68	19
Europe	18	29	19
World	38	57	41

Based on these indicators, 57% of the world’s potentially productive area is located in drylands, which carry 41% of world population. On the criterion based on area, over 90% of North Africa and the Near East are vulnerable to desertification, and at least 50% of land in four of the seven regions. On the criterion based on population, North Asia (east of the Urals) has an even higher risk than North Africa and the Near East.

At country level, in order to prevent exaggeration of desertification risk, countries lying largely or entirely in the hyperarid zone were first assigned a risk of zero. These nine countries are listed in Table 7.

Having excluded the hyperarid zone, desertification risk at country level was assessed on the two criteria above (Appendix 3, Table 3a-g). Globally, 25 countries have over 90% of their potentially productive land located in drylands, of which 12 have 100% (Table 8). Based on the criterion of population, 23 countries have >90% of their people living in drylands.

TABLE 7

Countries with >95% of territory in the hyperarid zone

Djibouti	Kuwait
Egypt	Oman
Libya	Qatar
Western Sahara	Saudi Arabia
	United Arab Emirates

TABLE 8

Countries with high vulnerability to desertification

Drylands 100 % of agriculturally productive area

Afghanistan	Jordan	Pakistan
Armenia	Mauritania	Senegal
Botswana	Mongolia	Somalia
Cyprus		Zimbabwe

	Drylands 90-100% of	agriculturally productive area
Botswana	Senegal	Iraq	Mongolia	Cyprus
Chad	Somalia	Jordan	Armenia	Finland
Eritrea	Gambia	Syria	Pakistan	Israel
Mali	Zimbabwe	Yemen	Uzbekistan	Iran
Mauritania	Afghanistan	Kazakhstan	Namibia	Macedonia

Population on drylands >90% total population

Botswana	Namibia	Afghanistan	Yemen	Cyprus
Burkina Faso	Niger	Iran	Kazakhstan	Israel
Chad	Somalia	Iraq	Mongolia	Macedonia
Eritrea	Zambia	Jordan	Pakistan
Mauritania	Zimbabwe	Libya	Armenia