Chapter 3 - Sources of data

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Global assessment of soil degradation (GLASOD)
Other sources of data
Variations in data and the need for definition of degrees of severity

Global assessment of soil degradation (GLASOD)

Under an international project, Global assessment of soil degradation (GLASOD), an attempt has been made for the first time to map the severity of degradation on a world scale, as the World map of the statue of human-induced soil degradation (Oldeman et al., 1990). The scale at the Equator is 1:15 000 000, becoming 1:13 000 000 at 30° latitude. The project was conducted by the International Soils Research and Information Centre (ISRIC) under the aegis of UNEP.

A standardized methodology, including definitions, was developed through international consultation. Data for individual countries was provided by the leading experts available, moderated by ISRIC with the aim of standardization. The GLASOD assessment (as it will be called) includes all the types of land degradation covered in the present study, somewhat differently subdivided but in ways that are compatible with the classification adopted here. This lest feature is of the highest value for filling what would otherwise be gaps in data from other sources.

At the same time, the authors of GLASOD acknowledge that there are certainly deficiencies in this first output, and that the World Map should be regarded as a first approximation. For some countries of South Asia, there exist other estimates, mainly governmental, of the extent of degradation, derived by somewhat different means. This range of sources provides the opportunity to compare data with the objectives first, of seeing how consistent these are, and secondly, to obtain best estimates.

For these reasons, it was decided in the present study:

1. To take the GLASOD classification of types of degradation as the basis for development of that used here, which is simplified and partly regrouped. The equivalence between GLASOD types and those of the present survey is given in Table 3.
2. To adopt the GLASOD definitions for degrees of severity of degradation (see below).
3. To treat the GLASOD assessments of the extent of degradation as a starting point or standard, against which other estimates can be compared; but not necessarily to adopt them as the best estimates where evidence suggests otherwise.

TABLE 3 - Correspondence between GLASOD types of degradation and those of the present study

* The GLASOD classes of Eo overblowing, Cp pollution, and Fs subsidence of organic soils were not reported for map units of South Asia. The class Pa aridification was included Guidelines for the GLASOD study and reported on South Asia Data sheets, but is not included in GLASOD maps.

TABLE 4 - GLASOD definitions of degrees of degradation

The degree to which the soil is presently degraded is estimated in relation to changes in agricultural suitability, in relation to declined productivity and in some cases in relation to its biotic functions. Four levers are recognized:

In the present study these same definitions are employed, but are referred to as "degree", "degree of severity" or "severity of degradation", all with the same meaning.

Degrees of severity of degradation

As will be made clear in later discussion, the definition of the degree, or severity, of degradation is of the highest importance. The definitions used in the present study are the same as those of degrees of degradation in GLASOD. In the present state of knowledge they are necessarily nonquantitative, although they contain guidelines for quantification.

In view of the importance of these definitions, they are given in full in Table 4. In abbreviated form the degrees of degradation are:

Light: somewhat reduced agricultural productivity.
Moderate: greatly reduced agricultural productivity.
Strong: unreclaimable at farm lever.
Extreme: unreclaimable and impossible to restore (with present technology).

The class 'Extreme' was not reported for any map unit in South Asia (one data sheet contained it originally, reduced on moderation to 'Strong').

In terms of their effects, the farmer is still using land with light and moderate degrees of degradation, but the boundary with strong degradation is the point at which land use has to be abandoned. Light degradation may not be clearly visible, but the farmer knows that yields (or other production) are longer than they might otherwise have been, or that additional inputs are necessary. Moderate degradation will often be visibly apparent, including stunted crops or sparsely vegetated rangeland, and yields are clearly and substantially longer. By definition, strong degradation means that the land has been abandoned , and no longer has potential for production.

Data for South Asia

For reason of cartographic necessity, the GLASOD World Map shows only the dominant form of degradation (as severity times extent) as coloured mapped areas, with the secondary form shown in the map symbol. Where, as happened widely, three or more forms of degradation were reported for the same map unit, only the first two appear on the map. This results in gaps when an attempt is made to abstract one form of degradation, say wind erosion, for all areas. This situation has recently been improved by the printing of maps of individual kinds of degradation, at a smaller scale, in the World atlas of desertification (UNEP, 1992a).

As part of the collaborative input to the present project, however, the complete original data sheets were made available, together with associated country maps and correspondence. These contain substantially more information than the published maps. Each data sheet (known as matrix tables) refers to a delineated map unit. For the unit, it gives:

• background information: physiography, soils, geology, climate, population, land use, vegetation;
• area of the map unit (square kilometres);
• a list of all types of land degradation identified, giving for each its type, degree (severity), extent (as percentage of the map unit affected), present rate, and principal causes;
• remarks, on each type and on the unit as whole.

Data on extent are given as five classes on a quasi-geometric scale, with bounds of S. 10, 25 and 50%. For the present study these were converted to a central value (using the geometric mean) and multiplied by the area of the unit to give a best estimate of the area affected by the type of degradation.

Treating one map unit with, say, 3 types of degradation as 3 records, and omitting units with no degradation, available data is as follows:

The records were put into a relational database, reviewed to remove errors, and analysed.

ISRIC/UNEP provided specially prepared maps showing the extent and severity of each type of degradation for the eight countries of South Asia.

Treatment of Bhutan

There are no GLASOD data sheets for Bhutan. The world map appears to treat this by extrapolation of conditions reported from adjacent countries to west and east, and the first procedure tried was to abstract this information and construct data sheets. However, this gave an estimate of area affected by water erosion over twice that of the FAO figure for total area under crops and pasture.

Whilst extrapolation is applicable to the physiographic zones, Bhutan has a much longer population density. It is reported that whilst there is a high hazard of erosion, including landsliding, "environmental planning precedes, and thereby hopefully prevents, environmental degradation" (Bhutan National Environmental Secretariat, 1992). It would be possible to assign zero degradation to the country, but this might give a false impression that no problem existed.

After discussion with FAO staff who have visited the country, a working assumption was made. This is that 10% of the reported area under crops and pasture is affected by water erosion, of which 9% is light and 1% strong, the latter representing landslides and gullies. This is intended to convert into figures the reported situation that the problem is not presently severe, but exists and should be guarded against in the future.

The total area is so small that this assumption does not appreciably affect regional totals. No other type of degradation has been reported for Bhutan.

The GLASOD assessment: results

In the presentation of results, for the purpose of broad regional comparison the countries have been grouped into a dry zone, with predominantly semi-arid and aria climates, and humid zone countries. Using the database, India was divided into dry and humid regions, the dry region being taken as all map units with rainfall not exceeding 750 mm per year (mainly the State of Rajasthan and western parts of Harayana, Gujerat and Punjab). Thus dry zone refers to Afghanistan, Iran, Pakistan and the dry region of India, humid zone to Bangladesh, Bhutan, Nepal, Sri Lanka and the humid region of India.

In the tables, areas are given in units of 1000 hectares. For discussion in the text, values are for the most part rounded to millions of hectares.

It should be noted that whilst "severity" is used in a specialized sense on the GLASOD map legend, in the present study, "degree", "degree of severity" and "severity" of degradation are all used with the same meaning.

Other sources of data

The starting point for estimates of the type, severity and extent of degradation is the report of the regional expert consultation Environmental issues in land and water development (FAO/RAPA, 1992). This includes a regional overview, and country reports for Bangladesh, India, Nepal, Pakistan and Sri Lanka. Other recent reviews containing data for more than one type of degradation are FAO/RAPA (1990) and ESCAP (1990a). Data for Afghanistan and Bhutan are qualitative only, and were obtained mainly from the respective UNCED reports (Afghanistan, Ministry of Planning, 1992; Bhutan, National Environmental Secretariat, 1992). Data sources for specific types of degradation are cited in context below. A valuable recent guide to sources of environrnental data is the annotated bibliography 1992 Directory of country environmental studies (World Resources Institute, 1992).

Most of these data ultimately derive from surveys or estimates by Government institutions: soil survey, soil conservation and irrigation management departments. Most of these estimates were initially obtained with care and effort, either from surveys or by assembly of estimates submitted by state and district branches and officers. They suffer, however, from a failure properly to define the degree of severity of the degradation for which an area is reported. As a consequence, different estimates may vary by a factor of two, or sometimes more.

The same data may be copied many times. Secondary publications sometimes do not make clear their sources (or even, in a few instances, units!). For all these other sources the data are highly non-uniform, both in availability and nature, as between the countries of the region.

For these reasons, the decision was made to take the GLASOD survey as a standard, which is then compared with other estimates. Only where there appears to be clear evidence of a data bias in the GLASOD survey have its results been modified from other sources to obtain best estimates as used in the present study.

Variations in data and the need for definition of degrees of severity

A major finding of the present comparative review is that large variations exist between different estimates of areas affected by degradation. A lack of surveys, and different methods used, is a contributory factor to this problem. The major cause, however, is believed to be the lack of precision in defining what is being surveyed.