Background

The World Atlas of Desertification was published by UNEP in 1992 as the result of a cooperative effort between UNEP's Desertification Control Programme Activity Centre (DC/PAC), the Global Environment Monitoring System (GEMS) and the Global Re-source Information Database (GRID). GRID compiled and/or derived most of the global and regional databases, produced the maps and carried out the data analyses and tabu-lations for the Atlas, assisted by a Technical Advisory Group on Desertification As-sessment and Mapping composed of various international experts.

The Atlas includes information and many maps derived from the Global Assessment of Human-Induced Soil Degradation (GLASOD), as conducted in 1990 by the International Soil Reference and Information Centre (ISRIC) at Wageningen, The Netherlands, on behalf of UNEP. Aside from GLASOD's data on soil degradation, and in order to capture the multi-dimensional nature of global desertification processes, other data layers relating to global climate and vegetation were compiled by GRID for inclusion in the 1992 World Atlas of Desertification. Both the source climate data and advice on the production of all climate surfaces were obtained from the Climate Research Unit of the University of East Anglia (UEA/CRU), U.K.

The Global Humidity Index is based on a ratio of annual precipitation and potential evapotranspiration (these data layers are described elsewhere) as P/PET, and largely follows the classification used in a 1984 UNESCO study. The Global Humidity Index surface shows mean annual potential moisture availability for the period 1951-1980, classified into four aridity zones and one humid zone, defined in this data set as follows:

The nomenclature for the aridity zones is standard in the scientific literature, and each is characterized by a series of general criteria (see the World Atlas of Desertifications page 5 under "Dryland Types"). The data set also includes a sixth zone for cold tundra and mountains, defined as those areas which have more than six months of an average temperature below 0 degrees and not more than three months where the temperatures reach above 6 degrees centigrade (roughly equivalent to Koeppen climate zones Dc, Df and E).

There were two reasons for having this additional zone. The first is that the objectives of GLASOD were global in nature, but focussed upon the arid regions of the tropics and sub-tropics. The second reason is that the Thornthwaite method, which was used to estimate PET, is known to systematically underestimate PET in more arid regions and seasons. For the same reason, in this data set the hyper-arid zone boundary was also adjusted upward to 0.05, compared to 0.03 in the UNESCO map.

The humidity index data set was used in combination other data sets (e.g. GLASOD) to highlight land degradation processes in susceptible dryland areas, and statistics were derived from these overlays to yield area estimates of degradation in various aridity zones.

Deichmann and Eklundh state in their report (1991) that there is no objective way to compare this humidity index surface with the older UNESCO product for at least five reasons:

difference in the observation period, as the UNESCO map is apparently based on a "timeless" climatology;

difference in the PET estimation method (UNESCO used Penman);

station network used (number/distribution of climate stations);

timing and method of interpolation used (i.e., UNESCO interpolated the final sur-face, rather than the input climate data layers);

so-called "knowledge-based" adjustments in the UNESCO map.

In contrast, the GRID humidity index surface is based on standardized climate data from a known time period (1951-1980), and the specific methodology used is well-documented. Thus, the GRID product is not only replicable, but could be compared with similar maps generated in the future which are also time-dependent.

Data format

The original humidity index data were in a generic raster format (no longer available), and were converted to ARC/INFO vector format and smoothed using the 'SPLINE' command. While the Geographic Projection data (latitude/longitude coordinate system) are generally distributed, the Desertification Atlas and GLASOD project made use of the Mollweide equal-area projection for deriving statistical information, and the Van der Grinten projection for display and analog map purposes.

The Humidity Index data set as processed by GRID is contained in five ARC/INFO coverages and 'EXPORT'-format data files. Following is the list of five coverages/data files (.E00) which users may select among:

HINDXVDG Global Humidity Index in Van der Grinten projection, HINDXGEO Global Humidity Index in Geographic Projection, both with outer border but not including coastlines.

HINDTMOL Global Humidity Index in Mollweide Projection, HINDTVDG Global Humidity Index in Van der Grinten Projection, HINDTGEO Global Humidity Index in Geographic Projection, all three with coastlines and outer border

These are all written on a single computer compatible tape (CCT) using the ARC/INFO 'TAPEWR1TE' command, with a blocksize of 8000, a logical record length of 80 (blocking factor 100), normally at 6250 bpi in an uncompressed 'EXPORT' format. They can be read from tape to disk with the 'TAPEREAD' command and re-'IMPORTed' using the following commands:

IMPORT COVER HINDXVDG.E00 HINDXVDG IMPORT COVER HINDXGEO.E00 HINDXGEO IMPORT COVER HINDTMOL.E00 HINDTMOL IMPORT COVER HINDTVDG.E00 HINDTVDG IMPORT COVER HINDTGEO.E00 HINDTGEO

The following shows the most important items found in the PATs (polygon attribute tables) of the Humidity Index data set, and their description:

The source document for the Global Humidity Index data set is as follows: Deichmann, Uwe and Lars Eklundh. July 1991. "Global digital data sets for land degradation studies: a GIS approach". GRID Case Study Series No. 4; UNEP/GEMS and GRID; Nairobi, Kenya; 103 pages (see pp. 29-32).

Additional references include the following:

• Koeppen, W. 1931. Das Geographische System der Klimate. Handbuch der Klimatologie. Volume 1. Berlin, Germany.

• UEA/CRU Report. August 1990. Phase II Global Humidity Index for 1930-59 and 1960-89. Draft version.

• UEA/CRU Report. September 1990. Additional Data for Phases I and II.

• UNEP. 1992. World Atlas of Desertification. Edward Arnold, London (UK), 69 pages (see especially pp. 4 to 9).

• UNESCO. 1984. Map of the world distribution of arid regions. Paris, France.

The coverage DUA1:[VAXELAS.RGWITT.CRUEA.HINDTGEO] contains:

The coverage boundary is:

Xmin: -180.000	Xmax: 180.000
Ymin: -57.019	Ymax: 72.014

General
Polygon Topology is present
The Coverage has not been edited since the last BUILD or CLEAN
Coverage size: 1374 blocks. Coverage date: 22-SEP-1993

The main INFO files and items are: (files other than PAT,AAT,TIC,BND are not displayed!)

HINDTGEO.PAT AREA PERIMETER HINDTGEO# HINDTGEO-ID DISSOLVE HIX-ZONE HIX-NAME

CLASS LAND FUDGE FUDGE-ZONE SUSCEPT-ZONE

HINDTGEO.AAT FNODE# TNODE# LPOLY$ RPOLY# LENGTH HINDTGEO# HINDTGEO-ID

HINDTGEO.TIC IDTIC XTIC YTIC

HINDTGEO.BND XMIN YMIN XMAX YMAX