The AEZ methodology for land productivity assessments follows an environmental
approach; it provides a framework for establishing a spatial inventory and database
of land resources and crop production potentials. This land resources inventory
is used to assess, for specified management conditions and levels of inputs,
the suitability of crops/LUTs in relation to both rain-fed and irrigated conditions,
and to quantify expected production of cropping activities relevant in the specific
agro-ecological context. The characterization of land resources includes components
of climate, soils, landform, and present land cover.
Inherent in the methodology is the generation of a climatic inventory to predict agro-climatic yield potentials of crops. The Global AEZ study uses a recent global climatic data set compiled by the Climate Research Unit at the University of East Anglia. The database offers a spatial resolution of 30 minutes latitude/longitude and contains climate averages for the period 1961-90 as well as year-by-year data of the period 1901-1996. These are used to characterize each half-degree grid-cell in terms of applicable thermal climates, temperature profiles, accumulated temperature sums, length of growing periods, moisture deficits, etc.
Adequate agricultural exploitation of the climatic potentials and maintenance of land productivity largely depend on soil fertility and the management of soils on an ecologically sustained basis. Hence, the climatic inventory was superimposed on FAO's Digital Soil Map of the World (DSMW). The DSMW is derived from the FAO/UNESCO Soil Map of the World at scale 1:5 million and presents soil associations in grid-cells of 5 minutes latitude/longitude, forming the basis of soil information in Global AEZ. The composition of soil associations is described in terms of percentage occurrence of soil units, soil phases and textures. Therefore, each 5 minute grid-cell is considered as consisting of several land units.
Terrain slopes were derived from the GTOPO30 database developed at the USGS Eros Data Center, providing digital elevation information in a regular grid of 30 arc-seconds latitude/longitude. At IIASA rules based on altitude differences of neighboring grid-cells were applied to compile a terrain-slope distribution database (for each 5 minute grid-cell of FAO's DSMW) in terms of seven average slope range classes.
The individual GIS layers with
attribute data and distributions at 5 minutes latitude/longitude constitute
the land resources database. The key components of this database include: the
FAO DSMW and linked soil association composition table, the slope distribution
database derived from GTOPO30, and an ecosystem database derived from the USGS
30 arc-second seasonal land cover data set providing distributions in terms
of twelve aggregate land-cover classes for each 5 minute grid-cell. The DSMW
has been made the reference for constructing a land surface mask, i.e., a binary
layer that distinguishes grid-cells as land or sea, respectively. Also, each
5-minute grid-cell is uniquely assigned to an administrative unit (a country
or region).