10. Agro‐ecological Zones classification - un-fao/gaezv5 GitHub Wiki

Introduction

The agro-ecological zones (AEZ) methodology provides a framework for establishing a spatial inventory of land resources compiled from global/national environmental data sets and assembled to quantify multiple spatial characteristics required for the assessments of land productivity under location-specific agro-ecological conditions. The land resources inventory includes spatial layers of historical and future climate, soil, terrain, land cover, population density, livestock density, protected areas/areas of high biodiversity value, and administrative boundaries.

On the basis of the available GAEZ v5 land resources information a workable number of AEZ classes was specified for the purpose of targeting users who may need relatively broad-scale tendencies for planning and analysis. AEZ definitions and map classes follow a rigorous methodology and an explicit set of principles. The first principle applied is to make sure the AEZ map classes align with major climate zones, as determined in GAEZ v5, delineated based on historical data of the period 1981–2000 & 2001–2020. Second, AEZ map classes reflect broad ranges of length of growing period (LGP) boundaries reflecting different agro-environments. Third, agro-ecological zones classes include generalized information regarding prevailing soil/terrain limitations derived from the GAEZ v5 terrain and soil resources inventories.

Factors used for global AEZ classification

The AEZ class layer provides a uniform classification of bio-physical resources relevant to agricultural production systems. The inventory combines spatial layers of thermal and moisture regimes with broad categories of soil/terrain qualities. It also indicates locations of areas with irrigated soils and shows land with severely limiting bio-physical constraints including very cold and very dry (desert) areas as well as areas with very steep terrain or very poor soil/terrain conditions. The basic principles and criteria used for the compilation of the AEZ class layer are listed below.

Thermal climate classes

Based on the twelve thermal climates delineated in GAEZ v5 Module I, six aggregate categories of major thermal climate (TC) classes, TC1 to TC6, were used in the definition of AEZs in GAEZ v5, namely:

• TC1: Tropics, lowland;
• TC2: Tropics, highland;
• TC3: Subtropics;
• TC4: Temperate climate;
• TC5: Boreal climate, and
• TC6: Arctic climate.

The six climate classes represent broad latitudinal belts based on monthly temperature conditions at sea level (using a uniform lapse rate of 0.55°C/100 m).

Thermal zone classes

Thermal zone (TZ) classes reflect actual temperature conditions (see also Chapter 3, section 3.3.2) and are used to characterize and sub-divide major thermal climate classes into more specific temperature regime classes (TRC). Six thermal zone classes were distinguished:

• TZ1 Warm: monthly Ta ≥ 10°C for all months; average annual Ta ≥ 20°C;
• TZ2 Moderately cool: monthly Ta ≥ 5°C for all months; monthly Ta ≥ 10°C for 8 or more months;
• TZ3 Moderate: monthly Ta ≥ 10°C for 5 or more months; number of days with mean temperature above 20°C (LGPt20) is 75 days or more; accumulated temperature during the period with Ta≥10°C exceeds 3000 (dd);
• TZ4 Cool: monthly Ta ≥ 10°C for 4 or more months; average annual Ta ≥ 0°C;
• TZ5 Cold: monthly Ta ≥ 10°C for 1 to 3 months; average annual Ta ≥ 0°C, and
• TZ6 Very cold: monthly Ta < 10°C for all months and/or average annual Ta < 0°C.

Note, thermal zone classification starts with testing for TZ1 and subsequently, when conditions are not met, testing continues for TZ2, TZ3, TZ4, etc. Testing for TZ3 is done only for grid cells in the temperate thermal climate.

Thermal zone TZ1 occurs in tropical lowland and subtropical thermal climates. There is no frost risk for perennial crops, no hibernation for annual crops and the climate allows foremost the cultivation of crops adapted to warm temperatures. Depending on moisture supply a wide range of crops can be grown. This includes tropical lowland maize and lowland sorghum as well as pearl millet and sugar cane (crop group C4-I) and a wide range of annual and perennial C4 crops, varying from annuals such as indica rice, soybean, groundnut, sweet potato and yam to perennials like oil palm, cocoa, coconut, robusta coffee and rubber (belonging to crop group C3-II).

Thermal zone TZ2 occurs in tropical highlands and in subtropical thermal climates. Heat provision is less than in TZ1 and mean monthly temperature can be less than 10°C for up to 4 months, though mean monthly temperatures stay above 5°C in all months. In tropical highlands, thermal zone TZ2 allows a range of crops to be grown adapted to moderately cool temperatures. These include crops like highland maize and sorghum as well as highland phaseolus bean and arabica coffee. At the margins with tropical lowland also some crops of crop group C3-II can be grown, e.g., tobacco, sunflower, soybean and various vegetables. In the subtropical thermal climate some temperature seasonality occurs and when water is available thermal zone TZ2 can support sequential multi-cropping such as the prominent wheat/rice double cropping system, as for instance practiced in northern India and in parts of eastern China.

Thermal zone TZ3 is considered only in the temperate thermal climate to subdivide the vast temperate region into a moderate and a cool temperature regime class. The moderate zone of the temperate thermal climate provides at least 5 months with monthly temperatures above 10°C and at least 75 days with average daily temperatures above 20°C, conditions which are sufficient to cultivate a thermophilic annual crop like cotton, tobacco or japonica rice with a crop growth cycle length of 5–6 months. The wide range of crops that can be grown in TZ3 include also soybean, groundnut (crop adaptability groups C3-II); maize, sorghum, foxtail millet (crop group C4-II); and wheat, barley, white potato, bean, rapeseed and sunflower (crop group C3-I).

The cool thermal zone TZ4 occurs at higher altitudes in tropics and subtropics and at higher latitudes in the temperate thermal climate. Zone TZ4 cannot accommodate crops adapted to warm temperatures. Cultivation is mostly practiced with C3-I crops, including wheat, barley, potatoes or rapeseed. TZ4 imposes frost risks and therefore frost sensitive perennials like citrus or olive cannot be grown.

The cold thermal zone TZ5 is characterized by only 1 to 3 months with average temperature exceeding 10°C and/or average annual temperature is above 0°C. TZ5 occurs at higher latitudes in the boreal thermal climate or may occur at high altitudes in tropical, subtropical and temperate regions. When creating temperature regime classes, this zone is further subdivided depending on occurrence of permafrost classes (see Table 10–1). Agricultural use of TZ5 outside permafrost zones is limited to pastures and a few cryophilic crops. These crops comprise very short cultivars adapted to germinate and grow at marginal soil temperatures, e.g., specific spring wheat and barley varieties and early white potato.

The very cold thermal zone TZ6 is not suitable for cropping. Mean monthly temperatures are less than 10°C in all months and/or mean annual temperature is below 0°C.

Temperature regime classes

The delineation of ten Temperature Regime Classes (TRC), which are used to define AEZs, combines major thermal climate classes (TC1 to TC6) and thermal zone classes (TZ1 to TZ6) in a systematic way. Note that thermal climates represent latitudinal belts with characteristic patterns of day-length and temperature seasonality, whereas thermal zones characterize actual temperature profile conditions depending on altitude and latitude. Conditions in different TRCs relate to the thermal requirements of different crop adaptability groups. The forming of temperature regime classes TRC1 to TRC10, using classes TC1 to TC6 and TZ1 to TZ6, is summarized in the table below.

Table: Creation of temperature regime classes

TRC class	Class name	Thermal climate class	Thermal zone class
TC1	TC2	TC3	TC4
TRC1	Tropics, lowland	×
TRC2	Tropics, highland		×
TRC3	Subtropics, warm
TRC4	Subtropics, mod. cool
TRC5	Subtropics, cool
TRC6	Temperate, moderate
TRC7	Temperate, cool
TRC8	Boreal/Cold, no PFR		×
TRC9	Boreal/Cold, with PFR		×
TRC10	Arctic/Very cold		×

Moisture regime classes

The delineation of four moisture regime classes (Figure 10–2) makes use of the GAEZ v5 agro-climatic inventory and results of the GAEZ v5 daily reference water balance to define broad moisture regime classes following the established AEZ terminology:

• M1: delineates desert/arid areas where 0 ≤ LGP* < 60 days;
• M2: is used for semi-arid/dry areas with 60 ≤ LGP* < 180;
• M3: represents sub-humid/moist areas with 180 ≤ LGP* < 270, and
• M4: denotes humid/wet areas where LGP* > 270.

For areas with LGPt5 > 330 days the indicator LGP* is set to the average number of annual growing period days. When LGPt5 < 330 days, i.e., in areas with seasonal temperature limitations, the LGP* indicator is set as the maximum of LGP days and a function of the annual P/ET0 ratio, using a quadratic regression equation, which was estimated (and applied) in GAEZ v5 and is based on a data set which includes all grid-cells with LGPt5 = 365 days. This function, termed equivalent LGP, results in 60 days for a ratio P/ET0 ~ 0.15, in 120 days for P/ET0 ~ 0.40, 180 days for P/ET0 ~ 0.65 and 270 days for P/ET0 ~ 1.15.

Soil/terrain related classes

The delineation of agro-ecological zones in GAEZ v5 distinguishes five classes related to soil quality and terrain conditions. The mapping of classes uses the GAEZ v5 soil/terrain inventory, i.e., the data from HWSD v1.2.1 and a terrain slope distribution inventory by 30 arc-second grid cells, which was derived from original 3 arc-second SRTM data. The following soil/terrain related classes are distinguished:

• S1: represents very steep terrain where the sum of percentages of slope classes SLP7 (30–45%) and SLP8 (slope > 45%) exceeds in a grid cell a given target threshold (e.g. 75%) and the sum of slope classes SLP1 to SLP4 (i.e., terrain slopes ≤ 8%) is less than a maximum threshold;
• S2: denotes areas with hydromorphic soils, which includes all Gleysols, Histosols, as well as all gleyic and stagnic soil types of FAO’74 and FAO’90 classifications;
• S3: comprises grid cells with no or slight soil/terrain limitations;
• S4: is for areas with moderate soil/terrain constraints, and
• S5: denotes areas with severe and very severe soil/terrain limitations and so-called miscellaneous units of the soil database (e.g. rock outcrops, sand dunes, glaciers, etc.).

For class S1 a minimum threshold of 75% was used for the sum of SLP7 and SLP8 as well as a maximum threshold of 10% for the sum of SLP1 to SLP4. Class S2 was assigned when hydromorhic soils account for at least 67% in a grid cell.

For classes S3 to S5 the severity of soil/terrain limitations is quantified by a soil/terrain suitability index, which is calculated in AEZ as the weighted sum of the component soil/terrain suitability rating factors. It can be obtained by summing area weighted edaphic ratings for all occurring soil/slope class combinations in a grid-cell. The soil unit rating refers to a reference crop (e.g. grain maize) under low input assumptions. The resulting index values range from 0 (entire grid cell is not suitable) to 1 (entire grid cell is rated as very suitable, having no constraints). The algorithm uses two thresholds to subdivide the full suitability index value range into classes S3, S4 and S5. Class boundaries used were [0.667–1.000] for class S3, [0.250–0.667] for class S4, and the remainder, interval [0.000–0.250] for class S5. Note, class S5 includes also all areas evaluated as having continuous or discontinuous permafrost.

Selected special purpose land cover classes

The delineation of agro-ecological zones in GAEZ v5 distinguishes three land cover classes, L1 to L3, listed below. They are related to selected (special purpose) elements of the GAEZ v5 land cover inventory, which was derived from GLC-Share v1.1 and GMIA v5 (for a description see Chapter 2, section 2.3).

For constructing AEZs, three special purpose land cover classes L1 to L3, with very specific properties were extracted and mapped together with soil/terrain related classes, as shown in Figure 10–3:

• L1: relates to the dominance of inland water bodies in a grid cell, i.e., where the respective land cover share for water exceeds a specified threshold (e.g. 75% of a 30 arc-second grid cell);
• L2: maps areas where artificial surfaces dominate, exceeding a specified threshold (a minimum 75% of a 30 arc-second grid cell was used), and
• L3: denotes irrigated areas where the share of irrigated cropland in a grid cell exceeds a specified minimum threshold (e.g., 20% of a 30 arc-second grid cell) or where cropland exceeds a given minimum threshold (e.g. 40% of a 30 arc-second grid cell) and at least half of the cropland in a pixel is equipped for irrigation.

Agro-ecological Zones classes in GAEZ v5

The temperature regime classes TRC1-TRC10, moisture regime classes M1-M4, soil/terrain related classes S1-S5, and special purpose land cover classes L1-L3, described above, represent the different dimensions used for AEZ classification. These were combined step by step, following a priority scheme, to form 57 unique AEZ classes, as listed in the table below.

Note, since some special purpose classes are defined by minimum thresholds of occurrence (e.g., water; built-up/artificial surface; irrigated cropland), some of these land cover types, when occurring with low intensity in a grid cell, will not be shown. For instance, water in grid cells where the water share is less than 75% will not be assigned to class AEZ-57 but will be mapped, depending on context, as one class of AEZ-50, AEZ-53 to AEZ-55, or one of AEZ-01 to AEZ-48. Besides the water class (AEZ-57), this qualification applies also to classes AEZ-49 to AEZ-52 and AEZ-56.

The combinations are listed in the Appendix and below by class with some additional explanations:

• AEZ-01 to AEZ-06: combinations of TRC1 (tropics, lowland) with M2-M4 and S3-S4;
• AEZ-07 to AEZ-12: combinations of TRC2 (tropics, highland) with M2-M4 and S3-S4;
• AEZ-13 to AEZ-18: combinations of TRC3 (subtropics, warm) with M2-M4 and S3-S4;
• AEZ-19 to AEZ-24: combinations of TRC4 (subtropics, moderately cool) with M2-M4 and S3-S4;
• AEZ-25 to AEZ-30: combinations of TRC5 (subtropics, cool) with M2-M4 and S3-S4;
• AEZ-31 to AEZ-36: combinations of TRC6 (temperate climate, moderate) with M2-M4 and S3-S4;
• AEZ-37 to AEZ-42: combinations of TRC7 (temperate climate, cool) with M2-M4 and S3-S4;
• AEZ-43 to AEZ-48: combinations of TRC8 (boreal/cold, no permafrost) with M2-M4 and S3-S4;
• AEZ-49: dominantly very steep terrain; all grid cells where soil/terrain related class S1 occurs;
• AEZ-50: land with severe soil/terrain limitations; covers all areas of S5 except where set to classes AEZ-49 or AEZ-51 to AEZ-57;
• AEZ-51: land with ample irrigated soils; set for pixels where land equipped for irrigation exists and exceeds the specified thresholds for class L3;
• AEZ-52: dominantly hydromorphic soils; set for pixels where class S2 occurs, but excluding grid cells which were previously set to classes AEZ-49 or AEZ-51;
• AEZ-53: desert/arid; delineates all areas in the arid moisture class M1, except for special purpose land cover classes L1, L2, L3, soil/terrain classes S1 (very steep terrain) and S2 (hydromorphic soils), or thermal regime class TRC10 (i.e., arctic/very cold);
• AEZ-54: boreal/cold climate with permafrost; includes all pixels where TRC9 occurs (except for special purpose land cover L1 and L2, or soil/terrain classes S1 and S2, or moisture class M1);
• AEZ-55: arctic/very cold climate; includes all pixels where TRC10 occurs (except for special purpose land cover L1 and L2, soil/terrain classes S1 and S2);
• AEZ-56: dominantly built-up/artificial surface; is set in all grid cells where L2 occurs, and
• AEZ-57: dominantly inland water; is set in all grid cells where L1 occurs.

When grid-cell values match the conditions for more than one class, for instance a lake in a desert area, or very steep slopes in a cold climate, it is important that the classes are assigned step by step following a priority scheme to ensure consistency of classification.

The sequence followed to assign AEZ classes begins with checking conditions for special purpose LC classes AEZ-56 and AEZ-57. Then the conditions for classes AEZ-49, AEZ-51 and AEZ-52 are tested and assigned, if matching. Thereafter conditions for classes AEZ-53 and AEZ-54 are tested and assigned, when matching. For instance, a grid cell in a desert area meeting the hydromorphic soils criteria would be assigned to class AEZ-52 (dominantly hydromorphic soils), not to AEZ-53 (desert/arid) which comes later in the priority scheme. The next step is to check for very severe soil/terrain limitations (class AEZ-50). From there onward all combinations of thermal, moisture and soil/terrain conditions are unique and can be assigned to classes AEZ-01 to AEZ-48.

The robust principles of combining thermal and moisture regime classes with soil/terrain and special land cover characteristics can also be applied in regional or national studies. In such cases, it is usually necessary to refine class definitions to better reflect critical thresholds relevant to each application. Additionally, the integration of biophysical or socioeconomic features—such as unimodal vs. bimodal rainfall zones in East Africa—may help define appropriate territorial units for analysis and planning.

Climatic requirements of crops have a major influence on their distribution across AEZ classes, particularly those distinguished by thermal and moisture gradients. Differences in crop suitability across AEZ classes are especially evident for sugar crops (sugar beet and sugar cane) and root/tuber crops (potato, sweet potato, cassava, yams, and cocoyam). For example, sugar beet thrives in cool subtropical and temperate zones, while sugar cane dominates tropical and warm subtropical zones. Potatoes are prevalent in cooler regions, whereas sweet potato and cassava are more suited to tropical climates. Yams grow primarily in humid tropical and warm subtropical regions.

Tree-covered land predominates in humid tropical, subtropical, and wet temperate or boreal zones, while grasslands and shrublands are more common in semi-arid and arid environments.

The AEZ framework in Module I also introduces a multiple cropping zones classification (see Chapter 3, Section 3.5.1), featuring eight distinct classes that reflect whether 1, 2, or 3 sequential crops may be cultivated annually and whether these are cryophilic or thermophilic. For example, the “Zone of triple rice cropping” represents areas where conditions permit the cultivation of three successive wetland rice crops. This classification is developed separately for rain-fed (MCR) and irrigated (MCI) systems, with heat availability and temperature seasonality as key differentiators.

Cropland in lowland tropics and warm subtropics has ample thermal availability to support three thermophilic cropping cycles, provided that irrigation or sufficient year-round rainfall is also available. In these regions, water and input availability are the primary limiting factors for productive land use.

In contrast, cool temperate and boreal zones typically allow for only one cryophilic crop, regardless of water conditions. Meanwhile, highland tropics and moderately cool subtropics can support two thermophilic crops under adequate water supply.

The moderate temperate zone generally supports a single thermophilic crop, though in some areas, double cropping is possible with at least one cryophilic crop.

This zonal differentiation, based on thermal and water regime constraints, provides a methodological foundation for understanding the potential for multiple cropping and for refining agro-ecological analysis at national or regional levels.

See the GAEZ v5 dataset on AEZ classification, 57 classes on the FAO Agro-Informatic Data Catalog (link).