Stepwise review of suitability analysis procedures

Crop suitability is a result of both agro-climatic and agro-edaphic evaluation. The combination of the agro-climatic suitabilities with the agro-edaphic suitabilities is based on the fact that the former results assume ideal soil and terrain conditions, while the latter evaluation assumes ideal agro-climates. In fact, the results of the agro-climatic suitabilities are successively modified, according to edaphic suitabilities, to provide overall crop suitability.

The calculation procedures have been grouped into five steps:

(1)	Climate data analysis
(2)	Crop-specific agro-climatic assessment and potential biomass calculation
(3)	Application of agro-climatic constraints
(4)	Edaphic assessments
(5)	Various applications (e.g., calculation of land with cultivation potential)

Step 1 calculates and organizes climate-related parameters for each grid-cell, i.e.,

Altitude
Latitudinal climate
Presence of cold break
Continentality index
Mean annual temperature
Mean annual minimum temperature
Mean annual maximum temperature
Temperature profile: number of days in intervals of five degree centigrade steps from <-5ºC to >30ºC separately for periods of increasing and decreasing temperatures
Thermal growing periods: number of days >0ºC, >5ºC, >10ºC
Begins and ends of thermal growing periods
Accumulated temperature during thermal growing periods
Mean temperature during thermal growing periods
Aridity index (precipitation over reference evapotranspiration)
Aridity index during growing period
Total number of growing period days
Number of growing period days when full crop water requirements of reference crop are met
Total number of wet-days, i.e., growing period days with excess moisture
Number of growing periods
Begin and end of dormancy period
Length of individual LGPs
Number of days in each LGP when crop water requirements can be fully met.
Number of days in each LGP with excess moisture
Begin and end dates of each LGP
Temperature profile during growing period
Accumulated temperatures (above 0°C, 5°C, 10°C) during growing period
Average temperature during growing period
Multiple-cropping zones classification for rain-fed and irrigated conditions.

Step 2: All the 154 LUTs (148 crop/LUTs and 6 grass/pasture legume LUTs) are “grown”. The LUTs are tested starting successively each day during the permissible window of time (separately determined for irrigated and rain-fed conditions). The highest obtained yield defines the optimal crop calendar of each LUT in each grid-cell. The CROPWAT methodology (FAO, 1992a) is used to run crop-specific water balances and to account for yield losses due to water deficits. Calculations are done seven times: once for irrigation conditions, and six times for rain-fed conditions assuming in the soil moisture balance calculations an available water-holding capacity (AWC) of respectively 150, 125, 100, 75, 50 and 15 mm/m. The following information is stored for each grid-cell after Step 2:

Maximum attainable biomass and yield (determined by radiation and temperature)
Estimated actual crop evapotranspiration (ETa)
Accumulated crop water deficit during the growth cycle (i.e., ETo - ETa)
Attainable water-limited biomass and yield.

Step 3: Specific multipliers are used to downgrade yields for what is defined in AEZ as agro-climatic constraints. This step is carried out separately to make the effect of the workability, pest and diseases, and other constraints transparent. The results of Step 3, agro-climatically attainable yields, are stored by crop/LUT for each grid-cell.

Step 4 performs the edaphic assessment and combines the agro-climatic results in accordance with the soil information. The FAO digital soil map of the World (DSMW), with a grid-cell size of 5 arc-minutes latitude/longitude, is used for the assessment, defining soil characteristics (soil type, soil texture and soil phase) and proportions of different soils in each mapping unit. For terrain-slope conditions, a slope distribution was derived from the 30 arc-second GTOPO30 digital elevation database (EROS Data Center, 1998). The slope characterization has been aggregated to the grid-cells of the DSMW in terms of seven classes, as used in SOTER (van Engelen and Ting-tiang, 1993): 0-2%, 2-5%, 5-8%, 8-16%, 16-30%, 30-45% and > 45%. Soil and slope rules are applied separately for rain-fed and irrigated conditions. As a result, for each 5 minute grid-cell and each crop/LUT an expected yield and suitability distribution (5 classes VS, S, MS, mS and NS) regarding rain-fed and irrigation conditions are obtained. Results are stored separately for dryland and naturally flooded soils (Fluvisols, and Gleysols with 0-2 % slopes).

Step 5: The databases created in steps 1 to 4 can and have been used to derive additional characterizations and aggregations, such as:

Calculation of land with cultivation potential is involving an aggregation over individual crop/LUTs to estimate how much land is potentially suitable for crop cultivation. Results in map form have proven useful for spatial verification purposes.
Tabulation of results by ecosystem type: the USGS 1km land cover data set has been aggregated to twelve major ecosystem classes and subsequently to 5 minute DSMW grid-cells. The resulting ecosystem distribution was matched with the assessed land with cultivation potential in each DSMW grid-cell, providing information about what suitable extents are under which current ecosystems. This information was useful to compare and verify, for example, the overlap between potential cultivable land and land shown as being currently in use for cropping.
Quantification of climatic production risks by using historical time series of suitability results. For each crop/LUT and grid-cell this has resulted in information on average crop yield, number of crop failures, standard deviation of expected yields, ratio of average yield versus yield of average climate.

As discussed above, the structure of the suitability analysis procedures allows stepwise review of results. As an example of a verification sequence, a selection of intermediate results in map form for 120-days grain maize, grown under rain-fed conditions at high input level, is presented as follows:

	(Step 1)	Growing period days
	(Step 2)	Maximum attainable potential biomass and yield (determined by radiation and temperature)
	(Step 2)	Attainable (water limited) potential biomass and yield
	(Step 3)	Agro-climatically attainable yield potentials (including the effect of agro-climatic constraints)
	(Step 4)	Expected yields after application of edaphic constraints
	(Step 5)	Maximum expected yields potentials across 13 grain maize types

The results, obtained after completion of each of the above steps, have been used in the process of checking and validating the proper functioning of the various procedures. The intermediate and final results have been helpful for the verification against research data, crop statistics, expert knowledge, etc.

Various modes have been pursued for ‘ground-truthing’ and verifying results of the AEZ suitability analysis. Apart from consulting expert knowledge and agricultural research institutes, results have been systematically compared with research data and agricultural statistics. In particular the following activities have been conducted intensively by IIASA and FAO staff:[1]

Confirmation of estimated potential crop distribution and yields against quantitative and qualitative occurrence of these crops in national and sub-national agricultural statistics.
Comparison of limits of AEZ potential crop distribution with limits to actual distribution of agricultural land (e.g., by comparison with spatial land use/land cover databases and crop distribution maps).

[1] It should, however, be understood, that in the light of improved knowledge any part of the Global GAEZ suitability procedures and the model parameters will be scrutinized and may be subject to updating by FAO and IIASA. Also, the model and model parameters are expected to benefit refinement as result of follow-up applications.