Where irrigation matters

The results from Global AEZ have been examined to highlight areas where irrigation can make a significant contribution to land productivity [1] . After processing each land unit of the land resources inventory, individual 5-minute latitude/longitude grid-cells were marked according to the potential impact of irrigation on extents of suitable land and production of cereals. The grid-cell results were aggregated according to impact classes by regions and countries.

The analysis provides interesting insights regarding the potential role of irrigation in the various regions:

Full exploitation of potential irrigable land increases the gross extent of suitable land (VS+S+MS) for cereals by 6 to 9 percent [2] for the world as a whole over, and above the land potentially suitable for rain-fed cereal production. Regional results vary substantially, namely between 3 percent (for South America) and 69 percent (for Western Asia).
The impact of irrigation is more pronounced on the increase of potential production than on potential area. The cereal production potential increases by 30 to 40 percent. [2] The regions with the largest relative increase are Oceania (basically Australia), Northern and Southern Africa, Central and Western Asia.
In about 17 percent of gross suitable areas (under rain-fed and irrigation conditions), the share of supplementary or full irrigation would increase potential output by more than 50 percent above rain-fed levels. In another 11 percent of gross suitable areas, potential output would increase with irrigation between 20 and 50 percent.
Overall, application of irrigation has a slightly higher impact in the developed countries than in developing countries, both for increases in extents of suitable areas (13 and 7 percent respectively) and for potential output (47 and 38 percent respectively for developed and developing countries).

Algorithm

For each land unit within each 5-minute latitude/longitude grid-cell in the land resources inventory, the algorithm proceeds in 6 steps:

Step 1:	Determine the crop (or multiple crop combination) which maximizes expected cereal output under rain-fed conditions;
Step 2:	Determine the crop (or multiple crop combination) which maximizes expected cereal output under irrigation conditions;
Step 3:	Determine the fraction of land in each 5-minute latitude/longitude grid-cell that is assessed as very suitable or suitable under irrigation. Test whether the irrigable share exceeds a specified minimum threshold SHmin;
Step 4:	Combine rain-fed and irrigated production so as to maximize total output in each grid-cell;
Step 5:	Determine the ratio of potential cereal output under rain-fed and irrigation conditions to cereal potential under rain-fed conditions only, and
Step 6:	Aggregate results by country and region into 6 irrigation impact classes according to the following scheme:
	Areas where rain-fed cereals can be cultivated but irrigation is impossible or irrigable share is below a specified threshold SHmin ; The irrigable share in a grid-cell exceeds the minimum threshold SHmin; irrigation increases potential cereal output of the respective gridcell by less than 20 percent above rain-fed levels; As for 2, but contribution of irrigation to grid-cell production is 20 to 50 percent; As for 2, but contribution of irrigation to grid-cell production is 50 to 100 percent; As for 2, but contribution of irrigation to grid-cell production is > 100 percent, and No rain-fed production possible and no or little suitability under irrigation

By definition, there is no or little contribution from irrigation to the production potential in areas grouped in impact classes 1, 2 and 6. On the other hand, the potential contribution from irrigation is particular important in impact classes 4 and 5. Both the impact of irrigation on extents of suitable land and cereal production potential were quantified. Results were compiled for different levels of minimum threshold SHmin of 1%, 5%, 10% and 25%. Summaries are available in the form of tables and maps.

Potential impact of irrigation on global cereal suitability and production

Irrigation threshold SHmin	Land suitable for irrigation:		Contribution of irrigation to:
	Share in total suitable land (percent)	Share of impact classes 4&5 in total irrigable land (percent)	Total suitable land under rain-fed and irrigated conditions (percent)	Potential production under rain-fed and irrigated conditions (percent)
	Share in total suitable land (percent)
1 percent	34.6	52.1	8.8	41.7
5 percent	33.2	53.3	8.4	40.0
10 percent	31.2	54.9	7.8	37.4
25 percent	24.6	59.9	5.8	29.2

	Irrigation impact classes for areas suitable for cereals.

	Potential impact of irrigation on regional cereal suitability and production
	Rain-fed share of total area suitable for cereals, by irrigation impact class
	Rain-fed share of total potential cereal production, by irrigation impact class

[1] For the assessment of irrigated land productivity potentials, it has been assumed that (i) water resources of good quality are available, and (ii) irrigation infrastructure is in place (see Agro-edaphic suitability analysis ). In other words, the assessment identifies areas where climate, soils and terrain permit irrigated crop cultivation but does not undertake a quantification of water availability within a watershed. However, suitability in hyper-arid (LGP = 0 days) and arid regions (LGP < 60 days) is limited to specific soil conditions, such as Fluvisols and Gleysols.

[2] Depends on assumptions regarding minimum size of irrigable land tracts considered ( SHmin ).