Box 7. FAO’s estimations of the ratio between rainfed and irrigated yields For the purpose of the indicator 6.4.1 calculation on water-use efficiency, this yield ratio enables a more accurate estimation of agricultural production, and thus gross value added (GVA), by referring to water use in irrigated agriculture. The yield ratio is equal to the yield of a rainfed crop over the yield of the same irrigated crop, using the formula:
Where: Yri = ratio between rainfed and irrigated yields Yr = mean yield in rainfed conditions Yi = mean yield in irrigated conditions. In countries where there is no rainfed production, Yri is null because Yr is also null. The practical implication on the agricultural GVA is that no rainfed agriculture portion is discounted, and the whole agricultural GVA is included for the sectoral water-use efficiency of agriculture. The initial definition of indicator 6.4.1 presented in the previous report took into consideration a very coarse approximation of the ratio between rainfed and irrigated yields. This approximation was based on the idea that 20 percent of the world’s crop areas were irrigated but produced 40 percent of the global crop production, and that rainfed areas represented 80 percent of the surface area but yielded only 60 percent of total production (FAO, 2007), resulting in a ratio of 0.375. However, this estimation did not reflect the diversity of the irrigation situation across countries, such as the widely differing benefits of irrigation between humid and arid areas. More accurate and country-specific estimations were therefore needed to provide a better parameter for disaggregating rainfed and irrigated agricultural GVA. Using the same 2006 baseline data as the 2012 FAO study Agriculture towards 2030–2050, the difference between rainfed and irrigated situations was calculated for each crop with available data from each country. This means that the yield ratio cannot be calculated when: •
a crop is not cultivated in both conditions, i.e. irrigation and rainfed within the country
•
at least one of four needed data (area and production under rainfed conditions and area and production under irrigated conditions) is not available.
In general, countries could calculate a yield ratio for 5–10 crops. Once the yield ratio was calculated for all possible crops within a country, an average value was calculated at the national level, weighted on the area of each crop (to give more importance to the main crops).
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PROGRESS ON CHANGE IN WATER-USE EFFICIENCY - 2021
