Table 1.
Summary of models that estimate global irrigation requirement.
Fig 1.
Comparison between the CRU-TS v.4 ET0 (Penman-Monteith, monthly data) and the monthly average ERA5-based Hargreaves-Samani ET0.
The comparison was made considering all the cells around the world where the harvested area is at least 90% of the pixel, for the months of January (a) and July (b).
Fig 2.
Boxplots and area-weighted means of percentage differences between monthly ET0 (Computed HS and PM from CRU-TS).
Boxplots show values corresponding to 25%, 50% and 75% of harvested lands for six climate zones: Tropics (orange), Sub-tropics summer rainfall (yellow), Sub-tropics winter rainfall (green), Temperate sub-continental (blue). The grey boxplot describes the ET0 alignment over the least irrigated zones: Oceanic Temperate, Continental Temperate, Oceanic Boreal, Sub-Continental Boreal, Continental Boreal, Arctic. The horizontal dimension of the boxplot is proportional to the percentages of irrigated areas per climatic region, reported in the pie chart (c). Asterisks: area-weighted means of annual percentage differences. Fig 2d shows the GAEZ global agro-climatic classification of surface lands.
Fig 3.
Spatial distribution of irrigation requirement and precipitation surplus.
Cell-average water depths [mm], as a ratio between the total volume cumulated over the year within the cell and the cell area.
Table 2.
Crop-related summary of estimations and irrigated areas.
Fig 4.
Monthly volumes of crop-specific irrigation requirement [km3/month].
Rice volumes refer only to the evapotranspiration requirement.
Fig 5.
Monthly variability of climate forcings and model results, over three intensively harvested areas.
The selected regions belong to different agro-climatic zones: Central Valley (California, sub-tropic winter rainfall), Po Valley (Italy, temperate sub-continental) and Punjab (India, sub-tropic summer rainfall). The variables on both the axes have been quantified considering the monthly harvested AEI, as shown in the lowest part of the plots.
Fig 6.
Distribution of irrigation requirement volumes by classes of agricultural water risk.
Classification of the World Resources Institute: Low risk (0–1), Low-Medium risk (1–2), Medium-High risk (2–3), High risk (3–4), Extremely High risk (4–5).
Table 3.
Comparison of global estimations of irrigation requirement.
Fig 7.
Comparison of national irrigation requirements and withdrawals and aggregation by classes of national efficiency.
Blue circles, red crosses and green squares are countries where I<W, grouped by classes of E. Purple triangles are countries for which the model estimates I higher that actual W. The bold continuous line indicated the condition of I = W; classes of efficiencies lower than 1 are limited by dashed lines, while the two dotted lines delimit classes of efficiencies higher than 1. For E<1, the labeled countries have at least 106 ha of areas equipped for irrigation.
Fig 8.
State of California: Comparison between crop-specific irrigation requirement estimations (I), data of applied water (AWCDWR) and evapotranspired applied water (ICDWR).
The AW and ICDWR data are from the “California Department of Water Resources”, referring to year 2000.