There are four major cropland area maps and statistics at the global level. One study pri- marily used multisensor remote sensing (Thenkabail et al. 2009a; Thenkabail et al. 2009c).
The other three studies used a combination of national statistics and geospatial tech- niques (Goldewijk et al. 2009; Portmann, Siebert, and Dửll 2009; Ramankutty et al. 2008;
Siebert and Dửll 2008, 2009). The total global cropland areas estimated by these four stud- ies ranged between 1.3 and 1.54 billion hectares for the nominal year 2000. Only two stud- ies, reported in four peer-reviewed papers (Portmann, Siebert, and Dửll 2009; Siebert and Dửll 2008, 2009; Thenkabail et al. 2009a; Thenkabail et al. 2009c), separated the irrigated areas from rain-fed areas. The global irrigated area estimates, without considering crop- ping intensity, ranged between 312 Mha (Portmann, Siebert, and Dửll 2009) and 399 Mha (Thenkabail et al. 2009a). Only Thenkabail et al. (2009a) estimated the irrigated areas by considering cropping intensity as well, at 467 Mha. However, these studies varied sig- nificantly in providing precise spatial location of cropland areas and separating irrigated areas from rain-fed areas. Furthermore, none of the studies provide a proper assessment of crop type and/or crop dominance or irrigation by its source. A proper assessment and precise estimates of these aspects of global croplands are crucial given that 60–90% of all human water use is by croplands.
The global crop water use varied between 6685 and 7500 km3 yr−1; of this about 70%
is by rain-fed croplands (green water use) and 30% by irrigated croplands (blue water use). However, irrigated croplands use blue water (water in rivers, reservoirs, lakes, and pumped groundwater from the saturated zone). About 80% of all blue water currently used by humans goes to irrigated areas. This highlights the need for continued focus on irrigated croplands and their water use for enhancing global food security.
Uncertainties in global and regional cropland areas, their water use, and the precise geo- graphic location of these parameters are quite high at present. The need for high-resolution remote sensing products that can provide a greater geographic precision, crop types, and cropping intensities (by using high spatial resolution with high temporal resolution data) remains crucial for ensuring water and food security in the future.
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Environmental Applications:
Air, Water, and Land
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Remote Sensing of Aerosols from Space: A Review of Aerosol Retrieval Using the Moderate-
Resolution Imaging Spectroradiometer
Man Sing Wong and Janet Nichol