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The aggregated 17-class map provides broad categories of irrigated agriculture, rainfed agriculture, and few other classes of importance. Often, many users would just need such broad classes. The disaggregated classes provide discrete classes and are often invaluable for detailed LULC assessments at Global, regional, National, and local levels. The class labeling in disaggregated classes of GMLULCA are only indicative and can be improved based on further knowledge about the class/es in consideration.
Unlike the GMIA and GMRCA, no attempt was made to calculate sub-pixel areas (SPAs) for GMLULCA. Like various other studies of LULC, discussed soon after Table 1 below, GMLULCA provides full pixel areas (FPAs)-see Table 1. In comparison to USGS 1-km 1992-93 AVHRR data (see Loveland et al., 1999, and Loveland et al. 2000) based study, the IWMI LULC results using SPOT 1-km and AVHRR 10-km data at the end of the last millennium showed (Table 1): (a) croplands increased by nearly 14 percent, (b) shrublands, grasslands, and savannas decreased by nearly 16 percent, and (c) forests increased by nearly 6.2 percent. No attempt was made to calculate areas for different seasons. The GMLULCA FPAs are determined for June-October which is the major cropping season of the World.
The first 8 classes in aggregated GMLULCA 17-class map were irrigated areas (class 1-5 pure irrigated, class 6-7 supplemental, and class 8 LULC with irrigation fragments). The full pixel areas (FPAs) of these classes during June-October were: 537.1 Million hectares for class 1-5, 319.8 million hectares for class 7-8, and 343.7 million hectares for class 8. This sums up to about 1.2 billion hectares of FPA irrigated. In reality, through sub-pixel de-composition technique (see GMIA page and area calculation pages in this web site), we established actual irrigated area to be 318 million hectares. Thereby, on an average, only 26.5 % (0.318/1.2) of the pixel is actually irrigated. The exact fraction will vary from class to class, with classes such as class 1 or 2 having significantly higher fraction (SPA) of irrigated compared to classes 6-8.
The full pixel areas (FPAs) of IWMI GMLULCA at the end of the last millennium versus USGS LULC for 1992-93 for 3 key classes are shown in Table 1.
In the past, the most commonly used global land use/land cover (LULC) datasets were primarily non-remote sensing based, produced using data from various maps at 100-km grid by Matthews (1983), and 50-km grid by Olson and Watts (1982), Olson (1994), and Wilson and Henderson-Sellers (1985). More recently, AVHRR and MODIS sensor data have been widely used to produce global LULC. The 1992-93 AVHRR 1-km data were used by USGS (see Loveland et al., 1999, Loveland et al. 2000) and University of Maryland (DeFries et al. 1995, DeFries et al. 1998) to produce Global LULC datasets. These data also were used by IGBP (see see Loveland et al., 1999, Loveland et al. 2000). The most recent LULC products are from Boston University using MODIS (Friedl et al., 2002, Zhan et al., 2000). It is well known that no two global datasets match (DeFries and Townshend,1994) as a result of differences in methods, data sources, data types, data calibration, and data acquisition modes. Also, class definitions and how classes are aggregated to meet user requirements or target groups. The disaggregated 75-class IWMI GMLULCA can be used to derive classes that match the class names of USGS and MODIS LULC classes. The particular strength of IWMI’s GMLULCA will be in its emphasis on irrigated and rainfed croplands and in establishing their sub-pixel areas (SPAs).
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