The IWMI’s Global Map of Rainfed Cropland Areas (GMRCA) is a by-product derived when working on IWMI’s Global Map of Irrigated Areas (GMIA). The datasets approaches, and methods used to produce GMRCA are, to a great extent, similar to producing GIAM. Thereby, we refer the reader to detailed documentation on GIAM made available in this web site.

The Global Rainfed Croplands were estimated at 1.132 billion hectares at the end of the last millennium, from the GMRCA products (Biradar et al., 2007). This is 2.78 times the TAAI or net irrigated areas (407 Mha) of the World. The GMRCA area provided here is for the June-October period only. Like, GMIA it is possible to estimate seasonal Global Rainfed Cropland areas using the products and methods developed in this study. However, double crop rainfed is considered negligible. The total cropland is estimated as 1.539 billion hectares of which 1.13 billion rainfed and 0.407 irrigated.

The importance of rainfed croplands can not be over-emphasized. Rainfed croplands meet about 60 percent of the food and nutritional needs of the World’s population, are backbone of the marginal or subsistence farmers, and are increasingly seen as better alternative to irrigated agriculture as a result of its environmental friendliness and sustainability over long time periods. Rainfed agriculture has an history of roughly 10,000 years compared to about 6000 year history of irrigated agriculture (see World resources 1992-1999, and Mackenzie and Mackenzie, 1995). Literature shows that the World’s croplands increased from about 265 million hectares in year 1700 to about 1.4 billion hectares in 1990, of which rainfed cropland alone is about 1.2 billion hectares (Cramer and Soloman, 1993, Richards, 1990, Grubler, 1994, World Resources 1992-1999). Our estimate of rainfed croplands of the World, at the end of the millennium, is 1.13 billion hectares.

Most global digital maps (e.g., Loveland et al. 1999, Olson and Watts, 1982, Matthews, 1983) over estimate agricultural areas as a result of the pixel based area calculations (see Xiao, 1997, Cramer and Soloman, 1993). A pixel when classified as agriculture is automatically taken to have 100 % croplands in digital global maps. In reality only a certain percentage of a pixel is in cropland and that percentage can vary substantially. As a result the total agricultural lands estimated in various digital maps were 2.7 billion hectares by Olson and watts (1982) using a 50-km grid, 3.2 billion hectares by Matthews (1983) using 100-km grid, and 2.8 billion hectares by IGBP and USGS using 1-km grid (see Loveland et al. 1999). The FAO estimates based on Country statistics are closer to reality. The FAO statistics show cultivated areas at about 1.5 billion hectares (FAO, 2002). Grubler (1994) estimated that an increase of 1 billion arable lands would be needed for additional 5 billion world population in the 21st century.

The theoretical potential for cropland areas in the present climatic conditions and based on soil, climate, and topography are estimated at 3.29 billion hectares (Xiao et al. 1997) to 4.15 billion hectares (Cramer and Soloman, 1993). However, it must be noted that the productivity of a large proportion of these lands is limited due to poor soil fertility, soil depth, access to water, and disease (e.g., Tse-tse flies and the black fleas). Any increase will have to come from land conversions from forests and rangelands which will be environmentally costly (Richards, 1990) or from protected areas which is unacceptable.

In reality, cropland areas are shrinking in recent times as a result of soil degradation, urbanization, and desertification and global warming. Between the early 1960s and the late 1990s, world cropland grew by only 11 percent, while world population almost doubled. As a result, cropland per person fell by 40 percent, from 0.43 ha to only 0.26 ha. and reduced from 0.23 to 0.11 hectares (FAO, 2002). In future, 80 percent of increased crop production in developing countries will have to come from intensification: higher yields, increased multiple cropping and shorter fallow periods.

Thereby, tracking changes in spatial distribution and changing patterns of rainfed croplands is essential for understanding and planning food and nutritional demands of expanding populations of the World.

In this context, the IWMI’s GMRCA product-line provides a benchmark measure of Rainfed Cropland Areas of the World at the end of the last millennium. The sub-pixel area (SPAs) of GMRCA provides realistic estimates of the actual area cultivated unlike the full pixel areas (FPAs) of almost all other studies. The GMRCA product-lines have maps, images, area characteristics and calculations, snap-shots, and animations. In addition the satellite sensor data mega-files and the ground-truth data used to produce the GMRCA are made available.

There are two product-lines within GMRCA. These are:

1. Aggregated 9-class GMRCA map of the World; and
2. Dis-aggregated 67-class GMRCA map of the World.

The aggregated classes provide broad categories of rainfed cropland classes. Often, most users would just need such broad classes. The disaggregated classes provide a detailed picture and are often invaluable at regional, National, and local levels. For certain users, even at global level so that they can derive specific classes of interest to them. The class labeling in disaggregated classes are only indicative and can be improved.

References
Alexandratos, N. (ed.) 1995. World agriculture towards 2010, an FAO Study. Chichester, UK: John Wiley and Sons, and Rome: FAO.

Biradar C.M., Thenkabail P.S., Noojipady, P., Li, Y.J., Dheeravath, V., Velpuri, M., Turral H., Cai, X. L., and Ganguntla, O.R., 2007.. A Global map of rainfed cropland areas (GMRCA) using time-series data from multiple satellite sensors. International Journal of Remote Sensing. (in preparation). Cramer, W.P. and A.M. Soloman, 1993, Climatic classification and future global redistribution of agricultural land, Climate Research, 3, 97-110.

FAO, 2002. World agriculture towards 2015/30. Summary report. ISBN: 9251047618 FAO, Rome.

Fischer, G., van Velthuizen, H. & Nachtergaele, F. 2000. Global agro-ecological zones assessment: methodology and results. Interim report. Laxenburg, Austria: International Institute for Systems Analysis (IIASA), and Rome: FAO.

Grubler, A., 1994, Technology, In: Changes in Land Use and Land Cover: A Global Perspective, W.B. Meyer and B.L. Turner II (eds.), Cambridge University Press, New York, p. 287-328.

Loveland, T.R., Zhu, Z., Ohlen, D.O., Brown, J.F., Reed, B.C., and Yang, L. 1999. An analysis of the IGBP global land-cover characterization process. Photogrammetric Engineering and Remote Sensing. 65(9): 1021-1032.

Mackenzie, F. T., and Mackenzie, J. A. 1995. Our Changing Earth: An Introduction to Earth System Science and Global Environmental Change, Prentice Hall.

Matthews, E., 1983. Global vegetation and land use: New high resolution databases for climate studies, Journal of climate and applied Meteorology, 22:474-487

Olson, J.S., and Watts, J.A., 1982. Major World Ecosystem Complex map, Oak Ridge National Laboratory, Oak Ridge, Tennessee.

Richards, J.F., 1990. "Land Transformation" in B. L. Turner, ed., The Earth as Transformed by Human Action. New York: Cambridge with Clark University.

Xiao, X., Melillo, J.M., Kicklighter, D.W., McGuire, A.D., Tian, H., Pan, Y., Vorosmarty, C.J., and Yang, Z., 1997. Transient Climate Change and Potential Croplands of the World in the 21st Century. Joint Program on the Science and Policy of Global Change.Report #18:Massachusetts Institute of Technology.

World Resources 1992-1999: A Guide to the Global Environment, Oxford, 1992