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