South Asia is one of the major rice (Oryza sativa L .) producing regions of the world, with around 50 million ha under this crop. A substantial proportion of this area is under only a single crop, usually kharif (rainy) season rice, with the land remaining fallow during the following rabi (postrainy) season. This situation largely occurs for rainfed rice, where irrigation facilities for either rice itself or a post-rice crop are not available. Nevertheless, residual soil moisture, derived from the previously flooded rice fields that could support growth of a crop after rice, remains available well into the rabi season. Large areas of land lying fallow for a considerable proportion of the calendar year are a particular cause for concern in South Asia, for two main reasons. Firstly, the large and growing population of the region requires ever-increasing quantities of locally available food grains and fallow lands on which crops could potentially be grown represent under-utilization of agricultural land resources. Secondly, continuous cereal cropping, in this case rice, is unsustainable over time and some form of crop rotation or diversification is desirable for agricultural production system sustainability (Paroda et al. 1994, Hobbs and Morris 1996).

It would be desirable to increase the cultivation of legumes, which are already to some extent cultivated after rice in South Asia (Johansen et al. 2000b), in these rice-fallows for several major reasons. Firstly, there are growing shortages of legume grains in the region, as indicated by increasing imports. Secondly, legumes have ameliorative effects in cereal-based cropping systems and thus contribute to their long-term sustainability (Kumar Rao et al. 1998). The constraints and opportunities for legume cultivation in sub-tropical South Asia, including cultivation after rice, have recently been reviewed (Johansen et al. 2000b). Various technologies, including appropriate cultivars, are now available that would increase feasibility of legume cultivation after rice. Examples of application of these technologies to increase legume cultivation in rice-fallows are available, such as use of seed priming to promote chickpea (Cicer arietinum L .) cultivation after rainfed rice in the High Barind Tract of Bangladesh (Musa et al. 1999).

To assess the scope for increasing cultivation of legumes in rice-fallows of South Asia, better quantification of rice-fallow areas than is now available is required. Estimates currently available are derived from government-recorded crop statistics, by subtracting areas of rabi crops from area of kharif rice. This procedure can only indirectly approximate actual rice-fallow areas. Advances in the application of satellite imagery (SI) to quantify land use patterns have made it feasible to estimate more precisely areas of fallow lands after kharif season rice (Venkataratnam 1999). Further, geographic information system (GIS) technology can be combined with SI to relate spatial and temporal distribution of rice-fallows with the various climatic and edaphic variables that would determine legume adaptation and constraints to their cultivation. By incorporating socioeconomic information relating to legume cultivation after rice it is thereby possible to assess suitability of particular legumes for specified rice-fallow situations. This approach is attempted in the project described in this report. The South Asian countries of Bangladesh, India, Nepal, and Pakistan have been chosen for this study and recommendations for better utilization of rice-fallows through legume cultivation have been derived for these countries.