2. Crop diversification in irrigated agriculture: water management constraints.
In: Proc. of the Seventh Agric. Sector Symposium Sustainability Issues in Agricultural Development; World Bank, Washington, D.C., USA, ISBN 0-8213-0909-0, 1987, pp. 313-319
This paper reviews first the different water requirements of paddy and upland crops and reviews the general technical features of the two dominant surface irrigation methods, basin and furrow irrigation used in developing countries. Then the paper discusses the issue of improving irrigation facilities to make possible the shift from paddy cultivation to other crops and/or the adoption of mixed cropping. This review is limited to the aspects relevant to crop diversification and does not pretend to fully cover the above subjects.
Besides marketing considerations, diversification from rice paddy to non-paddy crops in irrigated agriculture is constrained by several physical and institutional factors such as soils, farmers' experience, credit, extension services and irrigation facilities. The issue of crop diversification is limited to surface irrigation which is the predominant method used for more than 90 percent of the 275 million ha currently irrigated in the world. In the vast areas of lands irrigated in Asia, surface irrigation methods are used almost exclusively for both paddy and upland crops.
The precise water control needed for diversified field crops requires in general extension of the tertiary networks, improvement and modernization of the main and distribution system, and in some areas, improvement of the drainage and flood control conditions.
The issue of improvement/modernization of irrigation systems to permit crop diversification has been complicated by the sharp drop in projection rice prices that occurred since 1982.
The 1990 rice price projections dropped from about 600 US$ in 1982 to 339 US$ in 1984/85 and recently below 250 US$. Most of the rice irrigation projects were viable in the early 80s including those for which all the infrastructure from storage or diversion works down to the on-farm water delivery works had to be built. Under the 1984/85 economic conditions, the viable investments in rice projects were those taking advantage of sunk costs in existing infrastructure.
In the case of Thailand a fast method for screening viability of projects was developed for the Irrigation Subsector Review issued in April 1986. It was found that development of the tertiary system at a low cost of US$ 600/ha is viable with only a modest paddy yield increase of 0.5 ton/ha in each season assuming a cropping intensity of 150%. To justify the investment required for an intensive tertiary system including land levelling (US$ 1000/ha) a yield increase of at least 0.8 to/ha for each season should be achieved.
With the most recent price projections, a detailed analysis of each project would be needed because of the sensitivity of the rate of return at these low rice prices. Investments required to improve the tertiary system together with improvement of the distribution system may no longer be justified unless there is a substantial increase in yields (above 1 ton per ha) and/or an increase in cropping intensity by making use of the water saved through more efficient operation. The conclusion is that in a number of cases the improvement of irrigation systems at both the tertiary and distribution level may not be economically justified for increasing rice production alone, without diversification to higher value crops. The investments required for crop diversification would have to be undertaken only when there is sufficient indication that all the other preconditions for crop diversification are met: market, marketing facilities, extension services, etc. The same conclusion may be valid for other rice surplus countries.
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Review, watershed management, stream corridor system, water resource quality, land-use impacts, costs and benefits, USAID, DESFIL
DICKINSON, J. and F. TRACY
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