Farmers’ efforts to diversify production are frequently hampered by inadequate irrigation systems. If diversification results in high-value crops being grown, irrigation system investments may be more economically viable as a result of the higher returns and the increase in water use efficiency. Diversification in irrigated agriculture often requires new or improved technologies for water delivery and drainage, with investment needed to modernize water control so as to increase the flexibility of water delivery and drainage. To ensure that farmers benefit from agricultural diversification, investments are required in marketing and processing facilities and services, such as research, extension, and credit, and incentive structures to facilitate growth of high-value crop production. Adoption of high-yielding crop varieties during the green revolution, combined with rapid expansion of irrigated areas from the 1960s to the 1980s, resulted in a significant increase in food production, especially rice production in Asia. Surpluses resulted in a decline in grain prices in domestic and world markets, and in response to this, farmers sought alternatives to cereal cultivation. Other factors spurring crop diversification include the availability of advanced irrigation technology; development and adoption of improved high-value crops; increased domestic and regional demand for fruits, vegetables, and livestock products; growth of private agribusiness in processing and marketing; and removal of distorting policies that favored selected crops. However, many farmers in rice-based agricultural systems experienced severe problems in diversifying to other crops, constrained by deficiencies in irrigation infrastructure and management at the farm level and by a poor policy environment and lack of support services for diversification. Because of the importance of rice, many issues discussed in this note deal with diversification of rice-based cropping systems. The lessons learned are also valid for increasing productivity of other irrigated agricultural systems. Irrigation, Drainage, and Crop Diversification Water delivery. Paddy and nonpaddy crops require different irrigation management. Both excess water and deficits adversely affect yields of nonpaddy crops, whereas rice does well with continuous irrigation and/or field-to-field irrigation, which has been the dominant method of irrigation in most of South and Southeast Asia. Basin irrigation, the method used for irrigated rice, is also used for other crops, such as groundnuts, maize, and soybeans, but is not suited to crops sensitive to wet soils or to soils that form crusts. Delivery of irrigation water to nonpaddy crops at discrete, variable intervals with precise flows is more complex than continuous delivery for rice. Flow rates must be carefully controlled to irrigate nonpaddy crops whether surface or pressure systems are used. On-farm irrigation and drainage networks. Many irrigation projects designed for rice production have a low density of irrigation ditches and farm drains. Such infrastructure is sufficient for field-to-field irrigation, but nonpaddy crops require direct plot access to irrigation and drainage that provide intermittent water supply and prevent soil saturation affecting crop production. The density of the tertiary system required for nonpaddy crop cultivation depends on factors including land slope, nature of soils, farm size, mechanization, and method of on-farm water application.     
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