General summary Asia

Irrigation

Irrigation potential

The irrigation potential for the region was estimated at 235 million ha, taking into account only the countries for which data were available. India and China account for about 76 percent of this total. The methods used to estimate the irrigation potential vary from country to country, which makes comparisons difficult. In addition, there are some contradictions between the figures given for the irrigation potential and the expected situation of the irrigation in the country. In India, for example, the irrigation potential, which is 113.5 million ha, is called ultimate irrigation potential. It corresponds to the gross area which could theoretically be irrigated in a year on the basis of the assumed design cropping pattern and a rainfall probability of 75 percent and represents 2.27 times the area under irrigation in 1993. This figure is a theoretical maximum. Indeed, it is considered that development of irrigation in India is about to reach its limits and that no major extension of irrigated lands is to be expected after the beginning of the twenty-first century.

In China, the figure for irrigation potential is 64 million ha and corresponds to the total area which could be brought under irrigation in the first half of the next century. As much of the additional land proposed for irrigation is located in the arid and semi-arid zones, reaching such a level would require a viable long-term strategy as to how to provide the amount of water necessary to irrigate these lands.

Irrigation development

In most countries of the region irrigation has a long history which is closely linked to the history of rice cultivation. Asia, in general, and the region covered by this survey in particular, represent the bulk of irrigation in the world. The region itself accounts for about 50 percent of the world's irrigation. High population density combined with the tradition of irrigated rice cultivation in all the tropical part of the region are the main factors explaining the importance of irrigation in Asia.

In many countries of the region, irrigation is viewed as an important input to the agricultural production systems. While irrigation development dates back several centuries, the twentieth century, and particularly its second half, has seen a rapid increase in what could be called modern irrigation development (Figure 1) and a majority of the countries have achieved self-sufficiency in cereal crops, mostly rice.

Table 5 presents the regional breakdown of areas under different types of water management.

The assessment of land under irrigation in the countries of the region is made particularly difficult by the different approaches used in the countries to compute irrigation. For some countries (Bangladesh, Bhutan) paddy fields, cultivated mainly during the wet season, are not considered as irrigated land. For the other countries where paddy rice cultivation is practised, all paddy fields are considered irrigated land. In most cases, schemes are designed primarily to secure rice cultivation in the main cropping season, although the need for intensification has progressively led some countries to design new irrigated schemes for year-round irrigation, e.g. Thailand, while Viet Nam has three rice crops a year.

In total, 37 percent of the land under cultivation in the region is irrigated (Table 5). This is the highest level compared to the other major regions of the world. DPR Korea has the highest level, with 73 percent of cultivated land under irrigation, followed by Japan with 65 percent and China with 55 percent. Bangladesh, Nepal, Republic of Korea and Viet Nam have more than 40 percent of cultivated land under irrigation. Other tropical countries of south Asia and the Islands have an average between 20 and 25 percent of their cultivated land under irrigation.

Scheme classifications also vary widely among countries. Classifications are made according to scheme size, type of management, or institutional set-up. Several countries make the distinction between schemes designed for wet season crops and schemes designed for dry season crops (Lao PDR, Philippines, and Bangladesh).

While most wet season rice irrigation is fully gravity irrigation (cascades from plot to plot), dry season cropping may require pumping in places. This is the case in Lao PDR where, due to the pumping costs, dry season rice cropping has not proved economic unless a very bad harvest has been recorded in the previous wet season. In the tropical zone, wet season irrigation is almost only paddy rice. It is usually considered as supplementary irrigation to an already abundant precipitation. During the dry season, a much larger diversity of crops are grown on irrigated fields. In Cambodia, Indonesia, Malaysia and Mongolia, a kind of flood control irrigation is practised with flood water being used to inundate paddy fields which are then cultivated with rice. In total, such practice concerns an area of about 1.2 percent of the total irrigated land in the region.

Irrigation techniques

Surface irrigation is by far the most widespread irrigation technique in the region. It includes all paddy rice cultivation and most of the other crops. In most countries, sprinkler or drip irrigation systems are reported to exist on very small, experimental plots. Table 6 presents the information on sprinkler and micro-irrigation for the countries for which this information was available.

Mongolia is the only country where sprinkler irrigation represents a significant part of the area under irrigation as large schemes were systematically equipped with sprinkler irrigation in the 1980s.

Origin of irrigation water

Surface water is the major source of irrigation water in the region, except for Bangladesh and India where groundwater is widely used. Irrigation systems are generally grouped as:

• systems supplied through surface reservoirs;
• pumping from rivers;
• pumping from groundwater.

The percentage of power irrigated area is more important in Bangladesh, China and India, with 83, 54 and 53 percent respectively, while in Bhutan and Myanmar the corresponding figures are only 2 and 3 percent respectively (Table 7).

Irrigated crops

Information on irrigated crops is scarce and incomplete. In many countries, no distinction is made between areas of irrigated and rainfed crops. Rice represents about 45 percent of all irrigated crop areas in the region and 59 percent of the rice is irrigated (Table 8). However, its regional distribution shows major trends: in the countries of the Far East, Southeast Asia and the Islands, rice represents systematically more than 90 percent of irrigated crops, as is also the case for Bhutan, Nepal and Sri Lanka. In these countries, the remaining 10 percent consists of some dry season cereals, vegetables or industrial crops. Irrigation has played an important role in rice production in the second part of the twentieth century, and by the end of the 1980s many countries of the region had achieved self-sufficiency in rice.

By contrast, India, China and DPR Korea have a much more balanced distribution of irrigated crops with rice representing only about one-third to one half of the total irrigated crop area. This reflects the cold or arid context of large parts of these countries. In India, the percentage of land under irrigated wheat is slightly higher than that under irrigated rice (31 percent against 30 percent), the rest being shared between a large variety of crops. Data for irrigated crops in China were not available but it can be estimated that it is shared evenly between rice, wheat and other crops; rice being the single most important irrigated crop. However, in India only 47 percent of the total harvested area for paddy rice is irrigated, while more that 92 percent of the harvested paddy rice in China is irrigated (Table 8).

More or less reliable and complete information on irrigated cropping pattern is available for ten countries. Table 9 compares the total area of irrigated crops with the area equipped for irrigation. Cropping intensity varies from 72 percent in Bhutan, to 132 percent in India and Malaysia with an average of 127 percent. Care should be taken, however, when comparing figures for different countries. In Bhutan, for instance, irrigation figures refer only to summer crops.

In Bangladesh (84 percent), irrigation is considered only for dry season cropping. The average irrigated cropping intensity for these ten countries is 127 percent.

Drainage, flood control and environmental and health issues