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Irrigation and drainage
Rice cultivation dates back about 2 000 years, and irrigation did not lag behind. Since then, land reclamation for rice cultivation has spread as the population has grown. In the early decades of this century, several irrigation schemes were undertaken as national projects. However, it is since the 1940s that a number of irrigation projects have been extensively carried out nationwide not only by the central government but also by local governments and Land Improvement Districts (LIDs) through subsidies.
In the 1940s, irrigation projects either modernized old systems or provided new ones often associated with land reclamation for paddy. They included the construction of dams for increased water supply and of weirs to unify intakes. Later integrated with farmland consolidation projects, many projects have had the objective of reorganizing irrigation networks to facilitate water distribution and meet increased water requirements as a result of improved drainage.
With the need to adjust to a changing residential environment, the development of irrigation has tended to move to the construction of pipeline systems instead of open channels, a fact which has also led to more effective water use. Due to the overproduction of rice in the early 1970s, resulting from an increase in yields and a decrease in consumption, efforts have been made to plant non-rice crops on paddy fields such as wheat, corn, beans and vegetables. The expansion of irrigation systems to dryland crops is being carried out in various parts of the country (Figure 2).
In Japan, irrigation is used predominantly for rice cultivation through basin irrigation. Cultivation techniques and mechanized farming require several drainages during one cropping season. This leads to water loss in the fields. Furthermore, the percolation rate in paddy fields through vertical drainage is relatively high, so leading to poor on-farm irrigation efficiency. However, water that returns to a river is re-used by other irrigation systems downstream. Thus, densely developed and stratified irrigation systems contribute to good water use efficiency at river basin level.
In 1993, the total area of land equipped for full or partial control irrigation was estimated at 3 128 079 ha. Almost all paddy fields are irrigated, and little rainfed land remains. In addition, irrigation for non-rice fields reached 346 668 ha in 1993 (Figure 3).
Groundwater is used mainly to satisfy supplementary irrigation water requirements during the low water season from April to September, and especially in August. The total area of land benefiting from supplementary irrigation from groundwater is estimated at 500 000 ha. More than 90 percent of it is paddy fields, the remainder being upland fields, orchards and grasslands. Irrigation facilities using groundwater are generally designed on a smaller scale than those based on surface water from streams and reservoirs.
Most irrigation systems for paddy fields are of the gravity type though some have pumping stations to lift water from rivers or other sources. For upland fields, sprinkler irrigation is usually applied. Drip irrigation is used mainly in greenhouses (Figure 4).
The central government finances one-third of the cost of construction through loans to local government and beneficiaries. Projects operated by local governments and LIDs receive subsidies from the central government of 50 and 45 percent of their cost respectively. The remaining costs are usually shared byboth. Users or LIDs operate and maintain the systems at their own expense including those projects completed by the central government, though there are some exceptions.
Drainage projects have been carried out successfully in flood-proneareas to protect land from excessive inundation. Farmland consolidation projects, implemented widely in the country, include the improvement of on-farm irrigation systemsand of surface or subsurface drainage. The improved drainage systems also contribute tothe conversion of paddy land to land suitable for non-rice cropping.