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Irrigation and drainage
Evolution of irrigation development
Irrigation from river diversion and reservoirs started in the north seven centuries ago. In modern times, canal construction for irrigation started at the beginning of the last century, in parallel with the creation of the RID. The aim was to maintain water in canals for irrigation and navigation, and to drain paddy fields during periods of flooding. Irrigation has traditionally been supplementary irrigation for the wet season. It is only recently that schemes have been designed for dry season irrigation.
The irrigation potential for the wet season can be roughly estimated as 12.2 million ha, considering both soil and water availability but excluding basin transfers (World Bank, 1985). New estimates consider that irrigation potential accounts for 9.5 million ha (Thai Hydrologist Assembly, 2007).
In 2007, the area equipped for wet season irrigation was an estimated 6 414 800 ha. In 2005, the regional distribution of irrigated area in the wet season was 54 percent in the central plain, 18 percent in the north, 14 percent in the northeast and 14 percent in the south. In 1995, the area equipped for wet season irrigation was an estimated 5 003 724 ha, of which 47 percent in the central plain, 24 percent in the north, 19 percent in the northeast and 10 percent in the south.
In 2007, the area actually irrigated was an estimated 5 089 914 ha, or 79 percent of the equipped area (Table 5).
Surface irrigation is the only technology used in the schemes. Sprinkler and localized irrigation are at an experimental stage only for fruit trees. Generally surface water is used, accounting for 90.9 percent of the total area equipped for irrigation (Figure 4).
Early systems were designed to operate at full capacity only in the wet season. The canal capacities and control regulators are inadequate for the increasing demand for dry season irrigation. Furthermore, irrigation water demand has to compete with demand from other sectors. This becomes a sensitive issue during the dry season. A certain flow of water must be maintained for navigation, to prevent saltwater intrusion, and to supply water for domestic and industrial purposes in the Bangkok area. In the dry season, water resources can no longer meet the increasing water demand from all sectors, and particularly from the irrigation subsector, which needs to withdraw more and more water because of the development of dry season irrigation. This water competition has led to poor agricultural performance in recent dry seasons.
Dry season irrigation is practiced on 60 percent of the equipped area, up from 18 percent in 1994.
Small-scale projects are those that can be completed within one year and without land compensation. The schemes that cannot be completed within one year, or that need land compensation are considered medium-scale. Schemes are classed as large-scale if there is a storage capacity of more than 100 million m3 or if they can irrigate at least 80 000 rais (12 800 ha). Irrigated areas can be divided into the three categories(Figure 5):
- There were 83 large-scale projects under the Royal Irrigation Department (RID) and operational by 2002 with a combined storage volume of 6 662 million m3. In 2007, the total command area was about 2.7 million ha. Water management in these projects is the responsibility of RID and water user groups.
- There were 607 medium-scale projects by 2002 with a combined storage volume of 3 191 million m3. In 2007, the total command area was 898 880 ha. Water management in these projects is also the responsibility of RID and water user groups.
- There were 10 606 small-scale projects by 2002 with a combined storage volume of 2 110 million m3. In 2007, the total benefit area was around 2.4 million ha. Water management in these projects is the responsibility of local governments and water user groups.
By 1999, there were 1 985 pumping projects. Essentially these are small-scale projects with electrical pumping from nearby waterways. In 2007, their combined command area was 460 000 ha, mainly in the northeast and north. Water management in these projects is the responsibility of RID and water user groups. Management responsibility is to be transferred to Local Governments.
Role of irrigation in agricultural production, the economy and society
In 2007, total harvested irrigated cropped area was an estimated 7 387 072 ha. Rice accounts for 6 268 080 ha (2 327 158 ha first season rice and 3 940 922 ha second season rice), or 84.9 percent of the harvested irrigated copped area, vegetables represent 83 421 ha (1.1 percent), sugarcane 256 016 ha (3.5 percent), other annual crops (tobacco, cotton, etc.) 37 396 ha (0.5 percent) and permanent crops 742 159 ha (10.1 percent) (Table 4 and Figure 6). There are also 233 033 ha of fish ponds, which are irrigated and not taken into account in the total, because they are not a crop.
Irrigation development costs US$3 647/ha as follows: construction of head work US$2 187/ha, conveyance system US$860/ha, field system US$600/ha and maintenance cost US$33/ha per year.
Poverty is observed to be concentrated in non-irrigated areas (or rainfed areas).