The water holding capacity of the soil is extremely poor. The central region is an intensively cultivated alluvial area. During the rainy season, rice covers the major part of the region. The central region accounts for about one-fifth of the total cultivated riceland of the country in the wet season. The northern region has almost 17 million ha.
Upland rice is grown in the lower altitudes of high hills and in upland areas.
Lowland rice is grown mainly in lower valleys and on some terraced fields where water is available. With limited ricefields under cultivation, there is always a shortage of rice for local consumption. In addition to the physical and weather problems associated with the different rice ecosystems, the main problems farmers face are stagnant yields and labor shortages. But the high quality, aromatic Thai rices, which command high prices on the world market, are low-yielding, traditional varieties. So the low yields do not necessarily reflect low input supply, poor water control, or any of the other constraints normally associated with low yields.