2.2. Outline of paddy field in the world

Rice, as one of the main staple for human being, is cultivated in various regions of the world from the wettest areas in the world to the driest deserts, with various conditions of natural environment including climate, topography, and soil conditions. For example, rice is grown in the area with more than 5000 mm of rain for one growing season, and with less, almost zero, of rainfall. The growing season average temperature of rice producing areas varies from more than 30C to less than 15C. Rice cultivation is observed in a higher mountain region with more than 2500 m above the sea level, as well as in ocean coast even in sea level region [1].

Most of the fields, where rice is produced, are flooded, or submerged by water, naturally or artificially during rice production period. To keep paddy fields submerged artificially, some infrastructures like reservoirs or ponds, intake and diversion works and canals are constructed. The infrastructures, after construction, are operated and maintained generally by local society, usually with some supports of the government. Rice is produced mostly in the fields with artificial water management with irrigation and drainage system, than in naturally

The stable water supply and continuous ponding in the fields are the base for much rice production. On the other hand, as mentioned above, they need hydraulic structures and the appropriate operation and management of the structures. The artificial ponding with stable and much water supply mostly results in better rice growth, while it might change the local environment both positively and negatively. In the case that the impacts of ponding and irrigation on the environment are positive, they are to be recognized as their "multi-functions." Recently, much water use for rice cultivation and necessity of water saving in paddy irrigation have been discussed often, and simultaneously the role of flooding in paddy fields has been

On the other hand, since rice production area has been reducing in some developed countries and regions with long history of rice cultivation, like Japan, Taiwan, and Korea, the role of rice fields and their flooding is to be reevaluated. In this chapter, the irrigated fields for rice are recognized as artificial and temporal wetland and reviewed comprehensively, focusing mainly

The words of "paddy field" are usually and widely used for the farmland, where rice is cultivated, and they generally imply the area flooded, like the definitions of Cambridge Dictionary as "a field planted with rice growing in water" [5], Collins English Dictionary as "a flooded piece of land used for growing rice" [6], and The Free Dictionary as "a field, often flooded with water, in which rice is grown" [7]. The definition of "paddy field," however, is intricate slightly. Fundamentally "paddy" means "rice" especially in the husk. Consequently, a "paddy field" means a field planted with rice. Some dictionaries describe that only "paddy" could mean "paddy field," without any word for indicates the space, like the definition of the

highlighted in terms of environmental conservation (for example, see [2–4]).

2. Definition and outline of paddy field in the world

2.1. "Paddy field" as a farmland with rice and flooded water

on their role for local hydrological environment.

flooded fields.

144 Irrigation in Agroecosystems

Generally, rice is a major food crop for the people in the world. Especially in the Asia region, rice is a staple food for about 2.4 billion people, and there the 90% of the world's rice is produced and consumed [10].

As summarized above, rice production and paddy fields are developed in a wide range of environments even in the arid region of the world and during the dry season. The paddy fields in dry areas sometime show very high and stable yields with much solar radiation.

The paddy fields or the environments of the rice production are classified usually based on the hydrological characteristics, since they are most essential condition to the production scheme. The most popular classification includes: (1) irrigated lowland, (2) rain-fed lowland, (3) flood prone, and (4) upland [1].

The first category "irrigated" paddy fields distributed in lowland are the area, where rice is grown in fields surrounded by ridges. Its water condition is managed by farmers, generally maintaining water depth as 5–10 cm. It covers about 90 million ha, as almost half of the world paddy area. The major portion of this irrigated area is in the Asian region.

The second category "rain-fed lowland" or "lowland rain-fed" is a field, where rice is also grown in fields with bunds, while they are flooded with rainwater for some period of a growing season. It covers about 50 million ha. There, water is flooded naturally by rain water, not fully controlled by the man-made irrigation system. These two types of paddy fields are usually predominantly puddled, and after it, rice seedlings are transplanted. These two types of paddy fields produce 75 and 19% of the world's rice production, as almost 95% of rice is produced in the area, of which water condition is fully or partly controlled by humans like farmers.

3. Significance of water ponding in paddy fields

Rice cultivation has some superiority on food production mentioned above. On the other hand, paddy fields, where rice is grown, need much water due to its flooding. The main reasons why paddy fields are flooded is that most rice varieties realize better growth and produce higher

Paddy Fields as Artificial and Temporal Wetland http://dx.doi.org/10.5772/intechopen.80581 147

In most cases, the water layer of some centimeters in a field is established usually after transplanting of rice seedlings and maintained until few weeks before harvesting. The typical water management of paddy field with standard depth of flooding for each growing stage is shown by

Actual water management practices on water application and flood depth control are affected

In the improved paddy field with stable water supply and enough drainage capacity, independent water management practices of farmers are implemented, where the farmers can apply and drain water whenever they want and they introduce advanced techniques and materials.

FAO [10], and it is summarized in Table 1 with supplemental explanation of GriSP [1].

6. drainage capacity (water conductivity of soil profile, groundwater table, etc.),

3. soil profile (water holding capacity, permeability, fertility, etc.),

Table 1. Typical water management of paddy field with standard depth of flooding.

4. fertilizer and chemicals (pesticide and herbicide), 5. irrigation water availability (timing and quantity),

3.1. Water management in paddy plot

yields in flooded farmland than in dry field.

by field conditions including:

1. cultivar of rice,

7. farm machinery, 8. labor inputs, and

9. other farming techniques.

2. climate and weather,

In the third category "flood-prone field," deep-water rice and floating rice are grown in the uncontrolled flood environments, suffering periodically from excess water and deep flooding, sometime with deeper flood of 100 cm for some certain part of the growing season. This covers about 15 million ha.

The last, forth category "upland," is a field where rice is grown under dryer conditions, without ponded water, and then it is not surrounded by ridges to keep water and not equipped with irrigation system. The area of "flood-prone" and "upland" is about 11 and 15 million ha, respectively [1].
