1. Introduction

Paddy cultivation plays a significant and vital role on rice production. Most of the global population depends on the 480 million tons of rice produced each year as the basis for their lives. While about 90% of the world's 160 million hectares of paddy fields are in Asian countries, mainly in monsoon regions, paddies are also seen in North America and Africa, even in dry regions where irrigation has reclaimed dry land for paddy fields.

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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].

Oxford Living Dictionary as "A field where rice is grown" [8], and of Merriam-Webster as "wet land in which rice is grown" [9], while it also show the meaning as "Rice before threshing or in

Paddy Fields as Artificial and Temporal Wetland http://dx.doi.org/10.5772/intechopen.80581 145

Since rice is usually grown in level basin flooded with water throughout most of the growing season, "paddy field" generally means "a field flooded with water for growing rice," and the definitions of "paddy field" in most of the current dictionaries include words of "rice" and

In Japan, the English words "paddy field" is translated to the Japanese word "suiden," while there this word "suiden" is used for flooded farm land, which distinguishes "flooded field" from "upland field" or "hatake" in Japanese. The upland field is not flooded and cultivated for normal crops like vegetables and flowers. Accordingly, in Japan, it is expressed that some aquatic crops like lotus and tatami are cultivated in "paddy fields." This Japanese case is recognized as an exceptional case. In this chapter, "paddy field" is to be used basically as "a

As mentioned below, actually, in considerable area in the world, rice is produced in fields without flooding. Then, some parts of paddy fields of the world are not identified as the

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

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

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

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

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

in dry areas sometime show very high and stable yields with much solar radiation.

paddy area. The major portion of this irrigated area is in the Asian region.

the husk."

"water" or "flood," as introduced above.

field planted with rice."

"wetland" with water submergence.

produced and consumed [10].

prone, and (4) upland [1].

2.2. Outline of paddy field in the world

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 flooded fields.

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

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 on their role for local hydrological environment.
