2. Rice production in China

Vietnam (6%), Thailand (4%), and Myanmar (3%), collectively account for 80% of the world's

Figure 1. Rice production of world top 10 countries in 2016 (based on data from www.statista.com; [2]).

Worldwide, agriculture accounts for 70% of all water consumption, far more than the 20% for industry and the 10% for domestic use (http://www.worldometers.info/water/). As water is needed throughout the rice-growing season, paddy fields altogether consume up to 90% of the total water used for irrigation in Asia [3]. Therefore, paddy water management is crucial to save water resources in the context of water quantity. Meanwhile, there have been frequent reports on environmental and ecological concerns related to paddy production. In particular, water quality issues have received increasing attention. Rice is commonly grown in regions close to inland streams and lakes, which is a double-bladed sword. On the one hand, such a landscape arrangement resulting from long-term human adaptation to the environment allows the most convenient and economic use of water resources in agricultural production. On the other hand, it generates risks of eutrophication in the streams and lakes where the ecological systems are sensitive to nutrients. Indeed, several previous studies identified phosphorus losses from paddy production systems as an important cause of eutrophication in the local, enclosed lakes in China (e.g., [4, 5]). This is because such regions are commonly characterized by enhanced, extensive hydrological networks between paddy fields and between the fields and their adjacent water

total rice production (Figure 1).

106 Irrigation in Agroecosystems

In China, rice is the first major food crop, owning a total planting area of 30 million hectares. The rice production of 207 million metric tons is equivalent to 34% of the total grain crop production [6]. Rice production is mainly concentrated in three geophysical regions: Yangtze River Basin (covering provinces of Hunan, Jiangxi, Jiangsu, Hubei, Sichuan, Anhui, Yunnan, Zhejiang, Chongqing, Guizhou, and Shanghai), Southeast Coastal Plains (Guangxi, Guangdong, Fujian, Hainan, Hong Kong, Macao, and Taiwan), and Northeast Plains (Heilongjiang, Jilin, and Liaoning; Figure 2). Specifically, the Yangtze River Basin accounts for 65% of China's total rice planting

Figure 2. Major rice-producing regions in China [7].

area, followed by the Southeast Coastal Plains (16%), the Northeast Plains (15%), and other regions (4%; Figure 3). Due to its nature of requiring large amounts of water throughout the growing season, over 80% of the rice planting areas are located in Southern China where annual precipitation ranges from 1000 to >2000 mm. Notably, the rice planting areas make up over 10% of the total provincial areas in Hunan, Jiangxi, Jiangsu, Hubei, Anhui, Shanghai, and Guangdong. In line with the patterns of planting areas, the Yangtze River Basin dominates the national rice production (65%), which is followed by the Northeast Plains (16%), the Southeast Coastal Plains (14%), and other regions (5%; Figure 3). In 2016, the top five rice-producing provinces, i.e., Hunan,

Heilongjiang, Jiangxi, Jiangsu, and Hubei, produced 51% of China's total rice. In combination, these five provinces receive a total of 180 billion m3 of precipitated water in paddy fields, which is

Water Quality in Irrigated Paddy Systems http://dx.doi.org/10.5772/intechopen.77339 109

Across the world, rice cropping systems vary from monoculture (e.g., northern China) to double (e.g., southern China) and triple cropping (e.g., India and Bangladesh) depending on climatic conditions. In most areas of China, the rice-growing season starts in spring with steeping paddy fields and the transplantation of rice seedlings and ends in the fall with the draining of the fields and harvesting rice grains. Despite large variabilities in paddy management by regional and local conventions as well as available technologies, rice cultivation is commonly characterized by the flooding of the paddy fields during most of the growing season along with intensive irrigation and fertilization. Flooding the rice fields is essential for most rice varieties to maintain good growth and achieve high yields. Figure 4 presents typical management schedules for paddy rice in Hubei Province, Yangtze River Basin, China. Due to favorable climatic conditions, the vast area of this province allows a double-cropping system represented as the rotation of rice with winter wheat (Triticum aestivum L.) or oilseed rape (Brassica napus L.). The rice-growing season starts in late May with irrigation to steep field for a few days, followed by plowing and basal fertilization prior to transplanting rice seedlings. During the process of transplanting rice in early June and harvesting of rice in late September, there are often a couple of fertilizer top dressings to meet rice's need of nitrogen. There are also a number of irrigation and drainage operations to

49% of the precipitated water received by all paddy fields.

maintain appropriate depths of ponding water.

Figure 4. Typical management schedules for paddy rice in Hubei Province, Yangtze River basin, China.

Figure 3. Annual precipitation, precipitation water volume, rice planting area, and rice production by provinces of China in 2016 [8, 9].

Heilongjiang, Jiangxi, Jiangsu, and Hubei, produced 51% of China's total rice. In combination, these five provinces receive a total of 180 billion m3 of precipitated water in paddy fields, which is 49% of the precipitated water received by all paddy fields.

area, followed by the Southeast Coastal Plains (16%), the Northeast Plains (15%), and other regions (4%; Figure 3). Due to its nature of requiring large amounts of water throughout the growing season, over 80% of the rice planting areas are located in Southern China where annual precipitation ranges from 1000 to >2000 mm. Notably, the rice planting areas make up over 10% of the total provincial areas in Hunan, Jiangxi, Jiangsu, Hubei, Anhui, Shanghai, and Guangdong. In line with the patterns of planting areas, the Yangtze River Basin dominates the national rice production (65%), which is followed by the Northeast Plains (16%), the Southeast Coastal Plains (14%), and other regions (5%; Figure 3). In 2016, the top five rice-producing provinces, i.e., Hunan,

Figure 3. Annual precipitation, precipitation water volume, rice planting area, and rice production by provinces of China

in 2016 [8, 9].

108 Irrigation in Agroecosystems

Across the world, rice cropping systems vary from monoculture (e.g., northern China) to double (e.g., southern China) and triple cropping (e.g., India and Bangladesh) depending on climatic conditions. In most areas of China, the rice-growing season starts in spring with steeping paddy fields and the transplantation of rice seedlings and ends in the fall with the draining of the fields and harvesting rice grains. Despite large variabilities in paddy management by regional and local conventions as well as available technologies, rice cultivation is commonly characterized by the flooding of the paddy fields during most of the growing season along with intensive irrigation and fertilization. Flooding the rice fields is essential for most rice varieties to maintain good growth and achieve high yields. Figure 4 presents typical management schedules for paddy rice in Hubei Province, Yangtze River Basin, China. Due to favorable climatic conditions, the vast area of this province allows a double-cropping system represented as the rotation of rice with winter wheat (Triticum aestivum L.) or oilseed rape (Brassica napus L.). The rice-growing season starts in late May with irrigation to steep field for a few days, followed by plowing and basal fertilization prior to transplanting rice seedlings. During the process of transplanting rice in early June and harvesting of rice in late September, there are often a couple of fertilizer top dressings to meet rice's need of nitrogen. There are also a number of irrigation and drainage operations to maintain appropriate depths of ponding water.

Figure 4. Typical management schedules for paddy rice in Hubei Province, Yangtze River basin, China.
