1. Introduction

Rice (Oryza sativa L.) is a staple food for roughly 50% of the world's population. Globally, rice is planted on a total of 155 million hectares of land, and annual rice production amounts to up to 480 million metric tons [1]. Nearly 90% of the rice is produced in Asian countries. The top seven rice-producing countries, i.e., China (30%), India (22%), Indonesia (8%), Bangladesh (7%),

© 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.

bodies, which elevate the risk of nutrient runoff from paddy fields to the waters. In the context of

Water Quality in Irrigated Paddy Systems http://dx.doi.org/10.5772/intechopen.77339 107

In this chapter, we reviewed water quality issues related to paddy rice production and discussed the potential strategies to reduce nutrient losses to the water environment. As China is the largest rice-producing country in the world, we focused the review and discussion on China's scenario.

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

water quality, both nutrient and water management are of great importance.

Even so, we included information from other countries whenever it was relevant.

2. Rice production in China

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

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

Vietnam (6%), Thailand (4%), and Myanmar (3%), collectively account for 80% of the world's total rice production (Figure 1).

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 bodies, which elevate the risk of nutrient runoff from paddy fields to the waters. In the context of water quality, both nutrient and water management are of great importance.

In this chapter, we reviewed water quality issues related to paddy rice production and discussed the potential strategies to reduce nutrient losses to the water environment. As China is the largest rice-producing country in the world, we focused the review and discussion on China's scenario. Even so, we included information from other countries whenever it was relevant.
