3. Water requirement of rice

Water requirement of rice crop is influenced significantly by environmental conditions such as climate. For example, in Bangladesh, with a tropical climate all over the country, more than 2000 liters of water is required to produce every kilogram of rice dry substance. In China, where rice production areas span from the cold Northeast to the subtropic and tropic South, such water requirement ranges from 400 to 1500 liters. Based on a 30-year meteorological data, statistics of crop growth stages, crop water requirement, and net irrigation requirement, Liu et al. [10] estimated the requirement of water and irrigation for the rice across China, using the FAO Penman-Monteith equation and crop coefficient method. Across the three major riceproducing regions, the rice crop requires 250–950 mm of water, which is greater than the 200–620 mm required for corn (Zea mays L.), wheat, or cotton (Gossypium spp.) crops. Likewise, rice requirement for irrigation (usually 70–500 mm) is also greater than the other crops (0–350 mm). It should be noted that rice water requirement and net irrigation requirement vary widely both between regions and within regions (Table 1), reflecting the spatial and temporal variability of water needs by the crop. Furthermore, water requirement and irrigation requirement also differ with rice varieties and growing seasons. Typically, middle rice and late rice need more irrigation

Water Quality in Irrigated Paddy Systems http://dx.doi.org/10.5772/intechopen.77339 111

Water quality problems evolve at both sides of the paddy systems, i.e., inputs of contaminants with irrigation water and exports of nutrients to the surrounding water environment. In the case of contaminant inputs with irrigation water, wastewater or reclaimed wastewater irrigation has generated particular concerns [11–14]. In a recent review on the impacts of wastewater irrigation, Amin et al. [11] concluded that even though wastewater is a valuable source of nutrients, it may contribute many emerging contaminants to the water environment. Indeed, wastewater may contain an array of contaminants such as heavy metals, pathogens, and organic contaminants, along with nutrients (e.g., those listed in Table 2). As a result, there is a potential risk of contamination of both shallow groundwater and surface water associated with wastewater irrigation [14]. In a field study, Cao and Hu [12] found that irrigation with copper-rich wastewater increased soil copper concentration in the surface soil layer (0–10 cm) by sixfold and reduced rice yield by 18–25% as compared with the control with normal irrigation water. Accumulation of copper in the surface soil greatly elevated the potential risks of copper pollution through surface runoff. Elsewhere, however, Kang et al. [13] found no adverse effects of reclaimed wastewater on both rice grains and the paddy fields after appropriate treatments of the wastewater. These results point to the importance of monitoring and treatment of wastewater before use for

In the case of nutrient exports to the surrounding water environment, the issue of water quality is closely related to water and nutrient turnover and management in the paddy systems. Budget of water in paddy systems involves water inputs in the forms of rainfall and irrigation and water outputs through evapotranspiration, runoff, and deep percolation (Figure 5). Rainfall is a

Nutrients Unit Range Contaminants Unit Range Total nitrogen mg/L 20–70 Total solid mg/L 390–1230 Total phosphorus mg/L 4–12 Total dissolved solid mg/L 270–860 Total organic carbon mg/L 80–260 Total suspended solid mg/L 120–400

Table 2. Typical nutrients and contaminants in untreated domestic wastewater (based on [15]).

Biochemical oxygen demand mg/L 110–350 Chemical oxygen demand mg/L 250–800 Total coliform Counts/100 mL 10<sup>6</sup>

Fecal coliform Counts/100mL 103

–109

–107

water than early rice, due to their prolonged growing seasons.

4. Water quality issues in paddy systems

irrigation.


Table 1. Water requirement and irrigation requirement of the rice crop in comparison to other crops in China (adapted from [10]).

It should be noted that rice water requirement and net irrigation requirement vary widely both between regions and within regions (Table 1), reflecting the spatial and temporal variability of water needs by the crop. Furthermore, water requirement and irrigation requirement also differ with rice varieties and growing seasons. Typically, middle rice and late rice need more irrigation water than early rice, due to their prolonged growing seasons.
