4. Water quality issues in paddy systems

3. Water requirement of rice

110 Irrigation in Agroecosystems

Yangtze River Basin (middle and lower portion)

Yangtze River Basin (upper

portion)

Southern Coastal Plains

from [10]).

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).

requirement (mm)

Early rice 400–580 80–300 Middle rice 500–800 150–420 Late rice 500–650 150–400 Spring corn 250–550 0–200 Summer corn 330–450 100–200 Cotton 450–620 50–300

Early rice 350–700 100–500 Middle rice 550–950 100–500 Late rice 400–700 100–350 Spring corn 300–500 10–250 Summer corn 300–450 20–100 Winter wheat 200–600 100–350

Early rice 400–580 70–300 Middle rice 450–570 90–250 Late rice 600–700 100–450 Spring corn 200–400 0–120 Summer corn 250–420 50–150 Cotton 450–520 30–180

Spring corn 200–500 10–220 Spring wheat 250–450 100–300

Net irrigation requirement (mm)

Region Province Crop Water

Hunan, Jiangxi, Jiangsu, Hubei, Anhui, Zhejiang,

Yunnan, Guizhou, Sichuan,

Guangxi, Guangdong, Fujian, Hainan

Northeast Plains Heilongjiang, Jilin, Liaoning Middle rice 250–750 80–450

Table 1. Water requirement and irrigation requirement of the rice crop in comparison to other crops in China (adapted

Shanghai

Chongqing

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 irrigation.

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


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

Figure 5. Water budget in rice production.

common, major water input to paddy fields. However, irrigation is usually needed to maintain an appropriate depth of ponding water enclosed by a constructed field berm (Figure 5). In addition to evapotranspiration, surface runoff is a major water output from paddy fields. Along with runoff water, phosphorus and nitrogen applied to rice or those in the soil materials are exported from paddy fields. Runoff occurs when the depth of the field ponding water is greater than the height of field berm. Runoff can be generated following small rainfall events when ponding water has been already substantial but more frequently during rainfall storms [16, 17]. Paddy soils are often heavily textured and have a plow pan beneath the surface soil (particularly for long-term cultivated paddy fields). Therefore, the amount of water percolating to the subsoil and out of the root zone is relatively small as compared to surface runoff. Nonetheless, Qiu et al. [18] reported that nitrate-nitrogen concentrations could reach 30–50 mg/L in the leachate from some paddy soils within 1–2 days after fertilizer applications. Due to the flooding nature of paddy fields, the surface soil is often water saturated with a predictably small change in soil water content throughout the paddy growing periods. The anaerobic condition may lead to an elevation of dissolved phosphorus concentrations in runoff water because when iron cation is transformed from iron3+ to iron2+ under anaerobic conditions, the phosphorous ions bound by iron3+ is dissolved. Moreover, artificial drainage that is made to prepare the field (Figure 4) forms a direct pathway for nutrient transport to surrounding water environments. Finally, paddy irrigation with nutrient-rich water (such as domestic wastewater; Table 2) can also greatly elevate risks of nutrient losses to the water environments.

Phosphorus and nitrogen applied to paddies with fertilizers and manures contribute to both short-term and long-term nutrient losses to the water environments. In a 3-year field study on the hydromorphic paddy soil, for example, Liu et al. [19] found that annual total phosphorus loss in surface runoff ranged from 0.63 kg/ha in the unfertilized rice-wheat rotation to 0.96–2.86 kg/ha when rice and wheat were fertilized with 50–230 kg phosphorus per hectare. In the same study, they found relatively smaller total phosphorus losses from the

Soil Rainfall

Irrigation

Phosphorus

Days between fertilizer

Total

Rainfall

Phosphorus

Days between fertilizer

Total phosphorus

loss (kg/ha)

application and the first

runoff

phosphorus

depth (mm)

rate (kg/ha)

> loss (kg/ha)

0.130.01

0.190.02

0.480.08

0.920.06

0.760.05

1.060.12

2.270.31

4.180.33

0.360.06

0.590.05

0.930.09

1.560.19

0.140.02

0.160.01

0.230.03

0.290.03

0.200.04

0.320.02

0.270.04

0.550.09

0.170.01

0.230.01

0.280.01

0.380.02

 468

 80

 conditions (mean

standard error, n = 4 for the

113

13

 468

 40

13

 468

 20

13

 468

 0

13

 667

 80

32

 667

 40

32

 667

 20

32

 667

 0

32

 439

 80

11

 439

 40

11

 439

 20

11

 439

 0

11

 547

 80

14

 547

 40

14

 547

 20

14

 547

 0

14

 688

 80

31

 688

 40

31

 688

 20

31

 688

 0

31

 449

 80

10

 449

 40

10

 449

 20

10

 449

 0

10

0.230.01

0.410.03

0.640.02

0.970.05

0.130.01

0.210.01

0.240.02

0.490.03

0.270.03

0.430.04

0.650.07

1.110.08

0.260.01

0.410.02

1.030.04

1.310.09

0.220.02

0.370.02

0.510.02

0.780.03

0.180.02

0.760.19

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

1.180.17

1.390.12

application and the first runoff

depth (mm)

Hydromorphic

 paddy 604

604 604 604 761 761 761 761 531 531 531 531 Degleyed paddy soil 548

1290

833

548 548 548 723 723 723 723 555 555 555 555

> Table 3.

hydromorphic

 paddy soil or 6 for the degleyed paddy soil; adapted from [19]).

Phosphorus

 losses in runoff from a rice-wheat rotation cropping system under different natural and management

1290

 150

49

1290

 75

49

1290

 30

49

1290

 0

49

868

 150

45

868

 75

45

868

 30

45

868

 0

45

833

 150

58

833

 75

58

833

 30

58

 0

58

 150

1

1290

 75

1

1290

 30

1

1290

 0

1

868

 150

2

868

 75

2

868

 30

2

868

 0

2

833

 150

5

833

 75

5

833

 30

5

833

 0

5

depth (mm)

rate (kg/ha)

Rice growing season

Wheat growing season


common, major water input to paddy fields. However, irrigation is usually needed to maintain an appropriate depth of ponding water enclosed by a constructed field berm (Figure 5). In addition to evapotranspiration, surface runoff is a major water output from paddy fields. Along with runoff water, phosphorus and nitrogen applied to rice or those in the soil materials are exported from paddy fields. Runoff occurs when the depth of the field ponding water is greater than the height of field berm. Runoff can be generated following small rainfall events when ponding water has been already substantial but more frequently during rainfall storms [16, 17]. Paddy soils are often heavily textured and have a plow pan beneath the surface soil (particularly for long-term cultivated paddy fields). Therefore, the amount of water percolating to the subsoil and out of the root zone is relatively small as compared to surface runoff. Nonetheless, Qiu et al.

paddy fields, the surface soil is often water saturated with a predictably small change in soil water content throughout the paddy growing periods. The anaerobic condition may lead to an elevation of dissolved phosphorus concentrations in runoff water because when iron cation is transformed from iron3+ to iron2+ under anaerobic conditions, the phosphorous ions bound by iron3+ is dissolved. Moreover, artificial drainage that is made to prepare the field (Figure 4) forms a direct pathway for nutrient transport to surrounding water environments. Finally, paddy irrigation with nutrient-rich water (such as domestic wastewater; Table 2) can also greatly ele-

Phosphorus and nitrogen applied to paddies with fertilizers and manures contribute to both short-term and long-term nutrient losses to the water environments. In a 3-year field study on the hydromorphic paddy soil, for example, Liu et al. [19] found that annual total phosphorus loss in surface runoff ranged from 0.63 kg/ha in the unfertilized rice-wheat rotation

hectare. In the same study, they found relatively smaller total phosphorus losses from the

–2.86 kg/ha when rice and wheat were fertilized with 50

–50 mg/L in the leachate from

–230 kg phosphorus per

–2 days after fertilizer applications. Due to the flooding nature of

[18] reported that nitrate-nitrogen concentrations could reach 30

vate risks of nutrient losses to the water environments.

some paddy soils within 1

Figure 5. Water budget in rice production.

112 Irrigation in Agroecosystems

to 0.96

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

113

hydromorphic

 paddy soil or 6 for the degleyed paddy soil; adapted from [19]).

degleyed paddy soil that ranged from 0.41 kg/ha in the control group to 0.70–1.49 kg/ha in the treatments with 50–230 kg phosphorus per hectare. Although the differences in magnitude of phosphorus losses in the two soils could result from different soil characteristics and rainfall patterns, both revealed increased phosphorus losses with greater phosphorus fertilizer application rates (Table 3). Furthermore, Liu et al. [19] found that the time interval between fertilizer application and the subsequent first large runoff event played a critical role in determining the annual phosphorus losses. Phosphorus losses greatly increased with decreasing time interval. The finding was supported by Guo et al. [20] who claimed that about 40% of total phosphorus loss from a rice-wheat rotation occurred within 10 days after fertilizer application to paddies.

5. Monitoring of water quality in paddy systems

In China, the approach for monitoring water quality in paddy systems has become standardized over the past 2 decades [12, 19, 29]. In the field, research plots are separated with plastic films down to 0.9 m in the soil profile and with soil berms up to 0.2 m on the soil surface to prevent flow of surface and shallow subsurface water between the plots (Figure 6). Soil berm is a common practice to maintain field ponding water for rice production. During the ricegrowing season, irrigation water is applied to individual plots through polyvinyl chloride pipe inlets when needed. During the non-rice crop-growing season in a double-cropping system, irrigation is usually not applied. Excessive ponding water is drained through shallow open ditches. Outside each plot on the opposite side of the irrigation water inlet, a cement pond is constructed to collect runoff water from every plot. Two water outlets of polyvinyl chloride pipe are installed on the wall of the cement pond, at depths of approximately 10 cm above and 10 cm below the soil surface, for collecting runoff water during the rice-and wheat-growing seasons, respectively. The runoff water collected in the pond is measured for volume and sampled for analyses of nutrients and sediments. Usually, one sample is taken after every regular runoff event and multiple samples during a large runoff event (i.e., rain storms).

Even though the approach described earlier is widely used such as in a national program to estimate nutrient losses from paddy systems across China, there has been an increasing interest in seeking alternative, simplified monitoring approaches. One potential approach is to monitor nutrient concentrations in the field ponding water. Liu et al. [19] found that concentrations of both total phosphorus and dissolved reactive phosphorus in surface runoff

0.88, p < 0.0001). In a follow-up study, Hua et al. [27] monitored different forms of phosphorus concentrations in field ponding water of five paddy soils over 2 years. They found that 2 weeks after fertilizer application is a critical period for phosphorus loss from paddies, which supported findings of others [19, 20]. Despite the large potential of monitoring field ponding water to save a lot of work associated with constructing runoff collection facilities, it should be

Figure 6. Monitoring of water quality in paddy systems: Research plots and field ponding water on the left and runoff

collection facility on the right.

<sup>2</sup> = 0.83–

Water Quality in Irrigated Paddy Systems http://dx.doi.org/10.5772/intechopen.77339 115

were significantly correlated with their concentrations in the field ponding water (r

In addition to "incidental" nutrient losses, overuse of fertilizers or manure also constitutes long-term risks of nutrient losses. A number of studies have demonstrated evidential buildup of soil phosphorus status and elevated degree of phosphorus saturation due to long-term phosphorus applications at rates exceeding crop needs [21–23]. In turn, phosphorus losses in surface runoff and leaching have been found to increase with elevated soil phosphorus status or degree of phosphorus saturation [22, 24]. This has been widely referred as legacy phosphorus issues [25]. Even though most of the research on this topic has been conducted on dryland soils, a few studies have reported that long-term excessive application of nutrients could enhance environmental pollution risk in paddy fields [4, 26]. In double cropping systems where flooded rice is planted in rotation with drained dryland crop, we can expect nutrient surplus from both rice and dryland crop-growing seasons [27]. It should be noted that China has the most intensive nutrient use in paddy systems among the world's top 10 rice-producing countries (Table 4). Therefore, there is a special need of better water and nutrient management to minimize nutrient losses from paddy systems.


#: Estimation of rice yield in China is made by the authors of this chapter.

Table 4. Rice yield and nutrient applications to rice in world's major rice-producing countries (data adapted from [28]).
