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Environmental impact of water-saving irrigation for rice

Mao Zhi, Department of Irrigation and Drainage Engineering, Wuhan University' of Hydraulic and Electric Engineering, Wuhan, China

SUMMARY

In the southern part of China over 70% of the cultivated area is under rice. A tremendous amount of water is used for the rice fields under the traditional irrigation method, termed 'shallow flooding irrigation'. Since the 1980s, in some provinces in south China, a new irrigation technique for rice, termed 'water-saving irrigation', has been advanced. In recent years this new technique has become widespread. The basic feature of this new irrigation technique is that there is no water layer above the soil surface in rice fields during the growing season of rice after the stage of recovering. This technique not only saves water and increases the rice yields, it also: reduces soil and water pollution; improves soil aeration; improves the field's microclimatic condition; reduces rice diseases and insect pests; and improves the regional water balance. This paper discusses the details of the water-saving irrigation technique for rice, and its potential environmental impact in south China.

Rice is one of the most important food crops in China. It is planted in almost all provinces, but about 60% of all rice is grown in the southern part of China. The traditional 'shallow flooding' irrigation method for rice uses a large amount of water. Since the 1980s the lack of water resources has become an important problem and the water-saving irrigation method has been tested and applied in southern China. The basic feature of this new method is that the soil surface is not submerged after the stage of recovering. Based on the analysis of observed data from experimental stations and from some typical areas in southern China, quantitative results of the environmental impact observed from spreading the water-saving irrigation method for rice will be discussed.

MATERIAL AND METHODS

There are many traditional irrigation methods for rice in southern China, but the major type is 'shallow flooding'. The typical water regimes for this method are shown in Table 1 (Mao Zhi, 1993). The basic feature of this traditional irrigation method is that a shallow water layer is kept on the soil surface of rice fields thoughout 70-80% of the whole growing season.

The typical water regimes on rice fields under water-saving irrigation in south China are shown in Table 2 (Mao Zhi, 1993). In some regions, when the air temperature is lower than 18°C during the early stage of tillering, a water layer of 20-40 mm may be kept under water-saving irrigation. The basic feature of water-saving irrigation is that there is no water layer on the fields throughout 75%-85% of the rice growing season. Since 1990, this new irrigation technique has been spreading widely in southern China. For example, it has been adopted in about 100 000 hectares in the Guangxi Autonomous Region and Hunan Province.

RESULTS AND DISCUSSIONS

Based on the analyses of the results of experiment and practice, the major environmental impacts are presented below.

Reducing soil and water pollution

Based on experimental data from lysimeters at the Guilin Irrigation Experiment Station in the Guangxi Autonomous Region, the mean values of evapotranspiration, percolation and water requirement of double cropping rice (early rice and late rice) under flooding and water-saving irrigation are shown in Table 3 (Mao Zhi, 1993). Compared with the percolation under flooding irrigation, that under water-saving irrigation drops by 345.6 mm (67.1%).

Data from seven irrigation experimental stations in four provinces in southern China, indicates that 50-80% of the percolation and seepage water from rice fields can be reduced by using the water-saving irrigation method (Mao Zhi, 1993).

Fertilizer is lost with the percolation and seepage water. Based on lysimetric data at the Guilin Station (Chen Weining, 1993), fertilizer loss from late rice fields with percolation water under flooding irrigation is shown in Table 4. The values in Table 4 show that a great quantity of fertilizer, especially the nitrogenous fertilizer, is lost with the percolation water under flooding irrigation. Because the percolation and seepage water is reduced by 50-80% under water-saving irrigation, fertilizer loss, especially of nitrogenous fertilizer, can be reduced by using this new method.

TABLE 1 - General standards of water regimes on rice fields under shallow flooding irrigation in southern China

Growing stages

Depth of water layer on rice field (mm)

Revival of green

10-30

Early and middle stages of tillering

20-40

Late stage of tillering

Drain, dry the field for 5-7 days

Elongating, booting, heading and flowering

20-60

Milk ripening

10-40

Yellow ripe

Drain, dry the field

TABLE 2 - General standards of water regimes on rice fields under water-saving irrigation in south China

Growing stages

Soil moisture (%) of saturation content) or depth of water layer (mm)

Revival of green

10-30 (depth of water layer)

Early and middle stages of tillering

80-100% (soil moisture)

Late stage of tillering

60-100% (soil moisture)

Elongating, booting, heading and flowering

80-100% (soil moisture)

Milk ripening

70-100% (soil moisture)

Yellow ripe

60%-100% (soil moisture)

TABLE 3 - Mean values of evapotranspiration, percolation and water requirement in whole growing season of double cropping rice (mm) at Guilin 1990-1992

Irrigation technique

Evapotranspiration

Percolation

Water requirement

Flooding irrigation

765.4

514.9

1280.3

Water-saving irrigation

688.8

169.3

858.1

Improving soil aeration

Based on the observed data from seven irrigation experiment stations in the Hunan, Jiangsu, Guangdong and Hubei provinces in southern China, the groundwater table in rice fields rises up to the soil surface and keeps this level during the period of submersion under the flooding irrigation, and it can be lowered to 0.3-0.8 m below the soil surface during the period of no submersion under water-saving irrigation. The soil redox potential in rice fields under water-saving irrigation is 120-200% of that under flooding irrigation (Mao Zhi, 1993). This means that under water-saving irrigation the soil oxygen content is increased greatly, the quantity of edaphon can be increased, and poisonous compounds in the soil can be reduced.

TABLE 4 - Processes of fertilizer loss with percolation from late rice fields under flooding irrigation at Guilin 1992

Kinds of nutrient

Rate of percolation (mm/day)

Quantity of fertilizer loss with percolation water (mg/m2)

August

September

October

10

14

18

21

5

9

14

18

21

24

1

7

11

NH4+ (Nitrogenous nutrient)

3

9.6

10.5

9.1

11.9

8.7

8.6

9.3

8.7

10.4

9.4

10.4

9.2

9.4

6

17.9

18.4

16.9

18.2

14.8

13.1

13.1

15.8

12.6

15.0

14.6

15.0

10.7

PO43- (Phosphate nutrient)

3

1.1

1.1

0.9

1.3

0.9

1.2

0.2

0.7

0.5

1.0

1.0

1.2

0.9

6

4.9

3.2

3.9

3.2

1.6

2.1

0.9

1.2

1.3

1.6

1.3

2.2

0.8

TABLE 5 - Quantity of edaphons in early rice fields (unit: million/g of dry soil) at Changsha 1992

Irrigation technique

Water-saving irrigation

Flooding irrigation

Date of sampling

13 June

16 July

13 June

16 July

Aerobic bacteria

0.277

1.624

0.657

1.025

Anaerobic bacteria

0.021

0.015

0.035

0.017

Actinomycetes

0.422

0.288

0.161

0.094

Ammonificator

11.550

112.000

0.355

4.200

Nitrifying bacteria

0.100

0.011

0.009

0.010

Denitrifying bacteria

0.390

0.400

0.085

0.042

Organophosphorus bacteria

4.025

0.960

0.710

0.420

Cellulose-decomposing bacteria

0.040

0.040

0

0.004

Sulphoficator

0.116

0.400

0.004

0.001

Desulphoficator

0.413

0.008

3.570

1.176

According to the observed data from the Changsha Irrigation Experiment Station in Hunan Province (Table 5), under water-saving irrigation the quantity of ammonificator is 26 times greater than under flooding irrigation, while the quantity of organophosphorus bacteria is 6 times greater and that of cellulose-decomposing bacteria 10 times greater under water-saving irrigation than under the flooding irrigation (He Shunzhi, 1993). The other important edaphons in rice fields under water-saving irrigation are also more abundant than under flooding irrigation.

The condition of lower groundwater table, higher redox potential (higher soil oxygen content) and the greater quantity of edaphons is very favourable for the transformation and assimilation of the organic fertilizer, and it is also very favourable for reducing the poisonous compounds in the soil. Therefore, the character of soil in rice fields can be improved and the yield of rice can be increased by spreading the water-saving irrigation method.

Improving microclimatic condition of fields

Based on statistics of the experimental results from seven irrigation experimental stations in four provinces in southern China, the microclimatic impacts of using the water-saving irrigation method for rice are that the difference in air temperature at the row space of rice between day and night is increased by 1-5 °C, and the air relative humidity at the row space of rice is reduced by 1-5 percentiles (Mao Zhi, 1993). Data from the Yulin Irrigation Experiment Station in the Guangxi Autonomous Region (Table 6) indicates that the condition of agricultural microclimate in rice fields under the water-saving irrigation is favourable not only for rice growing but also for reducing diseases, insect pests, and the lodging of rice (Yan Jincui et al. 1993).

TABLE 6 - Air temperature and relative humidity on rice fields at 2/3 of the height of rice at Yulin 1992

Date of observation

6 June

9 June

Weather types

Overcast

Clear

Hour of observation

8

14

20

Average

8

14

20

Average

Temperature

Water-saving irrigation

26.0

28.8

26.2

27.9

24.7

28.0

25.1

25.9

Flooding irrigation

26.1

27.2

26.1

26.5

24.0

26.8

25.0

25.3

Relative humidity

Water-saving irrigation

96

93

96

95

92

85

85

87

Flooding irrigation

96

96

96

96

96

89

84

90

TABLE 7 - Condition of the disease and insect pests for rice at Yulin 1990

Variety of rice

Early rice

Late rice

Date of investigation

1 June

28 June

15 September

Name of disease and insect pests

Sheath and culm blight of rice

Rice planthopper

Rice leaf raller borer

Term

Total number

Number by disease

%

Total number

Number by insect pest

%

Total number

Number by insect pest

%

Water-saving irrigation

96

20

21

42

8

19

602

64

10.6

Flooding irrigation

206

93

45

82

53

65

595

482

81

Remarks

Total number - Total number of investigated clumps of rice
Number by disease - Number of clumps of rice by disease
Number by insect pest - Number of clumps of rice by insect pest

Reducing rice diseases and insect pests

The above-mentioned results of microclimatic impacts (the increase in air temperature difference between day and night and the decrease in air humidity) have shown that the fields' microclimatic condition is favourable for reducing rice diseases and insect pests. Based on the observed data from the Yulin Agrometeorological Station, a comparison of the diseases and insect pests on rice for the two irrigation methods is shown in Table 7 (Zhang Yefang, 1993). Values in Table 7 show that diseases and insect pests can be cut significantly under the water-saving irrigation. Thus, the quantity of pesticide used and the pesticide pollution in water, soil and rice can be reduced.

Improving the regional water balance and advancing the development of the regional economy

The above-mentioned results have shown that the water consumption and the irrigation water requirement of rice can be reduced by more than one-third under water-saving irrigation, and hence the energy requirement for irrigation can also be correspondingly reduced. In many rural areas in China, the local industries cannot be developed for lack of water and/or energy. The reduction in water and energy consumption under water-saving irrigation for rice helps solve the problem of regional water shortages and develop local industries and the regional economy. For example, according to statistical figures from the Yulin Prefecture in the Guangxi Autonomous Region, where water-saving irrigation for rice has been adopted in about 30 000 ha in the last three years, about 100 million cubic metres per year of irrigation water for rice has been saved. The farmers' average annual income has increased by 8-20% both from agricultural activities (increase in rice yield) and from the local industrial activities. As another example, in the Jiankou Irrigation District, with an irrigated area of 2100 ha in the Yulin Prefecture, before 1989 the land was cultivated (with rice) in only seven months (April to October) and lay fallow in the other five months due to lack of irrigation water. Since 1990 water-saving irrigation for rice has been adopted and the average gross irrigation quota for rice has been reduced by 180 mm. As a result, half of the agricultural land has been planted with vegetables in the winter and the farmers' average annual income has increased by about 27%.

CONCLUSION

In southern China, the use of the water-saving irrigation method for rice is increasing rapidly year after year. A positive environmental impact is obtained by adopting this new irrigation technique. Therefore, the water-saving irrigation method for rice should be more widely used because of its potential for saving water, increasing rice yield, and improving the water and soil environment conditions in rice irrigation districts.

REFERENCES

Cheng Weining. 1993. Study of the Impacts on Soil Character by Percolation from Rice Fields. Thesis for Master's Degree, Wuhan University of Hydraulic Engineering, China.

He Shunzhi. 1993. Impacts on ecological environment by Irrigation. Irrigation and Drainage and Small Hydro-Power Station, No. 10, Wuhan, China.

Mao Zhi. 1993. Principle and Technique of Water-saving Irrigation for Rice. Wuhan University of Hydraulic and Electric Engineering, China.

Yan Jincu, Tong Jianli and Jiang Zeai. 1993. The Water-saving Irrigation Technique for Rice in Yulin Prefecture. Proceedings of the Provincial Workshop on Spread of Water-Saving Irrigation Technique for Rice held at Yulin City, Guangxi, 1993, China.

Zhang Yefang. 1993. The Investigation on the Relation of the Spread of Water-Saving Irrigation Technique for Rice to the Diseases and Insect Pests. Proceedings of the Provincial Workshop on Spread of Water-Saving Irrigation Technique for Rice held at Yulin City, 1993, China.


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