On Purple Soil Slopeland




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Rainfall Runoff and Sediment Reduction Effect of Hedgerow Plants

on Purple Soil Slopeland
Cai Qiangguo

Institute of Geography, Chinese Academy of Sciences

Beijing, China

E-mail: caiqg@mx.cei.gov.cn


K. F. Andrew Lo

Department of Natural Resources, Chinese Culture University, Taipei, Taiwan


Abstract
Rainfall runoff and sediment reduction effect by hedgerows plants on purple soil slopeland were studied based on natural precipitation observations and simulated rainfall experiments. Measurement data revealed that hedge plants grown on 25 degrees slopeland can reduce runoff by 22-43 percent and sediment yield by 94-98 percent. These substantial decreases can be achieved at a cost of only 10-20 percent of the stone ridge horizontal terraces. Hence, it is an effective way to control runoff and soil loss in ameliorating slopeland utilisation in the Three Gorges Reservoir areas of Yangtze River, China.
I. Experimental plots and simulated rainfall experiment
Purple soils are mainly distributed in the upper reaches of the Yangtze River in China and are also seen in Provinces and regions south of the Yangtze River. Purple soils are the most fertile soil in the Sichuan Basin, exceeding 70 million mu (15 mu equals to 1 ha) in area. They are mainly used for rice, wheat, edible oil and sugarcane production (Division of Pedology, 1991). At present, erosion-prone areas account for 60 percent of the Three Gorges slopeland and exceeds 3 million mu or 22 percent of the cultivated area. It is one of the most important sources of soil degradation on cultivated land.
Zigui County is situated in the Xiling Gorge of the Yangtze River in the southwest mountainous region of Hubei Province. It is the head area of the Three Gorges with a 90-km long river course flowing through it. It covers an area of 2,427km2 with a population 422,000, of which agricultural population totalled 385,000 and a population density of 169 persons per km2. The available cultivated land area per farmer is 1.34 mu, and per capita farmland is 0.6 mu. Purple soil is the leading cultivated soil in the county, which is concentrated in the Zigui Basin and purple sandy shale zones of the adjacent areas (Editorial Committee, 1991).

The experimental plots are located in Hongshiyan Branch of the nursery for variety improvement of oranges, Bureau of Farm and Local Products of Zigui County, Hubei Province. Elevation is about 200 m. Slope aspect is southwest facing. Nine experimental plots were set up at the nursery. Four of them were used for studying the soil and water loss process of purple soils developed on residues of purple sandy shales at different slope gradients (20, 25, 30, 35 degrees). The parent rock is Jurassic Penglaizhen Formation. Soil thickness is about 50-70 cm. The other four plots of 25 degrees were used for observations of water and soil loss with different hedgerow plants, including Hierochloe odorata, Coriariaceae, Vitex negundo, and Neoleucaena glauca. The 9th plot was a check plot with normal farming practices. Five more plots were set up at Wangjiaqiao Soil and Water Conservation Experimental Station belonging to the Office of Soil and Water Conservation of Zigui at the mouth of Wangjiaqiao Basin gullies slope. These plots were used mainly for comparative studies between terraces and slopeland. During the last two years, in addition to year-round natural rainfall observations, simulated rainfall experiments were also carried out twice a year to observe rainfall-induced runoff and sediment yield processes to compensate for inadequate natural rainfall observations in the 14 experimental plots. Nearly 60 simulated rainfall experiments were completed in the 14 plots with down-sprayed rainfall simulator imported from Canada, and SPRACO cone-shaped sprayer made in USA The momentum obtained from the simulated experiments was roughly equivalent to 90 percent of the iso-intense natural rainfall (Cai and Lu, 1992).


This study conducts quantitative analyses on the data from 16 relevant simulated rainfall experiments for studying impacts of different land use types on soil and water loss processes on 25-degree purple soil slopeland. Crop characteristics, antecedent topsoil and simulated rainfall conditions of the 16 experiments are listed in Table 1.
Reduction of water and sediment by hedgerow plants
One year after hedging Hierochloe odorata, Coriariaceae, Vitex negundo and Neoleucaena glauca (mixed with day lily) on 25 degrees slopeland, apparent reduction effects of water and soil loss have been observed in contrast to slopeland with normal farming activities. Accumulated runoff and sediment yield under different precipitation duration was measured on five experimental plots by means of simulated rainfall under heavy rainstorm conditions. Linear regression analyses of accumulated runoff process and sediment yield process were conducted based on fitting actual experimental data. Results obtained in Table 2 are similar to many previous research studies (Cai et al., 1990; Moly, 1981). The accumulated runoff and sediment correlate well with rainfall duration. The determinate coefficient reaches 0.86-0.98.
Table 2. Line fittings of accumulated runoff and sediment yield process

from hedged slopeland and slope farmland.




Plot

Hedgerows

CR process

Determinate

SD process

Determinate

No. of

No.







Coefficient




Coefficient

Samples

94H-5

Hierochloe odorata

CR=211+13.7T

0.96

SD=37.7+3.2T

0.98

14

94H-6

Coriariaceae

CR=386+23.8T

0.93

SD=157.5+13.3T

0.99

14

94H-7

Vitex negundo

CR=310.8+15.9T

0.88

SD=61.7+4.7T

0.92

17

94H-8

Neoleucaena glauca

CR=284.5+18.4T

0.95

SD=185.3+106.4T

0.86

15

94H-9

Slope farmland

CR=261.5+23.9T

0.98

SD=25476+1886T

0.93

17

Note: CR: Accumulated runoff (l); SD: Accumulated sediment yield (g);

T: Duration of precipitation (min)


The slope of the fitted line obtained from Table 2 actually represents the rate of increase of accumulated runoff per unit time. Compared to the check plot (94H-9), different water reduction efficiency was gained in the hedged-in plots. Of which Hierochloe is the best followed by Vitex negundo. Runoff reduction per unit time is about 22 and 43 percent, respectively. The accumulated runoff process slope for the Coriariaceae hedged-in plot is similar to the check plot. The main reason is the survival rate of Coriariaceae planted in 94H-6 plot is very low. Hence, no perfect hedgerow belt was formed.
The normal plough and an artificial earth-up ridge of up a 10-cm-high have been formed after one-year hedgerow planting. This can stop sediment penetration, retain most sediment inside the hedgerows, and gain apparent sediment reduction efficiency. Table 2 indicates that slope of fitted lines for accumulated sediment yield in 94H-9 plot is 18 to 570 times higher than that of the hedged plots. This also shows that per unit time sediment yield reduction is about 94 to 98 percent. Hedgerows planted with Hierochloe and Vitex negundo are the best in hedging off

Table 1. Crop characteristics, antecedent topsoil and simulated rainfall conditions of the 16 experiments.



















Soil conditions at initial stage of

Simulated rainfall

Plot No. Area

Land use

Crops

Types of hedgerow

Coverage

Rainfall







(m2)







Plants

(%)

Water

Resistance to

Intensity

Duration
















Content(%)

Shear strength(kpa)

(mm/min)

(min)

F25S 2*5

Waste slopeland

Grass




30

9.55

5.6

0.65

30

F25B 2*5

Waste slopeland

Grass




30

28.4

4.24

1.29

30

H25S 2*5

Slopeland

Newly-Ploughed

0

21.68

2

0.54

30

H25B 2*10

Slopeland

Newly-Ploughed

0

27.7

2

1.33

30

94H-5 2*10

Slopeland

Ormosia

Hierochloe odorata

45

19.7




1.39

51.5

94H-6 2*10

Slopeland

Ormosia

Coriariaceae

20

10.14




1.54

50

94H-7 2*10

Slopeland

Ormosia

Vitex negundo

70

8.1




1.66

58

94H-8 2*10

Slopeland

Ormosia

Neoleucaena glauca

35

8.21




1.46

56

94H-9 2*10

Slopeland

Ormosia

None

20

9.55




1.46

53

95H-5 2*10

Slopeland

Wheat

Hierochloe odorata

90

17.82

4.34

1.19

50

95H-6 2*10

Slopeland

Wheat

Coriariaceae

90

5.47

9.02

1.29

68

95H-7 2*10

Slopeland

Wheat

Vitex negundo

90

19.8

6.97

1.12

63

95H-71 2*10

Slopeland

Wheat

Vitex negundo

90

19.85

6.6

0.76

94

D-1 5*10

Terrace

Orange




90

21




0.62

51

D-2 5*10

Terrace

Wheat




60

17




0.91

22.5

D-4 5*10

Slopeland

Wheat




60

23




0.58

63

sediment. Experimental results also indicate that it is possible to allow most water flowing through but sediment being trapped in the hedgerows. Sediment hedge-off effect is most apparent in heavy rainstorms.


Soil erosion is serious during heavy rainstorms on purple soil slopeland. Sediment yield module of an individual rainstorm reaches as high as 3600t/km2, close to the annual average erosion module of local purple soil slopeland. The main reason for this is that rill erosion often occurs on slope farmland under heavy rainstorms. Subsequently, severe rill erosion and headward rill erosion were formed within 15 minutes after the simulation experiment. This rapid development of rill system causes large amount of muddy flowing rill water. Rill erosion can only be developed with slope length of 15-20 m and on slope gradient of 16 degrees on Loess Plateau (Cai et al., 1995; Cai and Luk, 1995). The above experiments also indicate that coarse-grain purple soils with slope steepness exceeding 25 degrees and high-intensity rainfall are more susceptible to rill erosion than loess. During the experiment, rill erosion also occurred in isolated areas of the hedgerows plots planted with Coriariaceae and Neoleucaena glauca that were not well established. This is also the reason why sediment yield in plots hedged by these two plants is much higher than the Hierochloe odorata and Vitex negundo plots. Results imply that planting hedges on slopeland and shortening slope length are essential in mitigating soil erosion in the Three Gorges Reservoir area, especially on steep slopeland (Lal, 1988).
About 20-cm high hedgerows were formed by plough and earth-up stubble of various hedge plants after harvesting ormosia in 1994. Results of accumulated runoff and sediment yield processes obtained from heavy rainstorm simulation performed in April 1995 are listed in Table 3. Compared with experimental results obtained in fall of the previous year (Table 2), slopes of linear regression lines were further reduced indicating an increase in water and soil reduction efficiency of the hedgerows.
Table 3. Fitting of processes of accumulated runoff and sediment yield

on hedged slopeland in spring of 1995.




Plot

Hedgerows

CR process

Determinate

SD process

Determinate

No. of

No.







Coefficient




Coefficient

samples

95H-5

Hierochloe odorata

CR=47.4+3.8T

0.90

SD=33.8+2.6T

0.90

16

95H-6

Coriariaceae

CR=262.8+9.7T

0.84

SD=273.4+11.2T

0.87

18

95H-7

Vitex negundo

CR=289.9+19T

0.99

SD=83.7+6.3T

0.99

18

III. Comparison between Hedgerow Plants on Slopeland and the Terraces


Research results from both within and outside China indicate that infrequent heavy rainstorms are the major cause of serious soil and water loss. In Missouri USA, a single rainfall event made up 50 percent of the total five years' soil erosion amount. Marzue experimental station of Zimbabwe also showed that soil erosion amount from one or two heavy rainstorm events might exceed half of the annual amount (Hudson, 1981). Runoff depth and sediment yield may differ greatly with different slope length for a heavy rainstorm with a return period of ten years. In Table 4, the slope length of 94H-9 and F25B is 10 and 5 m, respectively. Comparisons of experimental result shows that the slopes of the accumulated runoff depth fitted lines differ over 1 time. The slopes of the sediment yield modules fitted lines differ over 120 times, and sediment yield per unit runoff depth differs 56 times. Soil erosion and sediment yield is, therefore, very serious when farming is practised on steep slopeland with slope length exceeding 10 m. It is particularly so as farming is usually practised along the slopes in the Three Gorges Reservoir area. Rainfall characterised by concentrated precipitation and heavy rainstorms are the reasons behind the serious soil and water loss in the Three Gorges of the Yangtze River. This experimental result may also provide essential basis for designing hedgerow intervals on steep slopeland, for checking the effectiveness and efficiency of stone ridge terraces, hedgerows and various other adopted prevention and control measures. It is of practical importance to include soil and water loss process under intense rainstorms.
The fitted accumulated sediment yield module process line from hedgerow planted less than two years and the stone-ridge terraces is compared in Figure 1. The figure shows that although the accumulated sediment yield modules from hedged steep slopes are several times higher than that of the terraces, their actual values are not great. Even with one year growth (Table 1) of Hierochloe odorata (plot 94H-5) and Vitex negundo (plot 94H-7), the actual accumulated sediment yield module is merely around 16t/km2 with a rainstorm intensity of over 1.39 mm/min. This value is quite tolerable in this area. With two year growth of Vitex negundo (plot 95H-7), its accumulated sediment yield module process line is quite close to that of the horizontal stone ridge terraces D-1 and D-2 under similar rainfall conditions. At the initial simulation stage, the sediment yield process in the hedged plots and the stone ridge terrace plots is quite similar. The accumulated sediment yield module is only 6 t/km2.
Figure 1. Process lines of accumulated sediment yield modules

of hedged slopeland and stone ridge terrace.



The accumulated sediment yield module usually exhibits good linear relationships with accumulated runoff depth. Calculations of the above-mentioned five simulated rainfall experimental results are listed in Table 5. The intercept values of the linear relationship are higher due to impact of steep slopeland. In other words, sediment yield module is normally greater at the initial runoff generation stage on hedged slopeland. But, their linear slopes do not differ much indicating similarity in sediment yield modules per unit runoff depth. These results also show that the sediment reduction effect of terraces and the hedgerow plants on slopeland is the similar. This is why the hedgerows are more superior. The actual input of the hedgerows is only 100 to 200 Yuan (US $1 equal to 8.3 Yuan RMB) per mu in contrast to over 1000 Yuan per mu for stone ridge terraces at present in the Three Gorges Reservoir. In the case of slopeland greater than 25 degrees, the cost for stone ridge terraces would be even higher.
Table 4. Fitting of accumulated runoff depth process and sediment yield module process of slope farmland.


Plot

Land use

Accumulated

Determinate

Accumulated erosion-

Determinate

Relation of erosion-induced

Determinate

No. of

No.




Runoff process

Coefficient

Induced sand process

Coefficient

Sand and runoff depth

coefficient

samples

F25S




Rd=3.04+0.26T

0.96

Sm=4.53+0.4T

0.94

Sm=0.052+1.52Hh

0.99

18

D-4

Slopeland

Rd=6.78+0.22T

0.96

Sm=35.02+1.49T

0.94

Sm=10.8+6.66Rh

0.97

36

94H-9

Slopeland

Rd=13.07+1.19T

0.98

Sm=1273+94.32T

0.93

Sm=271+80.13Rh

0.98

17

Note: Rd-Accumulated runoff depth (mm); Sm-Accumulated sediment yield modules (T/km2); T-Duration of precipitation (min)

Table 5. Relationships between accumulated sediment yield modules and runoff depth of hedged slopeland and terraces.




Plot

Land use

CD process

Determinate

SD process

Determinate

SD and runoff depth

Determinate

No. of

No.







Coefficient




Coefficient




coefficient

samples

94H-5

Hierochloe odorata

Rd=10.56+0.69T

0.96

Sm=1.74+0.16T

0.98

Sm=15.66+0.23Rh

0.99

14

94H-7

Vitex negundo

Rd=1554+0.79T

0.88

Sm=3.09+0.24T

0.92

Sm=1.64+0.29Rh

0.98

17

D-1

Orange terrace

Rd=1.57+0.096T

0.97

Sm=0.16+0.013T

0.99

Sm=0.056+0.13Rh

0.98

30

D-2

Terrace

Rd=0.95+0.087T

0.94

Sm=0.33+0.042T

0.84

Sm=0.08+0.49Rh

0.97

26

94H-7

Vitex negundo

Rd=13.07+1.19T

0.99

Sm=4.19+0.31T

0.99

Sm=0.63+0.33Rh

0.99

18

Note: Meanings of symbols are the same as in Table 4.

IV. Conclusion


In heavy rainstorms, the hedge plants can lead to runoff reduction of 22 to 43 percent and sediment yield reduction of 94 to 98 percent on 25-degree slopeland. Hedgerow plant is effective in ameliorating slopeland in the Three Gorges Reservoir of Yangtze River since it can substantially reduce soil loss on slopeland with a cost of only 10 to 20 percent of the stone ridge terraces.
References

Cai, Q., S. H. Luk, et al.. 1992: Experimental plots and Modelling experiments for process of runoff, soil erosion and sediment yield. In: Soil Erosion Management and Application of GIS in the Loess Plateau, Western Shanxi. Beijing, Science Press. (In Chinese, English abstract).

Cai, Q., Wu, S. and Chen, H. 1990: Effect of soil surface crusting on rainfall runoff and erosion process on the ploughed slopeland. In: Sino-Canadian Symposium of Experimental Researches on the Law of Soil Erosion in the Loess Plateau Region. Beijing, Water Conservancy and Electric Power Publishing House. (In Chinese, English abstract).

Cai, Q. and Luk, S.H. 1995: Soil loss influenced by slope length in the hilly loess region, China. International Journal of Sediment Research 10(4)., P.43-50.

Cai, Q., Ma, S. and Wu, S. 1995: Impact of slope length of loess slopeland on runoff generation and sediment yield processes. In: Soil Erosion and slopeland Amelioration and Utilisation in Zhangjiakou City at Upper Reaches of Yongdinghe. Environmental Science Publishing House, Beijing. (In Chinese)

Editorial Committee of Local Chronicles of Zigui County, 1991: Local Chronicles of Zigui County, China Encyclopedia Publishing House. (In Chinese)

Division of Pedology, Chengdu Branch of the Academy of Sciences, 1991: Purple Soil in China (1). Beijing: Science Press. (In Chinese)

Hudson, N.W. 1981: Soil Conservation. Cornell Univ. Press, London, Batsford.

Lal, R. 1988: Soil Erosion research methods. The Soil and Water Conservation Society, Ankeny, Iowa.

Moly, M. 1981: Erosion and environment (Environmental Sciences and Applications Volume). Oxford: Perfaman Press.







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