Spatial distribution characteristics of intercepted rainwater on rock desertified shrub slopes in th

Scientific Journal of Earth Science June 2013, Volume 3, Issue 2, PP.71-80

Spatial Distribution Characteristics of Intercepted Rainwater on Rock Desertified Shrub Slopes in the Northern Guangdong Region* Xinghu Wei 1, Xizhen Xu 2, Li Lei 3 1. Research Center of territorial Resource Environment and Tourism, Foshan Scientific and Technologic College, Foshan, 528000, PRC 2. The second middle school of Zhongshan City, Zhongshan 528429, PRC 3. School of Geographic Science, South China Normal University, Guangzhou 510630, PRC Email: 1#weixinghu1964@163.com; 2 xuxizhen@163.com; 3 leili0106@163.com

Abstract In this study, typical cone karst slopes in Jiulong Township of Yingde City, Guangdong Province were selected as the study area.

The plant species and residue amounts in the runoff experiment plots with different rock desertification degrees were surveyed.

The weight of plant, litter, limestone rocks, and bulk were investigated before and after rains. The results show that as the stony desertification aggravated, the exposure percentage of rocks increased, but the soil cover and thickness and vegetation cover decreased. The interception amount of the surface plant and residue show a trend of parabola, the maximal interception amount of the surface plant in slight stony desertification plot was 920g/m2,but significantly decreased in the very severe cases, was 160g/m2.The maximal interception amount of the residue in moderate stony desertification plot was 906.70g/m2,minimal in the very severe cases, was 306.7g/m2.Water storage capacity of soils gradually increased with the aggravation of rock desertification degree but tended to decrease in the very severe case. The maximal water storage capacity of soils in severely stony desertification plot was 170kg/m3, minimal in the potential cases, was 90kg/m3; The natural rainfall storage amount on the limestone rock averaged 32.77 kg/m3. The total amount of rainwater intercepted and storage on the surface basically showed an increase trend with the aggravation of the stony desertification; but the proportions of the interception amounts of plants, residue, soils and rocks in the total interception volume differed greatly. The interception amount of plants and residue within the different rock desertified plot occupied 9~23% of total storage amount ,but in the very severely rock desertified slope,the interception amount of plants and residue only occupied 2.3% and 4.4% of total storage amount respectively, and water storage amount of rock exceeded 88%; The total interception and stoarg amount of plants, litters and soils in the potential rock desertified sample plot occupied 81% or more; According to the simulate rainfall (35mm/h), the effectively intercepted rainfall and water storage of plants, litters, soils and rocks on the potential, slightly, moderately, severely and very severely rock desertified slopes were 2.94 mm, 4.95mm, 6.21 mm, 7.73 mm and 6.9 mm respectively, account for 8.4%,14.14%,17.74%,22.09%,19.71% of total rainfall amount.

Keywords: Rock Desertification; Rainwater; Interception; Spatial Distribution; Mountain Area in Northern Guangdong

1 INTRODUCTION Water cycle, carbon cycle and calcium cycle jointly determine the karst dynamic processes. Rainfall infiltration and surface runoff are the main pathway of the water cycle. Unlike non-karst areas, underground water plays an important role in ecosystem of the Karst region in South China. Adequate rainwater is the main dynamic force to mould the karst environment. Due to well developed cracks, karst caves, funnels, sink holes and wells in the karst environment the surface water seepage is serious, even if there is adequate rainfall, it mostly flows into the *

This paper is funded by the Natural Science Fund of Guangdong Province (S2012010009272).

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underground river. The karst landscapes in south China mainly occurs in Guizhou, Guangxi, Yunnan and Hunan provinces, the total groundwater resource volume in the four provinces occupies a quarter of China’s total groundwater resource volume, and the karst groundwater accounts for more than one half of the total groundwater volume. The vast karst region in south China is a water resource-rich region but also a serious water-deficient region.

The study on the karst water resources in China mostly focused on the groundwater resources. According to the connectivity of the karst water-bearing medium void space, void space size, water transfer capacity, and the function of the karst medium assemblage, Guo Chunqing classified the karst water-bearing medium into three types, namely water storage medium, water transfer-storage medium and water transfer medium. They mainly comprise by the karst poles, karst cracks and karst caves, and contruct a “pipe-reservoir” water storage system with the underground river togather. Lao and Jiang systematically studied the karst water system and groundwater resources in the Shiqi River basin. There are more research results about soil moisture in Karst area, such as, Du and Wang’s result showed: Soil water in karst shrub and karst secondary forest decreased with increasing soil depth, whereas soil water in thin shrub grass and non-karst shrub increased with increasing soil depth. Slope positions had little effect on soil water distribution along hillslope with the same vegetation type, which was caused by the particular complex topography and physiognomy of the karst area. Some researchers had done some soil moisture research in karst region. In Karst area, there are less studys on distribution of rainfall in vegetation-rock-soil system, especially in the different rock desertified land. But in other area, there are many researches about the effects of canopy, residue on distribution of rainfall, erosion, soil moisture, nutrition, etc. According to the Wang’ research result in loess plateau, the maximal interception amount of residue is 1.7~3.5 times its own weights. As for the rock desertified land, the surface runoff is the main power of soil loss, while the exposure percentage of rock, soil cover and thickness, vegetation cover, biomass and litter thickness can directly or indirectly affect the rainwater runoff processes and vertical infiltration processes. In each rainfall process, surface plants and litter and soil can intercept a certain amount of rainwater, then surface runoff and vertical infiltration occur. The rock exposure percentage, vegetation cover and soil cover of different rock desertification degree are different, thereby lead to the difference in runoff processes and vertical infiltration processes and further affect the rainwater transformation at the surface and subsurface space.

Understanding such a process and the spatial distribution characteristics of surface water has an important significance to the systematical analysis of water dynamic processes, water resources and their eco-environmental effects. Rock desertification degree is more essential for understand Karst environment change, the mountainous region in the northern Guangdong is a most concentratedly distributed area of the karst topography and also a most severely rock desertified region in the Guangdong province rainwater resource is more abundant in the region than in other southern karst region. The karst hilly shrub slopes in the region that we selected have better representativeness for understand the spatial distribution characteristics of surface rainwater.

2 MATERIALS AND METHOD 2.1 Description of study area The experiment plot is located in the Shijiao Village of Jiulong Township in Yingde City of Guangdong province (24°08.113′N, 112°51.855′E) at the elevation of 121 m, it is an isolated cone karst hill with a height of 70-80 m, a length of about 80 m in east-west direction and a width of 60 m. The geomorphologic type belongs to the Lianjiang karst plateau in the mountain and hill region in the northern Guangdong and Jiulong-Mingjin Basin subregion. Due to the effects of the main stem and tributaries of the Lianjing River, limestone and sandy shale are alternately distributed in the region, peaks and hills, “rocky mountains”(Karst landscape) which exhibit a discontinuous isolated distribution or partly continuous distribution and “soil mountains” (landscape based on Arenaceous shale )are also alternately distributed. The carbonate rocks in the region mainly consist of the Palaeozoic Carboniferous and Permian soluble rocks. The karst landscapes include exposed type, semiexposed type and concealed type. The carbonate rock type is dominated by limestone, followed by dolomic limestone and dolostone. The region belong to the south Asian tropical monsoon climate region, annual mean air temperature is 20.7℃, annual preipitation is 1879 mm and mainly falls in April~September, accounting for more than 70% of the annual rainfall. Surface and crevice soils are of black calcareous soil but the sub soils are of red calcareous soil. Vegetation is limestone shrubs, there are few trees in lowlands or local places with thick soil layer, such as Pistacia chinensis, Alangium chinense, Mallotus - 72 http://www.j-es.org/

tenuifolius and others are shrubs, especially many climbing shrubs and vines. Dominant species are Loopetalum chinense, Sageretia thea, Rose cymosa, and Bauhinia championii; where there is flat terrain and adequate sun light, there are many Vitex negundo, Alchornea trewiodes and Boehmeria longispicate; Alchornea trewiodes and Boehmeria longispicata also occur on the steep rock walls. There are some herbs in the shrub zone, such as Miscanthus floridulus, Imperata cylindrical, Dicranopteris dichotma and Bothriochloa intermedia; under the dense shrub canopy at the lower parts of the slopes there are Lophatherum gracile and Carex spp.

2.2 Experiment and method The experimental plots of different stony desertification degrees are arranged at the north piedmont of the karsts pinnacle, with a slope of 15-30°. Each of the experiment plot has an area of 3×4 m. According to Li Sen’s classification indicatiors of the rock desertification in northern Guangdong, the experiment field was classified into very severe stony desertification (rock exposure percent >90%), severe stony desertification (rock exposure percent 70-90%), moderate stony desertification class (rock exposure percent 50-70%) and slight stony desertification class (rock exposure percent 30-50%). In addition, a potential stony desertification ((rock exposure percent less than 30%) sample plot was selected as the control plot. Total 15 plots. The basic conditions of the experiment plots are presented in Table 1.

TABLE 1 THE BASIC CONDITIONS OF THE EXPERIMENT PLOTS UNDER DIFFERENT ROCK DESERTIFICATION DEGREE Rock desertification degree Potential rock desertification

Rock exposure percent (%) 8.3

Soil mean depth (cm) 11.8

Vegetation coverage (%) 96

Slope (degrees) 13°

Slight rock desertification

31.3

10.8

92

14°

Moderate rock desertification

54.8

6.3

74

31.5°

Severe rock desertification

88.8

3.11

52

17.5°

Very severe rock desertification

92.7

2.40

27

21°

The instrument used in the experiment is the BX-1 portable lateral-spray rain simulator made by the Water and Soil Conservation Institute, Chinese Academy of Science and the Ministry of Water Resources.

Soil depths were measured by the insert pin method, pins were inserted along the diagonal line and cross wire at equal space in the plots and each plot at least inserted 17 pins or more. The rock exposure level was calculated through direct measurement of the exposed rock area and with the surface soil as a plane the volume of the exposed rock was figured out.

The above-ground biomass was measured by cutting method and three sampling points were randomly selected in each plot using square net. 0.5 m×0.5 m quadrats were arranged, the above-plants were vertically cut off in sunny days, in the meanwhile the surface litter was collected, then they were placed in their original plots after their fresh weights were weighed to conduct simulating rainfall experiment (rainfall intensity 30-40 mm/h),three times repeat.

After the simulating rainfall, above-ground plant stalks and surface litter were weighed, respectively. The stones of various size were collected and weighed in fine days, then replaced in the plots, after 2 hours of simulation rainfall they were weighed and their volumes were measured. In addition, these stones were soaked in water for 8 hours till the water in it saturation, after that, they were oven-dried and weighed again to calculate the saturated water capacity of the stones per unit volume.

Soil saturated water measured by ring sampler. In different rock decertified sample plots different depths (0-10 cm) of soil samples were collected by ring sampler, each plot collected three samplers, filter paper was placed at the bottom of the ring sampler. Then it was put on the glass strip and then transversely placed in the porcelain pans, the pans were filled with water, the filter paper was folded downward to fully absorb water and stood for 6-8 hours till - 73 http://www.j-es.org/

reached water saturation and then weighed to obtain the saturated water capacity. After that, discharged the water in the pans, and stood for 8 hours until the gravity water was fully lost, and then weighed to obtain the field moisture capacity.

The experiment survey was conducted in June 2011, prior to the experiment the vegetation, including plant species, individual density, height, coverage and biomass under the conditions of different stony desertification degree were surveyed.

2.3 Analysis and calculation method The variance analysis and correlation analysis used SPSS 16.0. The calculation methods of main indexes are as follows: Maximum interception amount of precipitation by surface plants (litter) (g/m2)=[after rainwater saturated weight of surface plants (litter)-before rained weight of surface plants (litter)] Maximum interception amount of precipitation by surface plant [per unit weight (litter) (g/m2)]=maximum interception amount of surface plants (litter)/before rained weight of surface plants (litter) Simulation rainfall interception amount of limestone rock (kg/m3)=(rock weight after rain-rock weight before rain)/rock volume Natural soil saturated water capacity (g/cm3)=(saturated water-holding weight―natural soil weight)/cutting ring volume Maximum saturated water capacity (g/cm3)=(saturated water-holding weight―oven-dried soil weight)/cutting ring volume Maximum field water capacity (g/cm3)=(field water holding weight―oven-dried soil weight)/cutting ring volume

3 RESULTS AND DISCUSSION 3.1 Biological interception amount TABLE 2 THE INTERCEPTION OF PRECIPITATION BY SURFACE PLANT AND LITTER DURING A ARTIFICIAL RAINFALL EVENT ON THE SHRUB SLOPES OF DIFFERENT ROCK DESERTIFICATION DEGREE Rock

Mean weight of

Maximum

2

Maximum

desertification plant (g/m )

interception

interception

degree

amount of

amount per

Before Rain After rain

plant 2

(g/m )

Potential

1866.67a

Slight

2160a

Mean weight of surface 2

Residue(g/m )

unit weight

Before

After

plant(g/g)

rain

rain

interception

interception

amount of

amount per 2

residue(g/m ) Unit weight Residue(g/g)

666.67a

0.36a

1076a

1742.67a

666.67a

3080a

920a

0.43a

986.67ab

1666.67a

680a

2186.67a

3013.3a

826.67a

0.38a

1146.67a

Severe

2413.3a

3120a

706.67a

0.29a

1160a

0.15a

*

1040 a

Maximum

2533.3 a

Moderate

Very severe

Maximum

1200a

160b

**

Note: Different letters (a,b,c) represent the difference in significance

440b

*: 0.05 significance level

**

2053.33a 1840a 746.67b

0.61a 0.69a

906.67ab

*

680a *

306.67ac

0.79a 0.59a

*

0.69a

**: 0.01 significance level

Vegetation in the experiment plots are of limestone shrub, there are trees such as Pistacia chinensis and Alangium chinense in the potential stony desertification plot; Pistacia chinensis in slight and moderate stony desertification plots, and Clausena excavate in the severe stony desertification plot. In addition, there are other trees outside the quadrats, such as Bischofica javanica and Mallotus henryi. The survey area is dominated by shrub vegetation, especially many vines, such as Clematia henryi, Bauhinia championii, Rubus tephrodes, Cayratia japonica and Pueraria phaseoloides; there are also climbing plants such as Sageretia thea, Rose cymosa, Zanthoxylum planispinum and Loopetalum chinense; other shrub species include Vitex negundo, Boehmeria longispicata and Alchornea trewiodes; herbaceous plants include Dendranthema indicum, Ischaemum aristatum, Ophiopogon reverses, - 74 http://www.j-es.org/

Cyclosorus parasiticus and Selaginella tamariscina. The suface vegetation consists of three synusiums, i.e. tree synusium, shrub synusium and herb synusium. As viewed from the plant biomass (Table 2), no significant difference existed in the potential, slight, moderate and severe stony desertification plots (F=0.738), in contrast, it tended to increase, and only in the very severe stony desertification plot it significantly decreased. This is because there are trees present in various sample plots except the potential and very severe stony desertification plots. Furthermore, in localities where there are stones, especially larger stones, there are crevice soil or better soil-forming micro-environment created by intercepted slope runoff and soil, this is favorable to the development of trees and thereby occupy the space above the exposed stones. On the extremely severely rock desertified surface most soil is lost due to erosion, soil cover and thickness are too small to support the survival of trees. In the potential stony desertification plot soil layer is relatively uniform, soil cover is large, some pioneer plants including herbs and shrubs first occupy the favourable sites and spread outwards rapidly, especially some intermediate herbaceous and woody vines rapidly widen their space by means of other shrubs. As a result, the ground surface in the potential stony desertification area forms a certain thickness and density of mixed mulch which prevent trees from invasion. This is a main reason why the above-ground biomass in the potential stony desertification shrubs is smaller than that in the slight, moderate and severe stony desertification shrubs but the saturated water capacity is just opposite. The maximum intercept amount of the surface plants has a certain relation to the weight of the surface plants, and is also related to leaf blade size and smooth degree, stalks and leaves with awns or without awns and stem diameters of shrubs and trees. Apparently, there are numerous tree and shrub species, the surface interception amount of the slightly rock desertified shrubs is the largest because of their high and thick synusium, followed by the moderately and severely rock desertified shrubs, and the surface interception amount of the extremely severely rock desertified shrubs is significantly low (F=7.145). However, no significant difference exists in the saturated water weight of surface plant (F=1.393) and the unit above-ground plant interception amount (F=1.161) of different stony desertification degree.

The weight of the surface litter is related to the biomass and also depends on the plant types and the number of herbs and deciduous shrubs and trees. Except the surface litter in the slight stony desertification plot slightly decreased, no significant difference existed from the potential, moderate to severe stony desertification plots (F=2.833), but it significantly decreased in the very severe stony desertification plot. Clearly, the vegetation cover in the very severe stony desertification plot largely decreased, exposed stones resulted in the decrease of litter. The weight of surface litter after retained water and the surface litter weight showed a same trend (F=3.372), the interception amount of litter in the moderate stony desertification plot was the largest but in the very severe stony desertification plot it significantly decreased (F=1.977), however no significant difference existed in the interception amount per unit weight litter (F=0.311).

Correlation analysis (Table 3) showed that, there is an highly significant correlation between the surface plant weights before and after rain, the surface residue weights before and after rain, the surface residue weights after rain and the maximum interception amount of surface residue. But the difference between the surface plant weight before rain and the maximum interception amount of surface residue; the surface residue weight before rain and the maximum interception amount of surface plant is not significant, and the difference between other indexes is significant.

3.2 Interception amount of limestone stones Limestone texture is dense, with a density varied between 2.51-2.59 g/cm2. Compared to surface plants and litter, limestone has a small rainwater-absorbing capacity, and this is related to the rock surface roughness, rock crack, karst hole and karst mark but has no significant correlation with the rock size and weight (Table 4). From the weighing of oven-dried rock it was found that the weights of most tested rocks had little difference after being oven-dried and immersed in water, furthermore, almost no difference existed after 2h of rainwater immersion and 8h of water immersion. Therefore, from the rock weight after rain minus the rock weight before rain we obtain the natural interception amount of limestone rock. Considering the surface distribution characteristics of rocks, with per unit volume to measure the interception amount of limestone rock, after 2h of rainfall the mean interception amount of limestone rock reached 32.77 kg/m3.

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TABLE 3 PEASON CORRELATION ANALYSIS OF THE WEIGHT AND INTERCEPTION AMOUNT OF SURFACE PLANT AND LITTER BEFORE AND AFTER RAIN Surface

Maximum

residue

interception

weight

amount of

after rain

surface plant

(g/m2)

(g/m2)

0.948*

0.928*

0.85

0.958*

0.939*

0.935*

0.880*

0.958*

1

0.872

0.895*

0.880*

0.948*

0.939*

0.872

1

0.984**

0.910*

0.928*

0.935*

0.895*

0.984**

1

0.969**

0.85

0.880*

0.880*

0.910*

0.969**

1

Surface plant

weight before

weight after

rain

rain

(g/m2)

(g/m2)

1

0.988**

0.903*

0.988**

1

0.903*

Plant weight before rain（g/m2） Plant weight After rain（g/m2）

Maximum

Surface plant

interception amount of surface plant

Surface residue weight before rain (g/m2)

(g/m2)

Maximum interception amount of plant （g/m2） Surface residue weight 2

before rain (g/m ） Surface residue weight 2

after rain（g/m ） Maximum interception amount of surface

residue （g/m2） Note: *: 0.05 significance level

**: 0.01 significance level

TABLE 4 INTERCEPTION AMOUNT OF PRECIPITATION BY STONES Rock No.

Weight before Rain(kg)

Weight

after

Rain(kg)

Volume 3

(cm )

Simulation rainfall

Simulation rainfall

interception

interception

amount(g/kg)

amount(g/m3)

Density （g/cm3）

1

1.5

1.51

596.75

6.67

16.76

2.51

2

1.05

1.07

416.09

19.05

48.07

2.52

3

0.44

0.46

175.05

45.45

114.26

2.51

4

0.83

0.85

328.91

24.10

60.81

2.52

5

0.83

0.83

330.20

0.00

0.00

2.51

6

0.52

0.52

200.83

0.00

0.00

2.59

7

0.68

0.69

262.63

14.71

38.08

2.59

8

0.77

0.76

297.39

12.99

33.63

2.59

9

0.72

0.76

278.08

55.56

143.85

2.59

10

0.43

0.42

166.07

23.26

60.21

2.59

Mean

0.777

0.787

305.20

20.18

32.77

2.55

3.3 Soil interception amount The soil on the shrub slopes of different stony desertification degree exhibited a marked physical difference due to the difference of surface topographic feature, surface soil erosion and vegetation features. On the potential and slightly rock desertified slopes, soil layer was relatively thick and compact, soil cover and soil bulk density were - 76 http://www.j-es.org/

large although the surface occurred erosion. As for the moderately and severely rock desertified slopes, stone exposure percentage increased, rock and soil surface was not flat due to permanent erosion, rocks were convex and had cracks, in the microterrains such as lowlands residual soil cover and thickness were small, litter and rock weathering product were transported and deposited in residual soils by rainwater, hence soil organic matter content was high, soil structure was loose, soil bulk density was low, especially in the severe stony desertification case (F=11.766). On the extremely severely rock desertified slope, soil cover was small, soil gravel content was high although soil was loose, but soil bulk density significantly increased due to the effect of gravels. Owing to different bulk density, soil water holding capacity differed greatly. As viewed from the maximum water capacity, it was smaller in the potential and slight stony desertification cases than in the moderate, severe and very severe cases, the maximum saturated water capacity and the maximum field water capacity were the largest for the moderately rock desertified slope but smallest for the slightly rock desertified slope. However, no significant difference existed between maximum saturated water capacity (F=2.16). But the difference in the field water capacity was significant for the slight, moderate and severe stony desertification plots (F=2.678). This reflected the soil structural difference of different rock desertified slopes, especially soil porosity. In the soils of the moderately, severely and extremely severely rock desertified slopes both the capillary pores and non-capillary pores were more than those of the potential and slight ones. Such a character also reflected in the natural saturated water capacity and natural field water capacity. Due to fewer soil pores, poor air and water permeability of soil in the potential and slight sample plots, soil had better water retensiveness and therefore had high natural water content. The severely rock desertified plot has the smallest bulk density, best water permeability, poorest water retensiveness and lowest natural water content but the natural saturated water capacity was the highest.

TABLE 5 SOIL WATER CAPACITY OF SHRUB SLOPES WITH DIFFERENT ROCK DESERTIFICATION DEGREE Rock

Natural saturated

desertification

water capacity 3

Natural field water capacity(g/cm3)

Maximum saturated

Maximum field

Bulk

water capacity

water capacity

density

(g/cm )

(g/cm )

(g/cm3)

0.04a

0.45a

0.39a

2.09a

0.12a

0.06a

0.38a

0.32ab*

2.03a

Moderate

0.14a

0.08a

0.55a

0.48ac*

1.97a

Severe

0.17a

0.08a

0.54a

0.46ac*

1.59b**

0.15a

0.10a

0.54a

0.47a

2.48c*

degree

(g/cm )

Potential

0.09a

Slight

Extremely severe

Note: Different letters represent the difference in significance

3

*: 0.05 significance level

3

**: 0.01 significance level

3.4 Spatial pattern of surface rainwater interception over the shrub slopes of different stony desertification degree Because of different the exposed rock and soil surface, there would be different runoff infiltration processes on the karst shrub slopes. The rainwater firstly falls on the leaf and stems of plants and then falls on the rock and soil surface. After that, it is first absorbed by algae, mosses, cracks and karst pores. Once saturated, part of water flows into rock cracks and holes, and then reached lower part of rocks. Part of water enters rock-soil interstices along karst channels and seeps downward to form soil-water complex. Water fallen on soils is first intercepted by surface residue and then enters soil layer, once surface soil is saturated and water seeps downward, surface runoff occurs. If the water entered rocks or flowed over rock surface (including the contact surface between subsurface rock and soil) can not enter the underground river, it flows down the slope from rock surface and emerges at the surface to form surface runoff. Due to dense texture of limestone rock, rainwater is very difficult to be absorbed in a short time even for long time. Preliminary observations showed that if no cracks existed, the effective water-absorbing thick of rocks was smaller than 20 cm, therefore, based on the mean interception amount of stones (Table 4), we can calculated the - 77 http://www.j-es.org/

rock interception amount of 20 cm deep exposed rocks.

According to the calculated results (Figure 1), the total surface intercepted rainwater amounts in the potential, slight, moderate, severe and very severe stony desertification plots were 2939.32, 4947.4, 6206.93, 7733.62 and 6902.23 g/m2 respectively after the 60-80mm simulate rainfall, they roughly exhibited an increase trend. Except the potential stony desertification plot, the rock interception amount was the largest and tended to increase with the aggravation of stony desertification. But its rainwater-absorbing velocity was very slow even in the 20 cm depth of the exposed rocks, especially in the cases of heavy rain, if no cracks and hole existed, runoff occurred easily. Surface soil interception amount in the slight stony desertification plot was the larger and it decreased with the aggravation of stony desertification. In the potential stony desertification pot although soil layer was relatively thick, it was compact and had few pores, thereby reduced the soil water retentiveness, however the interception amount was significantly higher than that in the severe and very severe sample plots. The surface plant and residue interception amounts in various sample plots differed residue, except the extremely severely rock desertified slope. The interception amount of the former occupied 9-23% of the total interception amount, the surface plant interception amount and residue interception amount of the extremely severely rock desertified slope occupied only 2.3% and 4.4% of the total interception amount, and the rock interception amount occupied 88%.

From the calculation of the intercepted rainwater amounts of surface plants, residue, rocks and soils we can predict the accommodated rainwater volume in a certain depth range of the ground surface and estimate the rainwater distribution in the surface runoff and infiltration processes during one rainfall event. Taking the potential stony desertification slope as an example, surface plants, residue, soil and exposed rocks intercepted 2.94 mm rainfall, this means that the effectively absorbed rainfall is 2.94 mm in single rainfall process, excess rainfall can generate surface and subsurface runoff or vertically seep in the underground river. The intercepted rainfall of the slight rock desertified slope reached 4.95 mm, while that of severely rock desertified slope was 7.73 mm. The interception amount of exposed rocks accounts for 75% or more of the total amount, Rock absorbs water very slowly and it is very difficult to reach saturation in a short time (<1h). Therefore, for the heavy rainfall event, plants, residue and soils have a much more important significance to the rainfall interception.

Rainfall interception amount ďź&#x2C6;g/m 2 ďź&#x2030;and storage amount(g/m 3 )

7000

Surface plant interception amount (g/m2) Surface litter interception amount (g/m2)

6000 5000

Per unit volume soil storage amount (g/m3) 20 cm deep exposed rock storage amount (g/m3)

4000 3000 2000 1000 0 potential

slight

moderate

severe

extremly severe

Rock desertification degree

FIGURE 1 INTERCEPTION AMOUNTS OF RAINFALL BY PLANTS, RESIDUE AND WATER STORAGE AMOUNTS BY SOILS AND ROCKS ON THE SLOPES OF DIFFERENT ROCK DESERTIFICATION DEGREE

4 CONCLUSIONS (1) On the rock desertified shrub slopes in the northern Guangdong, the rock exposure percentage was increasing with the aggravation of stony desertification degree, the soil cover and thickness and vegetation cover decreased but plants exhibited a strong adaptability, especially the increase in climbing shrubs, herbaceous and woody vines increased the use of rock space, this to a certain degree weakened the effects of stony desertification. Therefore, the decrease in surface biomass on the extremely severely rock desertified slope was insignificant and the difference on other degraded slopes was less evident, or even increased in biomass on the severely rock desertified slope. The - 78 http://www.j-es.org/

surface residue showed the same trend, although it significantly decreased on the extremely severely rock desertified slope. The surface plant interception amount on the extremely severely rock desertified slope largely decreased, but it differed little on other rock desertified slopes. The residue interception amount of the moderatedly rock desertified slope was the largest but it significantly decreased in the very severe stony desertification plot.

(2) Soil bulk density on the shrub slopes of different stony desertification degree showed a significant difference. On the potential and slight stony desertification slopes soil layer was thick and compact, soil bulk density was high. On the moderately and severely rock desertified slopes there were residual soils in cracks and lowlands, soil cover and thickness were small, soil structure was loose and soil bulk density was low. On the extremely severely rock desertified slope soil cover was very small, soil gravel content was high and soil bulk density significantly increased.

The natural saturated water capacity gradually increased with the aggravation of stony desertification degree but it decreased in the very severe case.

(3) The density of rocks of different size differed little, their water absorption also showed no significant difference.

Natural rainfall interception amount averaged 32.77 g/m2.

(4) The total surface intercepted rainfall basically exhibited an increase trend with the aggravation of stony desertification. In the potential stony desertification plot the plant and residue interception amount occupied over 45% of the total interception amount, soil interception amount occupied 36% or more, while in the extremely severely rock desertified plot the two only occupied 6.76% and 5.22%, and the rock interception amount occupied 88%. On the potential, slight, moderate, severe and very severe stony desertification slopes the effectively intercepted rainfall by surface plants, residue, soils and exposed rocks were 2.94 mm, 4.95 mm, 6.21 mm, 7.73 mm and 6.9 mm respectively, account for 8.4%,14.14%,17.74%,22.09%,19.71% of total rainfall amount. However, the increase in rock exposure percentage caused by the aggravation of stony desertification reduced the rainwaterabsorbing velocity of the ground surface. Therefore, the intercepted rainfall by the potential and slight stony desertification slopes has a more practical significance.

ACKNOWLEDGMENT This project were supported by the National Science Foundation of China (Transference and circulation process of calcium within the rock-soil-plant and ecological effect in Karst mountain area, northern Guangdong province, Grant No. 31170486) and Action Programs of Western, C.A.S â&#x20AC;&#x153;Study on Maintenance mechanism and control technology of Karst Ecosystem service in southwest of Chinaâ&#x20AC;? No. KZCX2-XB3-10)

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AUTHORS Xinghu Wei (Lanzhou city, Gansu province, 1964- ), Male.

Xizhen Xu (Ganzhou city, Jiangxi province, 1986- ), Male, a

Doctor of Geography, professor. Graduated from the Northwest

teaching assistant. Graduated from the College of Geography,

Agricultural University, Bachelor degree,1988; Graduated from

South China Normal University, 2012, Master degree. The major

the Cold and Arid Regions Environmental and Engineering

field of study is technique and mechanism of ecological

Research Institute, CAS, doctor's degree, geography,2004; The

rehabilitation.

major field of study is Mechanism and process of land degradation.

- 80 http://www.j-es.org/