Guizhou Karst and rocky desertification

Guizhou is located in the center of East Asia, one of the three largest karst concentration areas in the world. Against the background of a geological environment with widespread carbonate rock distribution and a warm and humid monsoon climate, Guizhou covers an area of ??176,100 km2. , accounting for 73% of the exposed area of ??karst landforms, it is a plateau mountainous area with the largest distribution area and the strongest development of subtropical cone karst in my country and even the world (Photo 2-1). Many theoretical issues on the development of karst in the world have yet to be resolved based on the research results on karst in southern China (Sweeting, 1986).

1. Karst rocky desertification environmental landform types

Under specific natural geographical conditions, the rich and colorful karst landforms in Guizhou have a clear contrast between positive and negative topography. They accompany each other in the formation process and have certain causal connections. They coexist with each other in spatial distribution and have certain combination rules. This genetic connection and combination law not only reflects the development characteristics of landforms of different origins, controlled by different structures and at different development stages, but also lays the foundation for the classification of environmental landform types.

(1) Peak cluster depression karst rocky desertification environment series: a combination of conical peaks and depressions, valleys or canyons. The area occupied by positive terrain on the plane is greater than the area of ??negative terrain. The cone peaks (hill peaks) are connected at their bases, with a relative height of 100 to 250m. The peaks are uneven and inclined towards the regional terrain slope (Figure 2-1). Except for Maolan Nature Reserve and Yuntai Mountain, the peak slopes are mostly bare rock or rocky corners with severe rocky desertification. The underground is dominated by pipeline flow, sometimes forming underground rivers. In the Fengcong depression type, the depression is deeply sunken and closed, with polygonal features, cylindrical, funnel-shaped or basin-shaped, and varies in size. The height difference at the bottom of the depression is very different, and it gradually decreases towards the regional terrain slope; the bottom of the depression is severely rocky desertification, and the rocks are exposed; there are often buckets or sinkholes underground; in the peak cluster valley type, the valley is narrow and smooth, and is a depression Karst dry valleys that developed and evolved along the structural direction. Some are ancient river channels where early river networks were located. The valley bottoms are relatively flat. There are generally no modern surface rivers. Most of the rocks are exposed, and a few are covered with residual sediments and slope sediments. Doulin, Sinkholes are developed; in the Fengcong canyon type, the canyon is formed due to the strong uplift of the plateau and the rapid incision of the main river for hundreds of meters. The valley is narrow, the water is fast, the specific drop is large, alluvial deposits are not developed, the valley slope is steep, and the deep incision is " V”-shaped, box-shaped or even rift-shaped, the surrounding depressions have developed into deep depressions with exposed rocks due to rejuvenation, which complement the canyons. The ecological environment of this series is characterized by exposed karst, with evergreen broad-leafed and deciduous karst vegetation and lime soil. Mainly, there is strong leakage, deeply buried groundwater, surface water shortage and drought, thin soil layer, discontinuous distribution, and many drought and flood depressions. Human activities are dominated by dryland sloping farmland, with low degree of water conservancy, uncoordinated water, soil and fertilizer, large spatial changes, single agricultural structure, and low and unstable comprehensive production.

Figure 2-1 Shuicheng Zhongba Peak Cluster Depression

(2) Peak Forest Depression Karst Environment Series: The combination of conical peaks and depressions or troughs, the area occupied by the positive terrain It is approximately equal to the proportion of negative terrain. In fact, this is a series of transition between peak cluster depression karst and peak forest karst karst. Cone peaks (hills) are scattered in isolated shapes around depressions or valleys, with relative heights ranging from 100 to 200m. The peaks have small fluctuations and no obvious tendency (Figure 2-2). The peak slopes are mostly sparse shrubs, barren hills, grassy slopes or rocky corners with severe rocky desertification. There are good Fengshui woodlands developed near the villages. In the peak forest depression type, the depression is large and shallow polygonal, flat and open, covered with a thin residual layer, and often has buckets and sinkholes; in the peak forest trough type, the valley extends longitudinally, or A basal sloping valley formed by the merging of depressions along structural trends, or a modern valley resulting from the lateral broadening of a river, or both. The valley bottom is close to the base level, wide and open, with wells and springs widely distributed on the edge, river alluvial deposits developed, and farmland widely distributed. The ecological environment of this series is characterized by semi-exposed karst, with subtropical evergreen broad-leaved forests and limestone vegetation existing, yellow soil, yellow-red soil and lime soil alternately distributed, valley rivers rare, groundwater buried moderately and unevenly distributed. Human activities are mainly in dry land, with both floods and droughts, and the ratio of dam fields to dam soil has increased.

Figure 2-2 Xiaorapai River Slope Valley in Longgong Basin

(3) Fenglin Karst Environment Series: Combination of conical peaks and solvogens, basins or platforms, positive topography The area is much smaller than the area of ??negative terrain. Cone peaks (hills) are dotted in isolated shapes on the flat carbonate rock surface, with relative heights ranging from 50 to 150m. The peaks are even and have no obvious tendency (Figure 2-3). The peak slopes are mostly sparse woodlands, shrub slopes or sloping farmland with severe rocky desertification. There are good Fengshui woodlands developed near the villages. Modern underground river-like caves and caves are well developed at the base of the stone peak. The groundwater system begins to transform to the surface.

In the Fenglin lysogen type, the lysogen is a karst peneplain with a flat structure, which is flat and open, close to the base surface, with alternating light and dark rivers, numerous pools and lakes, covered with a thin residual layer, and farmland dams are widely distributed; in the peak Among the forest basin types, many basins are karst peneplains along structural slope valleys caused by neotectonic fault depression, and some are basins developed on the basis of syncline structures. Most of them are closed and wide, with centripetal water systems developed along the structural trend. Characteristics of extension include thick fluvial and lacustrine sediments and numerous paddy fields; among the Fenglin platform types, the platform system karst quasi-flat is caused by the uplift of neotectonic fault blocks or the deep incision of surrounding rivers. The platform is flat and open, covered with thin residual sediments. layer, many recent sinkholes and buckets developed along the fault. The ecological environment characteristics of this series are shallow-covered karsts, dominated by evergreen broad-leaved forests and yellow soil. Both the surface and underground water systems are well developed. The groundwater is shallow and relatively uniform. There is a relatively rich groundwater zone, and the soil layer is thick and The distribution is contiguous, the water retention and fertility are enhanced, and the water conservancy conditions are good. Agricultural utilization is mainly paddy fields, with both flood and drought cultivation, high degree of water conservancy, high multiple cropping index, diversified agricultural operations, and high comprehensive production volume.

Figure 2-3 Anshun Qiyanqiao watershed peak forest karst plain

2. Regional structure of karst rocky desertification environment

Guizhou is an uplift in the Sichuan Basin The subtropical karst plateau mountainous area between Guangxi and Guangxi hills is the eastern extension of the Yunnan-Guizhou Plateau. Due to the widespread distribution of carbonate rocks, the strong tectonic uplift of the recent Himalayan orogeny and the influence of river erosion in tropical and subtropical humid climate environments since the Tertiary Period have formed a significant karst plateau-canyon structure (Yang Mingde, 1985), that is, it is composed of two significantly different but closely related geographical units - the plateau area and the canyon area (Figure 2-4).

Figure 2-4 Structural map of Guizhou Karst Plateau Canyon

(1) Environmental characteristics of the plateau area. The plateau area is composed of the stripping surface as the core and is distributed on the watershed plateau surface of the upstream of major rivers and major tributaries, that is, it is distributed in the watershed areas of the two major river systems of the Yangtze River and the Pearl River. Neotectonic movements caused large-scale uplift, and the terrain rose 600 to 1,000 meters higher than the surrounding valleys. River source erosion has not yet reached it, making it an area of ??delayed or inherited karst development in Guizhou. The terrain is relatively high, with altitudes mostly between 2200-2400m (Weining and Hezhang areas), 1600-1800m (Liuzhi and Shuicheng areas), and 800-1200m (Guiyang and Anshun areas). The undulations are small, the relative height is generally less than 200m, and it is a wide and gentle watershed-type plateau area (Photo 2-2). Because the ground is relatively flat, large areas of cone-shaped peak forests and broad basins, valleys, and depressions form a set of peak forest landforms such as peak forest karst plains, peak forest basins, peak forest valleys, and peak forest depressions, with shallow coverage of karst. characteristics. The gentle hilly highlands on the original surface of the instant solution are covered with a weathered crust of leaching residual red clay that is several meters to tens of meters thick. Stone ditches and stone buds often develop under the crust and pinch out downwards, reflecting that it was formed under the soil. of. Of course, there is also a certain area of ??exposed karst plains at altitudes near the watershed, such as in the Anshun Northwest Airport area, Liuzhi Dayong and other places. Quaternary clay accumulation is only found in shallow depressions and valleys on the karstic plain surfaces, but It still shows that the topographic surface that makes up the original surface has a flat structure phenomenon. This may be because the weathering crust that was not originally thick was later denuded and exposed a large area of ??bedrock. The land type is subtropical lysate mounds. Due to the open terrain, good light conditions, and relatively high light-temperature product, it is conducive to the development of light-loving crops. The soil is mainly zonal yellow soil, red-yellow soil or yellow-brown soil, and the terrain has small fluctuations. , the valley is wide, the flow is slow, the groundwater is shallow, and the distribution is relatively uniform. There are often groundwater "enrichment zones" with good conditions for exploitation, so the conditions for water conservancy are good and the degree is high. The cultivated land is mainly dam fields and dam soil, which is relatively concentrated and contiguous. , the land reclamation index is high, reaching more than 30%, which is conducive to agricultural intensification and large-scale operations. And because of its early development history, convenient transportation, rapid urban development, and large scale, it is conducive to promoting the development of agricultural commodity economy. It is Guizhou's agriculture Relatively developed area.

(2) Environmental characteristics of the canyon area. The canyon area is mainly composed of deep valleys, mostly distributed in plateau edge canyons in the middle and lower reaches of major rivers and major tributaries. The neotectonic movement caused strong uplift, and the riverbed was generally cut down by 300 to 700m, indicating strong erosion, and it is a karst superimposed development area in Guizhou. Although the terrain is low, with an altitude ranging from less than 800 to 1,400m, it has large undulations and steep slopes, with the relative height often reaching about 1,000m. It is a relatively narrow deep valley area (Photo 2-3). Due to the broken and rugged ground surface, large areas of cone-shaped peak clusters and deep valleys and depressions form a set of typical exposed peak cluster landforms such as peak cluster valleys, peak cluster depressions, and peak cluster canyons. The rocky desertification is severe, and the soil layer is extremely thin and sporadic. Only in some depressions are loose Quaternary sediments of small thickness developed into young lime soil. Due to the structural reasons of the rock group, in some areas, the Permian Maokou limestone (P1m) is overlain by the basalt (P2β) caprock, forming a small area of ??buried karst. Once the river cuts down to the limestone, it will lead to the strong development of karst in the lower part, and even form a valley with two structures. The upstream is a ravine developed on the basalt, and the downstream is a karst valley where spring water is exposed to form a trickle. The valley slope Layered caves are developed, and the development of many caves is closely related to the seepage of water along the overlying basalt joints.

The land type is subtropical rocky mountain, karst depression and canyon land. Light, heat, water, air and soil are significantly redistributed according to the terrain, and the vertical differentiation is obvious. There is a saying that "a mountain has four seasons, and the sky is different ten miles away". Depressions, canyons, shady areas and Slope illumination time is short, light temperature product is low, ground cutting is strong, slope is large, soil layer is thin, discontinuous distribution, water and soil retention ability is poor, water and soil loss is serious, soil is generally lime soil and coarse bone soil with low degree of maturity. , mainly sloping farmland and dry land. Because the groundwater is deeply buried, the river water level is low, the water is low, the fields are high, the field soil is scattered, the utilization conditions are poor, the degree of water conservancy is low, and the field soil is prone to drought and waterlogging, there are many "looking at the sky" and "three fields". "The land runs away", so the land carrying capacity is low. Due to traffic congestion, poor external connections, the feedback effect of the closed self-sufficient small farmer economy ideology has promoted the "super stability" of the small farmer natural economy, and the long-term slow agricultural development and production With a single structure, backward management methods, poor economy and poor life, most of Guizhou's poverty-stricken areas are located in this type of landform area.

3. Current status and research significance of karst rocky desertification

Guizhou karst rocky desertification has been studied for a long time, but so far, due to limited research conditions, technical methods are not unified Due to other reasons, the data on rocky desertification status, area, and distribution are not uniform enough.

Xu Anren (1999) believed that the area of ??water and soil loss in Guizhou reached 43.5%, of which the area of ??water and soil loss in the Yangtze River Basin in the province was 67,000 km2, accounting for 58% of the basin area, and the annual sediment loss was 190 million tons, equivalent to As a result, 43,300 hectares of fertile soil with an average thickness of 33cm are washed away every year, resulting in the increasing expansion of rocky desertification area, which has now reached 13,300km2 (or 7.55%), and is increasing at a rate of 933km2.

Yang Mingde (1999) believed that Guizhou has experienced population expansion, forest area reduction, low coverage rate (20.8%), serious water and soil erosion (reaching 43.6% of the province’s area), and the development of rocky desertification. Rapidly (reaching 12.8% of the province's area), the cultivated land continues to decrease, at a rate of 114,000 acres per year, causing the per capita cultivated land to drop to 0.83 acres, the per capita cultivated land warning line set by the United Nations.

Luo Zhongkang (1999) believes that the current rocky desertification area in Guizhou Province reaches 2.257 million hm2, of which the rock exposure rate is more than 70%. There are 741,000 hm2 of stone mountains and 138,000 sand and gravel mountains that have shown rocky desertification landscape. hm2, 467,000 hm2 of rocky desertified mountainous areas with a rock exposure rate of 30% to 70%, and 35% of rocky desertified cultivated land. The above dry land that has been rocky desertified is 333,000 hm2, industrial and mining rocky desertified land and 25-35% of the land with potential rocky desertification trend in the short term, and dry land is 58.1hm2. The rocky desertified land is thus divided into rocky desert fossil mountains, sandstone mountains, rocky desertified cultivated land, >35. There are several levels: rocky desertification dry land, industrial and mining rocky desertification land, and dry land with potential rocky desertification trend in the short term.

Jin Xiaoqi (1999) believed that the current soil and water loss area in Guizhou's karst distribution area has reached 42,400 km2, and the average soil erosion intensity is 3169t/a?km2. Soil and water loss has intensified the process of karst rocky desertification. The existing rocky desertification area in the province reaches 33.86 million acres (24.15 million acres of severely rocky desertification and 9.67 million acres of karst slope cultivated land with potential rocky desertification), accounting for the total area of ??the province. 12.8% of the area, and it is still increasing at a rate of 1.35 million acres every year.

The karst rocky desertification in Guizhou has deepened the further deterioration of the ecological environment. Rocky desertification continues to engulf the land, reducing land productivity, reducing environmental capacity, and making it difficult for livestock to drink water. It has greatly affected the living conditions of local residents, posed new threats to human survival, and even caused local people to gradually lose their living conditions. In some areas, one area of ??land and water cannot support one person, which has become a typical example of the need to migrate through population and environment to fundamentally get rid of poverty. Karst rocky desertification has become a major problem facing Guizhou's ecological environment construction and economic and social development.