1. Environmental landform types of karst rocky desertification
Under the specific natural geographical conditions, the rich and colorful karst landforms in Guizhou have obvious contrast between positive and negative landforms, which are associated with each other in the formation process, have certain genetic connections, coexist in spatial distribution and have certain combination rules. This genetic relationship and combination law not only reflects the geomorphological development characteristics of different causes, different structural controls and different development stages, but also lays the foundation for the division of environmental geomorphology types.
(1) karst rocky desertification environment series of peak cluster depression: the combination of cone peak and depression, valley or canyon, in which the positive terrain area is greater than the negative terrain area. The bottom of the cone peak (mound peak) is connected, with a relative height of 100~250m ~ 250 m, and the peak is uneven and inclined to the regional topographic slope (Figure 2- 1). Except Maolan Nature Reserve and Yuntai Mountain, most of the peaks and slopes are exposed rocks or rock corners with serious rocky desertification. Underground is dominated by pipeline flow, sometimes forming underground rivers. Among the types of peak-cluster depressions, the depressions are closed in depth and have polygonal features, including cylindrical, funnel-shaped or basin topography, with different sizes. The height difference at the bottom of the depression is large, and the terrain slope to the region decreases step by step; The rocky desertification at the bottom of the depression is serious and the rocks are exposed; There are often Dou Lin or sinkholes underground; Among the peak-cluster valley types, the valley is narrow and gentle, which is a karst dry valley that evolved from a depression along the structural strike, and part of it is an ancient river channel where the early river network was located. The valley bottom is relatively flat, generally there are no modern surface rivers, most of them are exposed, and a few are covered by debris and slope deposits, and bucket showers and sinkholes are developed. Among the types of peak-cluster canyons, the canyon was formed by the intense uplift of the plateau, and the main river was quickly cut down by hundreds of meters. The valley is narrow and steep, with a large slope and undeveloped alluvium. The valley slope is steep and deep, in the shape of "V", box or even rift valley. The surrounding depressions have developed into deep depressions with bare rocks due to rejuvenation, which reflects that this series is a bare karst ecological environment, with karst vegetation and leakage mainly composed of evergreen broad-leaved and lime soil. Human activities are mainly dry land and sloping farmland, with low degree of water conservancy, uncoordinated water, soil and fertilizer, great spatial change, single agricultural structure and low and unstable comprehensive production.
Figure 2- 1 Shuicheng Zhongba Peak Cluster Depression
(2) Karst environment series of peak forest depression: the combination of cone peak and depression or trough and valley, and the area occupied by positive landform is roughly equal to that occupied by negative landform. In fact, this is a series of transition between peak-cluster depression karst and peak-forest karst. Cone-shaped peaks (hills) are scattered around depressions or valleys in an isolated manner, with a relative height of 100 ~ 200m, and the peaks are slightly undulating, with no obvious tendency (Figure 2-2). Most of the peaks and slopes are sparse shrubs, barren hills and grass slopes or stone corners with serious rocky desertification, and there are good geomantic woodlands near the village. Among the types of peak forest depressions, depressions are large and shallow polygons, flat and open, covered with thin eluvial deposits, and often have bucket showers and sinkholes. In the peak forest valley type, the valley extends vertically, or it is a bottom slope valley formed by the merger of depressions along the structural strike, or a modern valley formed by the lateral widening of rivers, or both. The bottom of the valley is near the base, wide and gentle, and the Jing Quan on the edge is widely distributed, and the river alluvial deposits are developed, and the farmland is widely distributed. This series of ecological environment is characterized by semi-bare karst, subtropical evergreen broad-leaved forest and limestone vegetation. Yellow soil, yellow red soil and calcareous soil are alternately distributed. There are few rivers in the valley, and the groundwater is moderately buried and unevenly distributed. Human activities are mainly dry land, with drought and flood coexisting, and the ratio of dam field to soil increases.
Figure 2-2 Small Rapeseed Gully in Longgong Gully
(3) Karst environment series of peak forest dissolution: the combination of cone peak and dissolution, basin or platform, positive landform area is much smaller than negative landform area. Cone-shaped peaks (mound-shaped peaks) are dotted on the flat carbonate rock surface in isolation, with a relative height of 50 ~ 150m, and the peaks are even without obvious inclination (Figure 2-3). Most of the peaks and slopes are sparse woodland, shrub sloping land or sloping farmland with serious rocky desertification, and there are good geomantic woodland near the village. Modern underground river caves and caves in Shifeng base are well developed. The groundwater system began to transform to the surface. Among the types of solution in peak forest, the solution is a kind of karst quasi-plain with horizontal organization, which is flat and open, close to the base surface, with rivers alternating with light and dark, many lakes and ponds, covered with thin eluvium, and widely distributed in farmland dams; Among the types of LAM Raymond basin, many karst quasi-plains stand along the structural slope caused by neotectonic fault depression, while others are basins developed on the basis of syncline structure, with broad sealing, centripetal water system developed and extended along the structural strike, thick rivers and lakes and numerous paddy fields; In the peak forest platform type, the karst quasi-plain of platform system is caused by the uplift of neotectonic fault blocks or the deep surrounding rivers. The platform is flat and open, covered with thin eluvial layer, and there are many modern tiankeng and Dou Lin along the fault. This series of ecological environment is characterized by shallow karst, mainly evergreen broad-leaved forest and yellow soil. The surface and underground water systems are well developed, the groundwater is shallow and relatively uniform, there is a relatively rich underground water belt, the soil layer is thick and continuously distributed, the water and fertilizer conservation are enhanced, and the water conservancy conditions are good. Agricultural utilization is dominated by paddy fields, with drought and flood coexisting, high degree of water conservancy, high multiple cropping index, high degree of agricultural diversification and comprehensive production.
Figure 2-3 Decomposition of Peak Forest in Qiyanqiao Watershed, Anshun
2. Regional structure of karst rocky desertification environment
Guizhou is a mountainous area of subtropical karst plateau between Sichuan basin and Guangxi hills, which belongs to the eastward extension of Yunnan-Guizhou Plateau. Due to the extensive distribution of carbonate rocks, A has been influenced by the strong tectonic uplift of Himalayan orogeny and river erosion in tropical and subtropical humid climate since Tertiary, forming a remarkable karst plateau-canyon structure (Yang Mingde, 1985), which consists of two distinct but closely related regional units-plateau area and canyon area (Figure 2-4).
Figure 2-4 Schematic Diagram of Karst Plateau Canyon Structure in Guizhou
(1) Environmental characteristics of plateau area. The plateau area is composed of strip-shaped surface, which is distributed on the upper watershed plateau surface of major rivers and tributaries, that is, the watershed area of the Yangtze River and the Pearl River. Neotectonic movement has risen on a large scale, and the terrain is 600 ~ 1000 m away from the surrounding valley, and the erosion of the river source has not yet reached, which is a backward or inherited karst development area in Guizhou. The terrain is relatively high, with an altitude of 2200 ~ 2400 m (Weining and Hezhang), 1600 ~ 1800 m (Liuzhi and Shuicheng) and 800 ~ 1200 m (Guiyang and Anshun). The fluctuation is small, and the relative height is generally below 200m, showing a broad and gentle watershed plateau (photo 2-2). Because the ground is relatively flat, a large area of cone-shaped peak forest and wide basins, valleys and depressions form a set of peak forest landforms such as peak forest dissolution, peak forest basin, peak forest valley and peak forest depression, which are characterized by shallow karst coverage. That is to say, the gentle hills on the surface of the solution are covered with residual weathering shells leached by red clay with a thickness of several meters to tens of meters, and stone gullies and stone buds often develop under the shells, which are inclined downwards, reflecting that they are formed under the soil. Of course, in the elevation near the watershed, there are also some exposed dissolved sources, such as the Northwest Airport in Anshun and Liuzhi Dayong. Quaternary clay can only be accumulated in shallow depressions and valleys on the source surface, but it still shows the phenomenon of cutting a flat structure on the topographic surface that constitutes the original surface, which may be due to the subsequent erosion of the originally not thick weathering crust, exposing a large area of bedrock. The land type is subtropical hillock-dissolving dam. Because of its open terrain, good lighting conditions and relatively high light and temperature products, it is conducive to the development of light-avoiding crops. The soil is mainly zonal yellow soil, red yellow soil or yellow brown soil, and because of its small relief, slow flow of valley width, shallow groundwater burial and relatively uniform distribution, there are often groundwater "enrichment zones", with good mining conditions, good water conservancy conditions and high degree. The cultivated land is dam land and yellow brown soil. It can reach more than 30%, which is beneficial to agricultural intensification and large-scale operation. 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 a relatively developed agricultural area in Guizhou.
(2) Environmental characteristics of the canyon area. The canyon area is dominated by deep valleys, which are mostly distributed in the valleys on the edge of the plateau in the middle and lower reaches of major rivers and tributaries. The neotectonic movement is strongly uplifted, and the riverbed is generally cut down by 300 ~ 700 m, which indicates that the erosion is strong and it is a karst superimposed development area in Guizhou. Although the terrain is low, the altitude is less than 800 ~ 1400 m, but it fluctuates greatly and is steep, and the relative height is often around 1000m, which is a relatively narrow valley depth area (photo 2-3). Due to the broken and rugged surface, a large area of conical peaks, deep valleys and depressions constitute a set of typical exposed peak-cluster landforms such as peak-cluster valleys, peak-cluster depressions and peak-cluster canyons. Rocky desertification is serious, the soil layer is extremely thin and scattered. Only in some depressions, there are Quaternary loose deposits with small thickness, which develop into young calcareous soil. Due to the rock structure, the Permian Maokou limestone (P 1m) is covered by basalt (P2β) in some areas, forming a small area of hidden karst. Once the river cuts into limestone, it will lead to the strong development of karst in the lower part, and even form two kinds of gullies. The upstream is a gully developed on basalt, and the downstream is a karst gully, and the spring water emerges to form a trickle. Layered caves are developed on the valley slope, and the development of many caves is closely related to the water seepage along the overlying basalt joints. The land type is subtropical rocky karst valley, and light, heat, water, gas and soil are obviously redistributed with topography, with obvious vertical differentiation. There is a saying that "there are four seasons in one mountain, and ten miles are different from the sky". The depressions, canyons and shady slopes have short illumination time, low product of light and temperature, strong ground cutting, large slope, thin soil layer, discontinuous distribution and poor soil and water conservation ability. Because of the deep groundwater, low river level, little water, high farmland, scattered farmland, poor utilization conditions, low degree of water conservancy, farmland prone to drought and waterlogging, and low land carrying capacity. Due to the closed traffic, bad external relations and the feedback of closed and self-sufficient small-scale peasant economy, the natural economy of small-scale peasants has always been "ultra-stable" and has been in the state of agricultural development for a long time.
3. Present situation and research significance of karst rocky desertification
The research on karst rocky desertification in Guizhou has a long history, but so far, due to the limitation of research conditions and the inconsistency of technical means, the data on the present situation, area and distribution of rocky desertification are not unified enough.
Xu Anren (1999) thinks that the area of soil erosion in Guizhou is 43.5%, of which the area of soil erosion in the Yangtze River basin in the whole province is 67,000 km2, accounting for 58% of the basin area, and the annual loss of sediment is 6,543.8+0.9 million tons, which is equivalent to washing away 43,300 hectares of fertile soil with an average thickness of 33cm every year, resulting in the continuous expansion of rocky desertification area, which has reached 65.44 million at present.
According to Yang Mingde (1999), Guizhou has experienced population expansion, forest area reduction, low coverage rate (20.8%), serious soil erosion (43.6% of the province's area), rapid development of rocky desertification (12.8% of the province's area), and the number of traps is decreasing continuously, with an annual reduction rate of/kloc.
Luo (1999) thinks that the current rocky desertification area in Guizhou Province is 2.257 million hm2, of which the rock exposure rate is over 70%, the rocky desertification mountain with rocky desertification landscape is 74 1000 hm2, the gravel mountain1380,000 hm2, and the rocky desertification mountain with rock exposure rate of 30% ~ 70% is 467,000 hm2. There are 333,000 hectares of dry land with rocky desertification, and there are 25 ~ 35 industrial and mining rocky desertification areas and dry land with potential rocky desertification trend in the short term, with an area of 58.438+0 hm2. Thus, rocky desertification land can be divided into rocky desertification mountain, rocky desertification mountain, rocky desertification cultivated land and > 35. Rocky desertification dry land, industrial and mining rocky desertification land, dry land with potential rocky desertification trend in the short term, etc.
According to Jin (1999), the area of soil erosion in Guizhou karst distribution area has reached 42,400km2, and the average intensity of soil erosion is 3 169t/a? Square kilometers. Soil erosion has intensified the process of karst rocky desertification. The existing rocky desertification area in the province is 33.86 million mu (serious rocky desertification is 24 1.5 million mu and potential rocky desertification is 9.67 million mu), accounting for/kloc-0.28% of the total area of the province, and it is still increasing at the rate of/kloc-0.35 million mu per year.
Karst rocky desertification in Guizhou has aggravated the further deterioration of ecological environment. Rocky desertification constantly devours land, reduces land productivity, reduces environmental capacity, and makes it difficult for livestock to drink water, which greatly affects the living conditions of local residents, poses a new threat to human survival, and even makes local people gradually lose their living conditions. In some areas, soil and water can not support one person, which has become a typical example of how to get rid of poverty through population and environmental migration. Karst rocky desertification has become a major issue in Guizhou's ecological environment construction and economic and social development.