The Study On The Characteristics And Mechanism Of Soil Erosion In Karst Valley Area, Chongqing

Comparied with non-karst area, soil leakage is a particular phenomenon in karst region. Traditional monitoring methods or USA universal soil loss equation can't reflect the amount of soil erosion truly. Therefore, we should clarify the laws of soil erosion well, then we can govern it and rock desertification. The purpose of this study is to explore the characteristics and mechanism of soil erosion in karst area and to establish soil erosion coupled model, which would be a scientific basis to govern soil erosion and rock desertification. Government made the call "to further enhance the management of soil erosion in Southwest China" in the Central Committee No.1 Document in 2011 and the requirements of "strengthening the comprehensive management of desertification" in Twelve·Five-Year" Plan. This indicates the importance of soil erosion management in karst region.The aims of this study are:(1) to study the characteristics of surface erosion in karst area through runoff plots monitoring in Nanchuan for two years; (2) to explore the proportion of surface and underground leakage on the small scale through one year monitoring based on paint scheme and piling and measuring the 137Cs content of 5cm layer soil sample in runoff plots; (3) to analyze the differences of soil nutrients with the change of slope altitude and profile by measuring soil nutrients and 137Cs content through the samplings (5cm layer) in the vegetable and abandoned land, and then to determine whether SOC can reflect soil loss phenomena; (4) to deduce the depth of surface soil leakage through the analysis of 137Cs distribution depth on the soil profile and 137Cs content of soil on cave walls in Zhongliang mountain karst valley area; (5)to estimate the loss ratio of surface and underground on the large scale with the ratio method by measuring 137Cs content both in surface soil and in ground river silt; (6)to build soil erosion coupled model based on the study of the characteristics and mechanism of soil erosion in karst area. The conclusions are as follows:(1) The nutrients except TP in the Calcium-rich region showed a similar pattern as those in non-karst area; Absolute amount of surface soil erosion in kast area is much lower than that in non-karst area, while the relative strength of erosion is high. Soil erosion and soil nutrient loss are correlated positively to rainfall in the karst region, which is similar to that in non-karst region. The correlation among surface soil erosion, rainfall and rainfall intensity is large in the kast valley area (up to 0.40 above). Soil nutrient contents are significantly correlated with rainfall intensity (correlation coefficient is above 0.60), so the total loss of soil nutrients is closely related with rainfall and its intensity as well. Surface runoff coefficient in the karst area is 1/7 of that in non-karst area, therefore the rainfall should be at least 30mm which can cause surface erosion in karst area while 10 mm in the non-karst region. The low rate of pedogenic weathering in karst area easily leads to rocky desertification. The loss of both organic matter and TN is the greatest and the order of the loss variation is:slope farmland> woodland> grassland> honeysuckle land. The loss performance of total phosphorus and available potassium content in various land use types is similar. The enrichment of soil nutrients displays differently:available K is the highest, total phosphorus is the lowest, and total nitrogen, hydrolysable nitrogen, organic matter is between them. Enrichment of soil nutrients in different land use types is different:that of sloping land is obvious (natural slope farmland has stronger enrichment than that of farmland moving soil to build up fertility), followed by woodland, honeysuckle, the grass land. Total phosphorus, organic matter and total nitrogen in various types of land profile presented such properties that the soil nutrient elements decrease with the increase of the depth. The content of organic matter and total nitrogen in low slope positions in various types of land use shows higher content than those that of high slope positions, but total phosphorus doesn't follow this rule.(2) Soil loss phenomenon in Karst valley area does exist but it is not found everywhere. The results show that the soil loss ratio vertically downward through fractures directly into the underground river is less than 1/4.There exists the phenomenon of soil filling capsule and leisure on a small scale. For example, among 9 runoff plots in Nanchuan, soil leakage phenomena of one plot is obvious, with the proportion of 96%. The measure of 137Cs content in soil profile indicates that 137Cs content generally rises with the increase of soil depth. In Zhongliang mountain, only one among the 8 soil profils experiences soil leakage with the depth about 70 cm. Soil erosion in Karst valley area go into ground river mainly through sinkholes, skylights and so on, which is consistent with the views of the scholar named Cao Jian-Hua. Soil leakage in Karst valley area occurs mainly in those places where there is strong human disturbance, high crack and rock exposure. Soil 137Cs content on the cave wall is almost 0 and that the depth of 137Cs content on No 3 vegetable land in Zhongliang mountain is only up to 70cm, which inferred that it's difficult for surface soil to enter into underground river through the cracks. There exists underground creeping and leakage phenomenon in karst region, but surface soil can't reach soil-rock interface and soil of soil-rock interface can reach underground river through cracks. The ratio of surface and subsurface soil erosion estimated by the ratio method is 75.45% and 24.55% through measuring 137Cs content in different land use types and underground river silt in the Zhongliang Mountain.(3) The distribution of nutrient contents in soil profiles and slopes can reflect the dual structure, and the changes of organic matter content can prove soil loss phenomenon. Soil nutrient content increases with the decrease of altitude in both the vegetable plots and the abandoned land in the place where there is no soil loss. The content of soil nutrient elements decreased with the increase of the depth, especially for TP. In the place with soil leakage, the soil nutrient elements would break the law that the content of nutrient elements increased with the decrease of the slope altitude, but still follows the properties that the soil nutrient elements decrease with the increase of the depth. The coefficient of soil surface clustering and soil nutrients variation in the area with soil loss are lower than those in no soil loss area. Soil leakage phenomenon can be proved with viscous grain,content of 137Cs and soil organic matter. 137Cs and soil organic matter with similar physical movement rule is proved by a good correlation between 137Cs and soil organic matter, so soil organic matter can prove soil leakage phenomenon. The content change of 137Cs is performed through the content change of the viscous grain, and the content change of soil organic matter is caused by soil preferential flow. (4) The traditional soil erosion model can not estimate the soil erosion amount of ground lossand reflect surface soil characteristics truly; therefore soil erosion coupled model in karst valley area of Chongqing is built by adding the adjustment coefficient, the bare rock ratio and surface soil erosion proportion. The models are as follows:Soil erosion coupled model in karst region:Y×δ+Y1-δWhere. Y=W/δ; Y refers to the total soil erosion amount in karst region,δis on behalf of surface soil erosion proportion in karst region;1-δrefers to underground soil erosion proportion in karst region; W refers to surface soil erosion amountSurface soil erosion amount:W=ARKDLSCP(1-a)Where, A is the adjustment coefficient; a is the bare rock ratio, and other indexes are similar to those of Liu Baoyuan's soil erosion model. A=B1×B2, B1 refers to the ratio of surface runoff coefficient in the Karst region to that of the non-karst area; B2 refers to the ratio of annual rainfall frequency causing soil erosion in karst region to that of non-karst area.Underground soil erosion amount:W'=W/δ×[1-δ]Where, W'is the amount of underground soil erosion andδrepresents the proportion of surface soil erosion.A relative error calculated with the estimated value of soil erosion model and the actual measure value, is from 0.85% to 38.45%. A relative error under 10% is dominant (66.67%), and that of 10%-20% and 20%-30% account for 16.67%,5.56% respectively, while that above 30% covers only 11.11%. Therefore karst soil erosion coupled model in this study can be used to calculate soil erosion in karst well.(5) Soil erosion coupled model shows that soil erosion in karst valley area is distributed in the region where human activities are frequent. The area and strength of soil erosion in Zhongliang mountain of ShaPingBa is 28.77% and 32.35% higher than those of Nanping in Nanchuan respectively, which explains that population and the contradiction between human and land are the main factors to cause soil erosion.Soil erosion area is mainly distributed near the elevation range of the human living place. The correlation between soil erosion intensity and slope shows that:soil erosion in Nanping and Zhongliang mountain karst valley area distributes mainly in the slope range of 5°-15°. where the ratio is 49.18% and 42.80%r espectively. The soil erosion in 15°-25°slope of Zhongliang mountain is up to 30.19%. while only 12.55% in Nanping. This indicates that converting cropland to forest in Zhongliang mountain should intensify in the farmland of 15°-25°slope range to reduce the soil erosion degree and slow down the process of rocky desertification.(6)The temporal and spatial variation is obtained based on soil erosion coupled modeh remote sensing data and GIS.Correlated relationship (correlation coefficient of 0.89) between soil erosion intensity and rocky desertification intensity is high using their comparison of two maps in 1988 and 2000. Soil erosion area decreases 29277.10 hm2 from 1988 to 2000 in karst area in Nanchuan. Soil erosion intensity decreases obviously, especially in moderate and above moderate. Rocky desertification area decreased 2042.72hm2 from 1988 to 2000 and that in the intent rocky desertification is particularly evident. Rocky desertification and soil erosion have causality relation on genesis and delaying relationship in time, for rocky desertification is the long-term result of soil erosion and soil erosion is intensity reflection in one stage of the course rocky desertification.In conclusion, soil nutrient decreased with the increase of depth in the place with soil leakage phenomenon or not. Soil nutrient increased with the decrease of slope altitude (except for total P) in no soil leakage place, while it doesn't obey the rule in soil leakage place. Good correlation between 137Cs and soil organic matter showed that soil organic matter can prove soil leakage phenomenon. Soil leakage phenomenon does exist, but surface soil cannot arrive at soil-rock interface and the soil on soil-rock interface can go through into underground river by fissure. Soil erosion coupled model in karst is built by adding adjustment coefficient and rocky desertification factor based on Liu Bao-Yuan soil erosion model. Research results can provide the necessary reference for governing pollution of underground river and afford scientific basis for accurate calculation of surface and subsurface soil erosion and afford reliable protection for governing soil erosion and rock desertification issues. Although we got Soil erosion coupled model in karst, a lot of experiences would be done to improve the laws of soil loss in the karst area later. Geological structure is complex,heterogeneous strongly here, Therefore only a large number of samples, a large number of experiments and extensive monitoring are finished, can we find soil erosion characteristics and mechanism well.