Study On The Process And Characteristics Of Water&Soil Leakage Loss In Shallow Fissures On Sloping Lands Of Karst Plateau

Fragile ecosystem in the Southwest Karst Area has been degraded greatly due to the special ecological environment and long-term human activities in the area.Water and soil are the most important restrictive ecological factors in the karst area.The combination of surface water and soil resources determines the stability of their ecosystems.The more abundant the soil and water resources,the more stable the regional ecosystem is,on the contrary,the more fragile it is.It is well known that the underground leakage loss of soil and water has become the most serious and urgent ecological environmental problem in the karst rocky desertification aream,where shallow pore fissures(hereinafter referred to as “fissure”)provide an important migration channel for the underground leakage loss of soil and water loss in this area.To reveal systemly the leakage loss process,characteristic and mechanism of water and soil in fissures on sloping lands in the karst area,taking fissures on sloping lands in karst plateau of Guizhou province as the research object,we have systemly analyzed the type and structure characteristics of fissures.Based on that,reseaches on process simulation of water and soil leakage loss in fissures,migration process of water in fissures and leakage mechanism of soil in fissures were conducted through the combined method of indoor simulation and field observation.Results presented could provide a theoretical basis for the implementation of the national ecological restoration of rocky desertification and ecological engineering.The main results are as follows:(1)Based on different classification basis,fissures can be classified into different categories.For example,fissures can be divided into pores and fissures in plane morphology and into funnel type fissures,vertical rectangle fissures,trapezoid fissures and oblique rectangle fissures in the form of section etc..Based on field investigations and geostatistics analysis,we found that the fissures are developed and distributed under different levels of rocky desertification.53% of fissures among 302 investigated belong to potential(20%-30%)and mild rocky desertification(30%-50%)levels.70% of fissures investigated support shrub-grass and even arbor-shrub-grass communities,moreover,71% of fissures selected present more than 30% of vegetation coverage.The width of 43% of fissures observed mainly concentrate between 25 cm and 75 cm,and the dip angle of 87% of fissures observed is higher than 50°.Soil types filled in fissure are mainly yellow soil and calcareous soil,in which the categories of fissure filled with yellow soil are single,vertical rectangle,and that filled with calcareous soil are diversity,including funnel type,vertical rectangle,trapezoid and oblique rectangle fissures etc..As the object of underground leakage loss,fissure fillings are different in material composition,water characteristics,structural characteristics and nutrient content among different types of fissures.(2)Based on rainfall simulation tests for researching fissure leakage process of water and soil,we found that heavy rain,gentle slope,developed fissures and severe rocky desertification level would aggravate the fissure leakage loss of water that is one of the main ways of water loss on karst slopes.The ratio of fissure leakage loss of water is higher than 50.8% under different rainfall intensities,slope gradients and bedrock bareness rates.Soil loss on karst slopes is a changing process with rainfall intensity and slope change,that is,soil loss changes from underground leakage loss to surface loss with rainfall intensity or slope increased.Soil loss is dominated by underground leakage loss for small rain intensity(15~30 mm/h)or gentle slope(5°~15°)conditions while dominated by surface loss for heavy rainfall(50 mm/h or above)or steep slope(20°~25°)conditions.The ratio of fissure leakage of soil is generally less than 48.4% under different fissure densities and bedrock bareness rates,in other words,the total soil loss on karst slope is dominated by surface loss,and there is a critical rainfall intensity(30~50 mm/h)for forming surface soil loss.Rainfall intensity is a leading factor determining the amount of fissure leakage loss of water and soil and the relationship between the two.Slope degree and fissure density have a certain effect on the fissure leakage loss of water,but do not play a leading role in the fissure leakage loss of soil.Bedrock bareness rate plays a leading role in the fissure leakage loss of water and soil,in which there is a critical bedrock bareness rate(30%)of soil fissure leakage loss.Fissure runoff yield rate behaves extremely significant(P<0.01)linear relationship with rainfall intensity,is insignificant positively correlated with fissure density and bedrock bareness rate,but extremely significant(P<0.01)negatively correlated with slope degree.Fissure leakage rate of soil behaves extremely significant(P<0.01)exponential function relationship with rainfall intensity and extremely significant(P<0.01)linear negative correlation with slope degree.The fissure leakage rate of soil present a trend that decrease firstly and then increase with increase of fissure density,which reaches the minimum value at 3% of fissure density.On the contrary,it shows a trend of first increase and then decrease with increasing of bedrock bareness rate,which reaches the maximum value at 30% of bedrock bareness rate.(3)Single ring infiltration method was conducted to study water infiltration process in fissures.We found that the soil water infiltration process in different fissures can be divided into two stages: rapid reduction and stabilizing stage.There are great differences in water infiltration process among different fissures,the infiltration rate changes between 0.05 mm/min and 5.89 mm/min affected by the properties of fillings in fissures(such as the total amount of soils and the existence of the Quaternary Red Clay bellow the soil layer),interface characteristics of rock on both sides of fissures and the connection of the lower part of fissures.It is found that rock-soil interfacial flow is the most important preferential flow form in fissures through dye tracer tests on different fissures.Dyed area ratio of soil + rock profile,soil profile,rock profile of fissures range between 0.12~0.48,0.06~0.43 and 0.02~0.16,respectively.The preferential flow of different fissures mainly occurs at the 20 cm soil layer.The preferential flow path is mainly the two sides of rock face;moreover,the rock-soil interface flow on the two sides can pass through the yellow clay layer with poor permeability.Besides,the rock face dyed can reach the depth of 50 cm and below.The development of fissure preferential flow is mainly affected by the undulating condition of the rock face,the degree of inclination,the fillings(including the yellow clay layer)and the plant roots etc..(4)A combining method of paint markings,REE tracer technique and soil particle analysis was conducted to study the leakage loss process of soils in fissures.It is found that rainstorm or downpour is the key factor of soil fissure leakage loss,and its leakage form is mainly soil creep.Soil creep displacements of different fissure at 104.5 mm rainfall event(including once rainstorm,twice moderate rains and several times light rains)changes between 1.0 cm and 2.5 cm,accounting for more than 62.5% of the displacement at 332.7 mm rainfall event,while the soil creep displacement of fissures just range from 0.2 cm to 0.3 cm at the larger rainfall of 181.5 mm(including once heavy rain,four times moderate rains and several times light rains).Variations in the concentrations of Ce,La,Sm and the particle content with different particle sizes in rock-soil interfaces and soils of six fissures selected under 3 precipitation events(104.5,151.2 and 332.7 mm)show that the fissure soil does not leak down uniformly,but some particles at the soil layers or rock-soil interfaces leak to the lower layer at random in the process of creep leakage loss.In other words,the occurrence of soil layers and particles are accidental,and the soil particles in the rock-soil interfaces and the soil layers of fissures have the possibility to leak down.Facts proved that the soil creep leakage loss in fissures is a complex process which is determined by the internal factors such as fissure structure,fillings characteristics and bottom connectivity,and such external factors as rainfall etc..