| Karst water resource has a significant influence on economy development of karst area in south-east China. Generally there is abundant rainfall and water resource, however, high non-homogeneity caused by long time and strong karstification which leads to double structures of surface and underground let water resource be a main controlling factor of economy development. To be a special water resource of karst aquifer system, overland flow stored or not stored by karst fissure owns high velocity. This water resource flows away quickly after rainfall hardly used by humans. Sometimes it can create sever inland inundation to influence local people's normal production and life. Overland flow is also the main factor of molding topography, soil loss and pollution of karst aquifer. Quantity of overland flow has a direct relation to the intensity of soil loss which measures the rate of surface erosion and underground leaking. Through the research of the mechanism of overland flow yield. we can understand the occurrence conditions and effect factors. The stony desertification severely limits local economy development, so it's very hurry to know the main style of soil loss in karst area. This could provide fundamental basis theory for stony desertification control which is be or being launched.Yaji karst experimental site, peak-cluster depression, is choosed to build runoff plot on hillslope scale in this research. The clear answer is given to whether there is overland flow or not. Then the runoff styles and conditions of overland flow and contribution to subterranean river are discussed. Though hydrochemistry of overland flow, waterlevel and hydrochemistry of soil boreholes, situ phenomenon, the mechanism of overland flow yield is realized initially. Combining with the physical structure of karst hillslope, we discuss the distribution of overland flow. All of this provide epistemic basis of overland flow and soil loss in karst area.The result shows there is overland flow in karst area but with a little quantity. There are five overland flow processes with the rainfall threshold of 64.4mm/day which mainly come from with a little local Hortonian flow. Using water balance model, if infiltration capacity is ignored, the relationship between rainfall for overland flow occurrence (P) and pre-rainfall (W) is P=33.5-0.0795W in yaji site. The runoff coefficient of overland flow ranges from 0.48% to 0.71% which is similar with other national conclusions. Most water infiltrate into underground aquifer. During the storm, the quantity of overland flow accounts for 3.5% of spring S31 in the appearance of overland flow which accelerate formation of discharge peak and change of hydrochemistry.The research of hydrochemistry of overland flow shows K+,Na+,C+,SO42- is stable while Ca2+,HCO3- change a lot. Though the analysis of Sr. overland flow contain some water stored a short while by karst fissures in yaji site. Some special hydrochemistry ions of overland flow including Ca2-HCO3,pH,T and Ec are monitored in bare-rock vadose zone and soil covered vadose zone. In bare-rock vadose zone, hydrochemistry of overland flow is stable with concentration of Ca2- ranging from 24mg/L to 38mg/L and concentration of HCO3- ranging from 1.3mmol/L to 2.2mmol/L. Overland flow contains Hortonian flow from the rock surface and saturation flow from fissures. Because of soil CO2 and rain dilution effect, hydrochemistry of overland flow changes violently and shows overland flow contains rain water, epikarst water and soil water. In earlier stage of rainfall, overland flow only contains rain water and soil water. Then epikarst water increases gradually and overland flow is all epikarst water in later stage of rainfall. This all indicates epikarst water has a close relationship with overland flow.The strong karstification of epikarst zone leads to fast epikarst flow which easily causes a great amount of lateral flow. The yield styles of overland flow can be divided into four types:1. Hortonian flow due to partial imperfect development of fissures in epikarst zone or low infiltration of soil.2. There is aquitard in soil or not fissures in rock surface leading to saturation of soil layer which finally create saturation flow.3. Partial epikarst water flow out caused by non-homogeneity of fissure development which leads to saturation of soil layer and partial epikarst zone and then saturation flow yields.4. Saturation flow caused by saturation of whole epikarst zone due to abundant lateral flow supply from hillslope. Development density of epikarst zone and thickness of soil layer are different due to the difference of slope degree and runoff condition. So different part of slope has different overland flow yield style which creates special " geomorphy effect" and "scale effect" in karst hillslope. Generally, Hortonian flow often occurrences in the middle of karst hillslope because of non-development or weak development of epikarst zone. Epikart zone develops moderately in gradual slope part. This can easily cause some epikast water flow out which can create saturation flow. The overland flow creating in this area will infiltrate again due to the improvement of infiltration. So the overland flow in this area is discontinuity. In the area around depression, saturation flow is caused by saturation of whole epikarst zone due to a great amount of lateral flow and is continuity. Saturation flow created only by rain and soil water distributes in slope toe and bottom of depression.Though the monitoring of overland flow quantity, hydrochemistry, waterlever and hydrochemistry of soil boreholes, we discuss the yield styles and difficulty degree of occurrence of overland flow. According to the evolution law of peak-cluster depression, the position of different overland flow yield styles is given. All of this has a significant role for us to recognize not only occurrence and distribution of overland flow but also soil loss in karst area. |
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