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The Desertification Hydro-process and Carbon and Nitrogen Loss in a Typical Catchment of the Karst Plateau-GorgeCN



Abstract:Karst rocky desertification in southern China is an extreme phenomenon of soil erosion and ecological degradation caused by karst hydrological process.The high heterogeneity of rocky desertification environment and complex hydrological structure limit the understanding of hydrological process and runoff generation,resulting in insufficient understanding of the mechanism of water and soil loss.The study of hydrological process and nutrient loss in rocky desertification area is the scientific basis of soil and water conservation,which is of great significance to the sustainable development of social economy.According to the theory of karst landform development,hydrological structure,hydrological cycle and stable isotope of hydrogen and oxygen,aiming at the scientific issue of how the typical rocky desertification environment affects the hydrological process of binary hydrological structure,the Dingtan catchment is selected as the research area in Zhenfeng-Guanling Karst Plateau Gorge area,which represents the overall structure of karst rocky desertification environment in southern China.Based on the geomorphological and hydrological structure of the catchment,through the positioning observation of the meteorological hydrology,runoff plot hydrology,fissure hydrology and catchment hydrology,combined with the stable isotope technology,the data analysis methods such as wavelet coherence analysis and binary mixed model are used,the characteristics of carbon and nitrogen loss and hydrological processes in slope,fissure and catchment were studied.This paper focuses on revealing the hydrological process and carbon and nitrogen loss mechanism of karst rocky desertification,so as to provide scientific basis for the optimal regulation of water and soil resources and ecological restoration.Firstly,the spatiotemporal dynamics of soil moisture in slope surface and its influencing factors,the effects of different vegetation types on hydrological processes of surface flow and interflow,runoff and sediment yield,and the effects of runoff processes on carbon and nitrogen migration and loss were revealed.The results show that the soil moisutre in the runoff plot increases gradually with the top to bottom of slopes.The soil moisture in different slope positions and different vegetation types has time stability,especially in rainy season.However,the difference of vegetation type and coverage will lead to short-term temporal instability of soil water in dry season.Most of the rainfall is loss on the slope by seepage.Deep seepage and interflow are the main runoff generation and loss paths.The proportion of rainfall loss through slope runoff is low(<10%);the rainfall loss is mainly through underground leakage(>40%).Drought interval and field capacity are the main factors affecting the proportion of soil water storage.The slope runoff mainly comes from the interflow,but the contribution of different depth and slope position to the runoff plots of different types was significantly different.Vegetation coverage and rainfall are the controlling factors of runoff and sediment yield.Most of the soil loss caused by rainfall erosion comes from the surface soil.The runoff generation process on the slope has a certain impact on the nutrient loss,and the amount of carbon and nitrogen loss varies with rainfall.There was a positive correlation between DOC and TSN in runoff.The loss of DOC and TSN was positively correlated with runoff,but not with rainfall.The results showed that the loss of rainfall water by evaporation and leakage was relatively large in abandoned land,and it was easy to cause the loss of soil organic carbon;planting peanut could effectively reduce the loss of soil and water and organic carbon.Secondly,the dynamic of fissure soil moisture and its influencing factors,hydrological process of vadose flow and its influencing mechanism were revealed.Soil physical properties have a significant effect on the hydrological process of fissure vadose flow.The hydraulic properties and connectivity of the upper layer soil are better than those of the middle-bottom layers,which affect the rainfall response rate and retention time of soil moisture at different depths.The seasonal variation of soil moisture in the upper of fissures shows obvious alternation of dry and wet,which has a strong correlation with the seasonal cycle;the seasonal variation of soil moisture in the middle-bottom is relatively stable,and the soil moisture in the rainy season is relatively high,and the retention time is relatively long.With the increase of depth,the hysteresis of soil water content peak in rainfall events is enhanced.Short-term continuous rainfall events will lead to more sensitive rainfall response of soil water,which increasing the movement rate of fissure vadose flow;long-term drought interval events will lead to the lag of rainfall response.During the rainfall period,the temporal stability of soil moisture existed among the three fissures on all time scales.The structure of epikarst has the function of regulating and storing the rainfall.There is a mixture of old and new water in vadose flow of the three fissures during rainfall event,and residence time of vadose flow in the middle-bottom of fissures is obviously longer than that in the upper.The proportion of new water in the upper of fissures is relatively high(>30%),while the proportion of old water below 100 cm depth is relatively high(>85%).The infiltration pattern of the upper is fast recharge preferential flow,while the bottom is slow recharge matrix flow.Thirdly,the rainfall response process of catchment runoff and its impact on carbon and nitrogen loss were revealed.The response rate of rainfall to runoff is very fast,its sensitivity of rainfall response is attributed to the comprehensive effects of terrain characteristics,rocky desertification landscape and underground infiltration system development.The difference of runoff generation process between runoff and confluence is affected by the different regulation and storage caused by the distribution characteristics of catchment topography.Terrain distribution characteristics and geomorphic types control the rainfall response process and dynamic change of runoff and total outlet confluence.The rocky desertification slope in the middle and upper reaches of the catchment accelerates the precipitation infiltration rate of the epikarst zone due to the development of a large area of exposed rocks and fissures,which has the characteristics of high permeability and low water holding capacity;while the depression soil in the lower reaches prolongs the water retention time of the epikarst zone,which has the characteristics of low permeability and high water holding capacity.The carbon and nitrogen loss is mainly the migration process of the dissolved nutrients carried by the runoff generation process.The hydrological process in the catchment has a strong impact on the variations of carbon and nitrogen concentration.Due to the difference of early hydrological conditions,the concentrations of DOC and TSN are affected by the first flush effect and dilution effect.The variations ofδD,δ18O,DOC and TSN of slope runoff and subterranean river in catchment distribution and rainfall events are similar.The concentrations of DOC and TSN were affected by dilution in the process of rainfall runoff,and the confluence was more obvious.Finally,the source of confluence and mechanism of runoff generation in the catchment,the influence of geomorphic characteristics and rocky desertification environment on hydrological process were revealed.During the rainfall period,the epikarst structure and water storage capacity control the recharge process of the catchment.There are many kinds of recharge paths in the confluence,and there is a mixed mechanism in the process of recharge.In most rainfall events,the vadose flow in epikarst zone and subterranean river are the main sources of confluence in the catchment.Due to the strong permeability of rocky desertification slope in the middle and upper reaches of the catchment,the proportion of rainwater directly forming slope runoff is less,and it mainly enters into the underground river system through fissure vadose flow;when the rainfall exceeds a certain threshold,under the influence of the geomorphic characteristics and terrain differences of the catchment,this part of the subterranean river formed by vadose flow overflows from the rock fissures,joints and springs in the lower reaches,thus forming the confluence of the catchment.The mechanism of runoff generation is different in different rainfall events.The runoff generation of infiltration-excess runoff and saturation-excess runoff is intermittent,and the main mechanism is saturation-excess runoff.In the typical karst rocky desertification area of plateau gorge,the soil erosion is serious,the rock exposed rate is high,the vertical development of fissures is obvious,the vadose flow speed of precipitation in the epikarst zone is extremely fast and the vadose flow is huge,which leads to the extremely low runoff and sediment yield of the catchment;under the influence of terrain difference,the runoff of the catchment is mainly supplied by the subterranean river formed by fissure vadose flow,which has a certain reference value for understanding the dual structure of soil and water loss of karst rocky desertification.
  • Series:

    (B) Chemistry/ Metallurgy/ Environment/ Mine Industry; (D) Agriculture

  • Subject:

    Environment Science and Resources Utilization; Fundamental Science of Agriculture; Agronomy

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