修正SWAT模型在喀斯特小流域的径流模拟研究——以羊鸡冲小流域为例

Runoff simulation of modified SWAT model in karst watershed:A case study of Yangjichong sub-watershed

在贵州喀斯特地区,地表径流通过土壤裂隙、岩溶管道等途径流失,限制了地表径流的形成。文章以龙里羊鸡冲小流域为研究区,基于2013-2019年的实测数据,修正SWAT(Soil and Water Assessment Tool)模型的裂隙流模块,修改了裂缝入渗的表示方法,对修正前后的日径流模拟进行对比分析。结果表明:率定期内纳什系数由0.32提升到0.60,决定系数由提高0.37提升到0.58。在验证期内,纳什系数由0.52提高到0.55,决定系数由0.56提高到0.60,修正后的模型与原模型相比具有更好的适用性。修正后模型的径流时间序列与实测数据更吻合,能合理估算研究区的入渗水量,为喀斯特小流域的水资源管理提供参考依据。

In the karst area of Guizhou,many factors can limit the formation of surface runoff,such as developed karst structures,strong rainfall leakage,rapidly changing hydrological processes,low water storage capacities,and lost surface runoff through soil fissures,karst pipes and other pathways.Therefore,for the protection of the ecological environment and the control of soil erosion in this area,it is of great significance for us to investigate the variation of surface runoff in karst areas and the law of flow production.The model of Soil and Water Assessment Tool(SWAT)is a distributed hydrological model developed by the Agricultural Research Center,United States Department of Agriculture(USDA-ARS),and it is widely used in the simulation and prediction of watershed runoff,sediment,water quality and so on in different environments.However,due to the dissolution and erosion in karst areas,the development of water systems and hydrological dynamics show great differences from non-karst areas.In general,scholars'modification of the SWAT model mainly focuses on the modeling of karst features,but the SWAT model has seldom been used in the study of the effect of soil fissures on runoff under wet conditions.Guizhou is located in the subtropical monsoon climate zone,with concentrated precipitation,alternating wet and dry conditions.The soil in Guizhou is characterized by strong expansion and contraction.Consequently,the fissure flow is developed from soil fissures during rainfall.Even after rewetting,the soil fissures will not close,leading to significant water infiltration,which in turn will affect surface-produced flows.However,in the original SWAT model,volume of fissures(volcr)under wet conditions will close by default,and hence water cannot infiltrate.In order to make the conditions of SWAT model closer to the real situation in the karst watershed,the calculation of the fissure volume in the SWAT model was modified in this study to balance the wet and dry soil conditions to prevent fissure closure and to ensure that soil fissures can be formed in wet soils as well.The modification process is as follows:the crack flow module was firstly activated;then the maximum crack volume of the soil layer was calculated;finally,the crack volume on the simulation day was modified in crackvol.f.Taking Yangjichong sub-watershed in Longli county as the study area,this study modified the fissure flow module of the SWAT model and the representation of fissure infiltration,based on the measured data from 2013 to 2019.Subsequently,a comparative analysis of daily runoff simulation before and after the modification was carried out so as to make the results of surface runoff simulation closer to the actual situation of the humid karst region.The study results show as follows:(1)The Nash coefficient increased from 0.32 to 0.60 and the coefficient of determination from 0.37 to 0.58 in the period of determining rate.In the validation period,the Nash coefficient increased from 0.52 to 0.55 and the coefficient of determination from 0.56 to 0.60.These data indicate that the runoff time series of the modified model is more consistent with the measured data,which can show a better applicability of the modified model than the original one.(2)There are some differences in the parameter sensitivity of the model before and after the modification,and the parameters SOL_BD,SOL_CRK,and CANMX are important to affect the soil fissure volume.(3)The modified model improved the water infiltration capacity of the soil.The soil moisture content was positively correlated with infiltration,and the surface runoff was negatively correlated with infiltration.The average daily infiltration increased from 0.23 mm to 0.84 mm,an increase of 3.65 times;the average daily surface runoff decreased from 3.98 mm to 3.14 mm,with a significant increase in infiltration and a decrease in surface runoff.At the same time,the soil water content increased from 139.53 mm to 158.30 mm,an increase of 18.77 mm.(4)The modified model showed a significant increase in infiltration and obvious seasonal differences,which was basically similar to the variation of rainfall,that is,the larger the rainfall is,the more infiltration recharge becomes,and the more significant infiltration is.The results of the study provide reference for water resource management in the karst sub-watershed.