Hydrologic Process Simulation In Karst Areas Of South China

There are a large number of karst areas distributed in southern China.The unique karst landform and hydrogeological conditions of the karst area,leading to significant difference in hydrological processes compared with other area.Therefore,it is difficult to adopt the existing hydrological models to accurately simulate the hydrological process.To this end,we should have a correct understanding of rainfall-runoff process in karst area.In order to efficiently utilize regional water resources,it has great significance and practical value that we establish suitable hydrological models for karst area.This paper took the middle and lower reaches of the Zuo River as the research area.Based on the analysis of topographic and geomorphologic features,meteorology and hydrology,soil and vegetation conditions in the research area,a SWAT model has been established.Performance efficiency of the model was assessed using Nash-Sutcliffe(ENS)and coefficient of determination(R2).The results shown that on the monthly scale,the value of R2 was above 0.59,and the value of ENS was above 0.57.On the daily scle,the value of R2 and ENS were all just above 0.50,which indicated that the simulation effect on the daily scale was worse than that on monthly.Through the calculation of water balance in karst area and clastic rock area,it can be concluded that:The recharge of groundwater accounted for 30.71%,which was higher than that in clastic area(14.07%).The proportion of underground runoff to total water yield was the largest(66.68%)in karst area,while in clastic rock area,the surface runoff was the highest(58.79%).In karst area,the groundwater resource modulus was between 29-43×104m3/km2 a,and the average was 35×104m3/km2 a.But in clastic rock area,the groundwater water resource modulus was just between 11-29x104m3/km2 a,and the average was 17.82×104m3/km2 a.The groundwater runoff coefficient was between 0.25-0.35 in karst area and that in clastic rock area was between 0.100.20.These indicates that there are great differences in hydrological process between karst area and non-karst area.In order to improve the applicability of the SWAT model on the daily scale,the SWAT model was loosely coupled with GA-BP model.Then the SWAT-GA-BP model was proposed in this paper.And the total daily runoff at the outlet of the basin was simulated.The simulated result shown that in the process of training,validation and test,the correlation coefficient(R)values were all above 0.90,which suggested that the simulated and measured runoff values in daily scale were fitted well.By comparing the simulation results of SWAT and SWAT-GA-BP models using predictive sample data,it was found that values of R2 and ENS of SWAT-GA-BP model(R2=0.82;ENS=0.85)were both higher than that of SWAT model(R2=0.82;ENS=0.85),the mean square deviation of SWATGA-BP model was 6.60,lower than that of SWAT model(7.62),which indicated that the simulation performance of SWAT-GA-BP model is better than a single SWAT model.In order to further study the process characteristics of surface and underground runoff in karst area,a SWAT-tank model was proposed in this paper.Firstly,the surface,soil and epikarst,aquifers with karst pipes and micro-cracks were generalized into water tank models respectively.The three water tank models were combined in series and parallel,and then loosely coupled with the SWAT model.The monthly simulation results of the S WAT-tank model showed that the rapid flow in karst condiut accounted for the largest proportion(59.86%)of the total runoff,and the slow flow in micro fissure accounted for the smallest proportion(7.28%).The runoff in soil and epikarst zone accounted for 19.19%,and the surface runoff accounted for 13.67%.The daily simulation results of each runoff were similar to that on the monthly scale.The results shown that the proportion of fast flow in the conduit was more than 50%,which accords with the unique runoff characteristics of karst area in southern China.In order to reveal the influence of land use change on the runoff,the land use change trend in the future was predicted based on the analysis of the land use change from 2000 to 2018.The SWAT-GA-BP model was used to simulate the response of total runoff under the land use change,and the SWAT-tank model was used to simulate the change of surface and underground runoff respectively.The results shown that under the influence of land use change from 2003 to 2017,the total runoff,runoff of surface,soil and epikarst,conduit and micro-cracks were all increased.In the future,if the forestland area increases and the cultivated land area decreases,all kinds of runoff will decrease.And all kinds of runoff will increase if the area of towns and other buildings increases.