Karst Valley Watershed Surface Rainfall Runoff Modeling Using HEC-HMS

Due to the intrinsic vulnerability, Karst environment could hardly be restored if destroyed. However, since the limited bearing capacity of Karst land, overexploitation of natural resources and weak consciousness of environment protection has led to a series of water environment and resource problems under the pressure from the population, such as soil and water loss, water quality deterioration and frequent drought and flood. These water environment and resource problems seriously influence the daily lives of the local people and the sustainability of the local economy. In order to realize the sustainable use of water resources and the inclusive development between human and nature, hydrology model is involved. Hydrology model is commonly considered as an effective tool and a useful method in hydrology process prediction and water resources management. It can provide a reference for the research of the water amount and water security by applying hydrology model into rainfall-runoff cases of Karst area.Qingmuguan valley is chosen as the study area for its typical Karst valley characteristics. Characteristic parameters that modeling requires are the quantitative descriptions of the Karst phenomenon and hydro-geological condition of the study area, which are acquired through the field survey, including define the watershed boundary through DEM data and site reconnaissance, measure soil infiltration rate by tension infiltrometer, monitor the hydrological and meteorological data by on-line instrument and transform the data into rainfall-runoff statistics. Based on HEC-HMS model, sensitivity analysis and parameter calibration procedures are used to determine the model parameters. Rainfall-runoff processes under different timescale and rainfall conditions are simulated by the calibrated HEC-HMS model. Conclusions are as follows:1. Runoff characteristics under different rainfall conditions show that:for higher rainfall intensity, runoff responds quicker with bigger surface runoff coefficient and the flood crest arrives earlier. Under the conditions of moderate rain and heavy rain, the rainfall-runoff process lines behave same in shape. Comparing with the runoff process line of torrential rain, their rainfall response times are longer and subsiding processes are slower, while it goes with a sharp shape of rainfall-runoff process line under the torrential rain condition.2. According to the analysis of actual measured rainfall and runoff data, the initial interception constant and the annual recession constant of the study area are determined. They are 1.5-6mm and 0.942 respectively.3. In the established HEC-HMS distributed hydrological model of this study case, CN value is a predominant parameter that influences the whole accuracy of the rainfall-runoff process simulation, especially in runoff amount simulation and flood crest amount simulation. For CN varies between-20%-20%, flood crest amount varies between -13.2%-11.8% dependently and the total runoff amount varies between -15.8%～13.5% coherently. Beside, concentration parameter and storage parameter of the watershed also influence the runoff amount and flood crest amount. For the concentration time varies between -20%-20%, the magnitude of the flood crest amount varies between -5.3%-3.7% and the magnitude of total runoff amount varies between -16%～-1.2% subsequently. For the storage time of the watershed varies between -20%-20%, the magnitude of the flood crest amount reaches -3.3%-3.7%, and the magnitude of the total runoff amount varies between -6.3%-0.4%. The arriving time of rainfall-runoff flood crest can be also influenced by the concentration parameter when it the reaches a certain threshold, although the lag time of the flood crest remains the key parameter.4. Model validation is verified by the rainfall-runoff process simulation under different rainfall conditions and timescales, the simulation results are close to the actual measured process line. Comparing to the actual flood crest amount, the errors of two simulated flood crest amount under different rainfall conditions are 4.8% and 9.8% respectively, and the errors of total runoff amount are 1.4% and 4.9% respectively. Nash coefficients are 0.93 and 0.81 respectively and their relative coefficient is greater than 0.9. The relative coefficient of longtime scale rainfall-runoff simulation is also greater than 0.8. These analysis results indicate that HEC-HMS model is applicable to the Karst valley rainfall-runoff simulation under the boundary conditions of moderate rain and heavy rain.5. By applying the model into annual rainfall-runoff predictive simulation, the simulated annual runoff curve represents the same variation trend between runoff production and rainfall. The simulated amount of the annual total runoff is 171mm, and its rainfall-runoff coefficient is about 22%.