Study And Application Of Hydrologic Model In Karst Basin

There are many carbonate rocks widely distributed in China, especially in southwest China. In these areas, different sizes and forms of karst underground river system, with addition of the basin surface water system, were known as karst watershed. The abundant water resource in these karst watersheds has become an important pillar of economic development. However, due to the complex mechanism of karst basin water cycle, the transformation of surface-groundwater and space variability of karst aqueous medium changed frequently. The very unevenly distribution of karst water resources in time and space caused water shortages in these areas. Thus, for rational planning of the development and utilization of regional water resources, assessment and modeling of hydrological processes are essential. Base on the former methods with superior difficulty and low accuracy, in-depth study of karst features and mechanism of karst basin water cycle, with respect to construction of the physical and mathematical model of karst hydrological processes, accurate assessment and prediction of river basin water cycle are the main study content of this thesis. And it has important theoretical and practical significance, especially to the rational utilization of water resources and underground karst water damage prevention.In this thesis, a typical karst watershed, Hamajing watershed, in southwestern China was studied. This watershed belongs to a small valley basin in Karst area, with a sound hydrological monitoring facilities and meteorological data. Based on "Water Resources and Hydropower Engineering design calculations norms (SL 44-2006) ", the design flood was estimated by the use of heavy rains in this area. However, according to the contract between actual measurement flood and design flood, the maximum peak measured values were only about one-fourths of the design flood peak. Although the norm proposed the use of "sinkhole rate" to calibrate designed flood, the amount of the corrected amount of flood still has large difference with the measured peak. Therefore, it is necessary to build a distributed hydrological model in karst areas.In the study area, there are different types of Karst morphology such as karst depressions, karst funnel, hole, karst conduit and other well developed karst forms. These forms are the product of water and carbonate rock interaction. The process is actually the transformation process of carbonate rocks by water. Some main controlling factors, including soluble carbonate rocks, water dissolution ability, permeability rock, water flow and so on, are interrelated.According to field investigation, Hamajing watershed can be divided into two kinds of combination types, i.e. peak cluster depressions and peak cluster valleys. Peak cluster depression contains some small closed basin without perennial surface water. Depressions which have thin layers of coverage and exposed bed rock are connected by underground rivers. And In the vertical profile, it can be divided into three main hydrological belts as subcutaneous belt, vadose zone and pipe flow belt. Peak cluster valleys, mainly slope Rift Valley, contain a plenty of soil coverage. Typically there are surface rivers developed in the area. Some of the sinkholes, funnel directly exposed on the surface. In the vertical profile, it can be divided into four hydrological belts with that overlay zone, subcutaneous belt, vadose belt and pipe flow.Karst pipes are a widespread groundwater storage and drainage systems in karst area, especially in South China Karst. Karst conduit flow is the underground water moving in the karst pipes. According to the formation mechanism, karst conduit is not only constrained by many factors such as lithology, structure, geomorphology, hydrology etc., but also have their own complex features. The Karst valley conduit of toad neck is developed from a single branch to a network flow. According to the field trials tracing, it takes 64.6h that surface water of Jiasha Creek go through underground river system to Wuliping river export, with an average flow rate of about 1115m/d. And it takes 26.5h that surface water of Xiangxiping go to Gaoyan underground river export, with an average velocity of about 1811m/d. That is, the inj ection of rainfall from Jiashaxi to the toad neck reservoir need around 4days, which means that the pipe flow rate is lower than the velocity of surface water.TOPMODEL model is constructed by spatial variation of terrain. Based on DEM derived topographic index (Lnα/tanβ), this model was used to describe the flow trends and reflect the impact of changes in the hydrological cycle terrain basin. According to the principle of gravity drainage of runoff along the slope movement principle, runoff area changes such as changes of saturated water in surface or underground area, were simulated by this model. TOPMODEL model structure and the concept is relatively simple with less parameters, taking advantage of the terrain data readily available, and can be used to calculate runoff yield in basin with no information. But in the Karst region, TOPMODEL has the following shortcomings:1) There is no direct contact between topographic index distribution and underground river flows out of the way. That is, the space distribution of groundwater systems is not controlled by topography, but mainly depends on geological underground karst fissures, pipes or underground river. Therefore, the truth will be distorted or failed relying on DEM model in karst water system.2) It is too simple to reflects the groundwater hydrology cycle in this model. In karst regions, groundwater karst aqueous medium are usually pipes or ground rivers, or their spatial distribution are complex and irregular, or its permeability is usually changes over time. But these are not to be described in this model.3) Due to the impact of ground river entrance, river water capacity is limited. The hysteresis of flood can not be resolved by this model.Base on these results of this study, the main conclusions obtained in this thesis are as follows.1) Toad neck Basin can be divided into three general tributaries and 108 sub-basins. The three tributaries are the Yuantou River in the east, the Long Beach river in the middle and Jiashaxi River in the west. There are both surface and underground river segments in all three rivers. Finally, they are converged into the toad neck reservoir through karst conduits. According to Strahler river classification method, constituted river basin structure with 54 headwaters and 5 classified levels of Hamajing watershed has been devided, which reflect this small size basin is a high magnitude water systems with complex structure.2) The toad neck basin can be divided into peak cluster depressions and peak cluster valleys. In peak cluster depression areas, sinkholes are developed in depression and there is no surface water exists. Surface water was converged into the main river through the dark underground river. On the flat bottom of depressions, red clay often deposited as relatively high moisture layer. The depressions drainage flows into the ground river through the main sinkholes or vent shaft. In Karst Valley region, the terrain is relatively flat with surface watershed, while valley with steep edges often covered with corrosion residual brown, red clay or drifting alluvium at the bottom. The water sources are from karst springs, eventually flowing into the underground river. Due to limited data base, distributed watershed model parameters are divided into two zones in order to facilitate the setting of the model parameters.3) The Hamajing watershed hydrology model constructed by the using of KARST-TOPMODEL was used to simulate the 11 July 2005 flood events. According to the measured data, the flood is equivalent to P=2% of the rainstorm (i.e. once in fifty years). There are three detention entrance was founded with flood hysteresis. Baihe Dong river entrance was the first one. Followed by Long Beach River river entrance, Jiasha creek was the last one. The largest capacity of flood hysteresis is 694,800 m3,563,900 m3 and 1,503,600 m3 respectively. Although the capacity of flood hysteresis is affected by the water capacity flowed from upstream river, the critical reason is the ratio of the area occupied by the relevant karst depressions. With high area ratio, the flood regulation and storage effect of karst depressions is stronger, and its weakening effect on the peak of flood is more obvious.4) By calibration and validation of three different flood process from 2003 to 2005 combined with fitting these year hydrological processes, it was proved that the improved KARST-TOPMODEL is reasonable. And its parameter calibration was reliable.5) By using of Hamajing watershed model constructed by the KARST-TOPMODEL, flood processes with different frequencies and rainfall intensity were simulated. When rainfall is small, the process of flood is consistent with detention process without considering the flood hysteresis. But as rainfall increases, the peak shape becomes increasingly flat and the flood process becomes longer. In other words, regulation and storage functions of karst watershed become apparent.The main feature of this thesis is reflected in three aspects.1) Base on field karst hydrogeology survey, combined with DEM model building, the whole basin was divided into 108 sub-basin. Then each sub-basin was calculated separately by using of TOPMODEL model. These results of the various sub-river basins were converged to calculate export. The model convergence was on the use of Muskingum method. The difference between karst underground pipes and surface runoff process can be detected.2) Set the blood hysteresis process in the detention program. It can make the simulation process is more closer as the actual process. And the regulation and storage functions of karst watershed become apparent.