| Yanzhou coalfield is located in the economically developed southwest Shandong plain area in China, where there are dense villages and developed river system, land resources is precious in contrast. Long time and high-intensive coal mining have resulted in wide range subsidence and the degree is very serious, which leads to varying degree of damage to buildings and structures, social infrastructures, cultivated land, vegetation, river system and so on. There appears different degree of ground subsidence and surface pond, thus affecting the normal watering of cultivated land and normal growth of vegetation, crops can't grow in the cultivated land without tillage conditions due to soil erosion, resulting in the destruction of original balance of agriculture ecosystem and disruption of the original relatively stable soil structure and geological environment of cultivated land. The impacts of coal mining subsidence on the river are mainly elevation reduction of levee, dam body deformation and cracking, thus affecting the normal use of the river, resulting in the river spills and loss of flood control capacity, etc. Meanwhile coal mining subsidence also had a certain effect on runoff and catchment in the Sihe basin, especially seasonal or even perennial depression water will appear in the subsidence area. The impact of coal mining subsidence on the runoff, catchment, river and peripheral agricultural production has become the main factor restricting the sustainable development of mining area.The distributed hydrological model Soil and Water Assessment Tool(SWAT) based on physical basis can realize more realistic simulation of the hydrological cycle and study the variation of hydrological due to the ability to consider the space differentiation of factors, such as precipitation, underlying surface and so on. In addition, the distributed input parameters and output results in the distributed hydrological model are more easily to be combined with remote sensing and GIS technology, the model can set land use/cover change and climate change scenarios flexibly to simulate hydrological responses to the different scenarios.In this study, the SWAT models of before coal mining subsidence, after coal mining subsidence and filling reclamation of mining subsidence land with Yellow River sediment are established respectively based on subsidence prediction theory and method. The measured daily flow data of Shuyuan hydrological stations belongs to the Sihe basin are used for the calibration and verification of SWAT model, reaching the conclusion that the SWAT model constructed applies to the Sihe basin, then the SWAT model verified is used to simulate and predict the runoff of Sihe basin. Before the establishment of the SWAT model, the basic database is needed to prefer for three stages of simulation, including spatial database and attribute database. The spatial database includes the DEM, the digitized river systems map, the land use map, soil type figure, etc. The attribute database includes the weather generator, the meteorological data(precipitation, temperature, radiation and relative humidity), land use database, soil attribute database, etc. The spatial database and attribute database prepared are used to build the SWAT model of Sihe basin, the runoff of Yanzhou coalfield in three stages that before coal mining subsidence, after coal mining subsidence and filling reclamation of mining subsidence land with Yellow River sediment are simulated respectively by using the built SWAT model, to achieve the river flow out, the total flow, the surface runoff, the underground runoff, the lateral inter flow amount and soil moisture content of three subbasin of coal mining subsidence area in three stages respectively.The SWAT model construction of before subsidence is based on the DEM map and land-use map of Sihe basin in 2009, among which the land-use map is extracted from the Landsat 4-5 TM remote sensing image by using ENVI software, the soil type figure in 2009 is obtained on the basis of soil data from the Harmonized World Soil Database(HWSD). The SWAT model construction of after subsidence is based on the DEM map of Sihe basin in 2014, among which the DEM map is obtained from the calculation of the grid format of coal mining subsidence is oline in 2010-2014 and original DEM map in 2009 of Yanzhou coalfield by using the raster calculator tools of ArcGIS software. The land-use map in 2014 is extracted from the Landsat 8 OLI_TIRS remote sensing image in 2014 by using ENVI software. The soil type figure in 2014 is obtained on the basis of soil data from the HWSD with corresponding modification of soil physical and chemical properties of the mining subsidence area in Yanzhou coalfield. The SWAT model construction of after filling reclamation of mining subsidence land with Yellow River sediment is based on the DEM of Sihe basin in 2009, assuming that all subsidence area of Yanzhou coalfield caused by coal mining subsidence in 2010-2014 is reclaimed by filling reclamation with Yellow River sediment, meaning that the topography of Sihe basin is restored to 2009's original terrain, thus the original DEM in 2009 is adopted. The land use map of after filling reclamation is achieved from the modification of Land use type in Yanzhou coalfield on the basis of land use map in 2014. The soil type map after filling reclamation is achieved through the modification from the soil type in 2010-2014 to that contains the Yellow River sediment of Yanzhou coal mining subsidence area, with corresponding modification of soil physical and chemical properties of filling reclamation region in the soil attribute database.The methods of field sampling, laboratory analysis and numerical simulation are mainly adopted to study on the influence of pre coal mining subsidence, post coal mining subsidence and filling reclamation of mining subsidence land with Yellow River sediment on the runoff, within Sihe and Baimahe basin in Jining city. The main conclusions obtained are as follows:(1) The SWAT model is applicable to Sihe and Baimahe basin in Jining city, with a high precision of runoff simulation.The Sihe and Baimahe basin in Jining city are selected as the study area, and Yanzhou coalfield is located in the No. 12, 13 and 14 subbasin. The measured runoff data of Shuyuan hydrological station from 1998 to 2007 are used for the calibration of the SWAT model, meanwhile the measured runoff data from 2008 to 2010 are used for the verification of the SWAT model, with a high precision of simulation. The conclusion that the SWAT model is applicable to Sihe and Baimahe basin in Jining city is achieved.(2) The measured historical precipitation, temperature and runoff data are analyzed, and the correlation analysis between precipitation and average monthly flow is made, meanwhile the correlation analysis between precipitation and groundwater level is also made.The precipitation and temperature data collected of Sihe basin in 1998-2014, measured flow rate data of hydrological stations in 1998-2010 and the real data of groundwater level of each well logging along the Sihe are analyzed, the correlation and variance between rainfall and flow of Sihe basin are analyzed by using SPSS software, those between rainfall and groundwater level are also analyzed in the same way.There are fluctuations in the rainfall of the Sihe basin, without greater difference in the study period. And there are also fluctuations without greater difference in the lowest and the highest temperature of the basin in the study period. But great fluctuations exist in the surface runoff of the basin over the time. The rainfall shows a highly significant correlation with the groundwater level mostly, with greater difference in the groundwater level of each well logging. Meanwhile the method of Least Significant Difference(LSD) is used for the difference analysis of the groundwater level of each well logging in various years, reaching the conclusion that there are various differences in the groundwater level of each well logging in different years.(3) The coal mining subsidence situations of Yanzhou coalfield from 2010 to 2014 are predicted by using probability integral method, reaching the conclusion that the coal mining subsidence situations of Yanzhou coalfield from 2010 to 2014 are mainly severe, and the distribution interval of surface subsidence value is 0.5-7.5 m, and the total area of coal mining subsidence of Yanzhou coalfield is 9.14 km2.The coal mining subsidence situations of Yanzhou coalfield from 2010 to 2014 are predicted by using probability integral method, achieving the coal mining subsidence isoline of Yanzhou coalfield from 2010 to 2014. And the coal mining subsidence isoline of those are analyzed to obtain the surface subsidence value and the coal mining subsidence area in the coal mining subsidence region, by using the statistical method of ArcGIS software. The distribution interval of surface subsidence value is 0.5-7.5 m in Yanzhou coalfield, among which the distribution intervals of surface subsidence value are respectively 4-7.5 m, 3.5-7.5 m and 0.5-7 m in No. 12, 13 and 14 subbasin. According to the land damaged degree classification standard of Yanzhou coalfield, the light subsidence, moderate subsidence and severe subsidence area are acquired in the study area, and the coal mining subsidence area of No. 12, 13 and 14 subbasin are summed respectively. The total coal mining subsidence area of Yanzhou coalfield from 2010 to 2014 is 9.14 km2, among which the light subsidence area is 2.06 km2, the moderate subsidence area is 1.27 km2, and the severe subsidence area is 5.81 km2. From 2010 to 2014, the coal mining subsidence area of No. 12, 13 and 14 subbasin are respectively 2.77 km2, 1.06 km2 and 5.31 km2.(4) The three stages of pre and post coal mining subsidence, after filling reclamation of mining subsidence land with Yellow River sediment are selected by using scenario analysis, the runoff changes of basin in three stages are simulated respectively through the SWAT model.The number of subbasin was reduced from 22 to 20 after the subsidence, and the number of subbasin of upstream doesn't change, and the number of subbasin of downstream increases with the changes of the runoff direction. Cultivated land and construction land decrease compared with pre coal mining subsidence, on the contrary, forest land, water area and unutilized land increase, it proves that coal mining has a significant impact on the land use type area of the basin. Due to coal mining subsidence, the potential evaporation and actual evaporation in the subbasin is increasing. The soil water content(SW), surface runoff(SURQ), underground runoff(GWQ), lateral interflow amount(LATQ) and water yield(WYLD) of the No. 12, 13 and 14 subbasin increase after coal mining subsidence, meanwhile permeability water(PERC) of the root zone in the time step decrease. The runoff depth and runoff coefficient of the No. 12, 13 and 14 subbasin increase after coal mining subsidence, and affected by the coal mining subsidence of the subbasin in the Yanzhou coalfield, the runoff depth and runoff coefficient of Sihe and Baimahe basin also increase. The runoff depth and runoff coefficient of the basin don't restore to the condition of pre coal mining subsidence, on the contrary, they continue increase after filling reclamation of mining subsidence land with Yellow River sediment. The logarithm of runoff coefficient and runoff depth all have significant correlation in the stages of pre subsidence, post subsidence and post reclamation. And the potential evaporation and actual evaporation of the No. 12, 13 and 14 subbasin increase in various degrees after filling reclamation of mining subsidence land with Yellow River sediment, it means that the potential evaporation of each subbasin after filling reclamation of mining subsidence land with Yellow River sediment doesn't restore to the condition that pre coal mining subsidence, however, the potential evaporation and actual evaporation in the stages of post coal mining subsidence and filling reclamation don't have significant difference. There exists little difference in the annual average water yield per unit area of the entire basin pre coal mining subsidence. The annual average water yield per unit area of the No.12 and 14 subbasin increase and is higher than the entire basin post coal mining subsidence. The annual average water yield per unit area of the No. 12 and 14 subbasin increase, and is also higher than the entire basin after filling reclamation. Therefore, the increase of the basin area has the ability to decrease annual average water yield per unit area, and it can be inferred that coal mining would have smaller impact on the annual average water yield per unit area with larger basin area.(5) Revealing the relational model that the deformation parameters of coal mining subsidence have impact on the basin runoff.The relational model that the deformation parameters of coal mining subsidence have impact on the basin runoff has been studied, the fitting degree between the maximum surface subsidence and the factors of the basin runoff is consistent with that between the maximum horizontal movement value and the factors of the basin runoff, the factors in descending order are as follows: underground runoff>water yield>runoff coefficient>surface runoff>average annual runoff depth, and the factors show a trend of decrease after the first increase. The fitting degree of the maximum surface incline value is consistent with the maximum horizontal deformation value, the factors in descending order are as following: average annual runoff depth>runoff coefficient>surface runoff>water yield>underground runoff, and the average annual runoff depth shows a trend of increase after first decrease with the maximum surface incline value and the maximum horizontal deformation value increasing, however, runoff coefficient, surface runoff, underground runoff and water yield show a trend of increase after first decrease, then decrease. The fitting degree between the factors of the basin runoff and the coal mining subsidence area in descending order are as follows: surface runoff>water yield>underground runoff>average annual runoff depth>runoff coefficient, and the factors show a trend of increase after first decrease with the coal mining subsidence area consecutive increasing.The fitting degree between ground movement and deformation parameters and the water yield in descending order are as follows: coal mining subsidence area >maximum horizontal movement value>maximum surface incline value>maximum surface subsidence>maximum horizontal deformation value. The water yield show a trend of decrease after first increase with the maximum surface subsidence and the maximum horizontal movement value increasing, showing a contrary trend with the coal mining subsidence area increasing, showing a trend of increase after first decrease, then decrease, with the maximum surface incline value and the maximum horizontal deformation value, and the impact of the coal mining subsidence area due to coal mining is the most significant.(6) Aiming at the changes of runoff of basin pre coal mining subsidence, post coal mining subsidence and after filling reclamation, some countermeasures and suggestions are put forward. Coal mining can be carried out from downstream to upstream, and strip shrubs or small trees can cover the coal mining subsidence area of the basin, of which the slope is larger, along the contour of the steep slope.It is suggested that the coal mining be carried out from downstream to upstream, which can reduce the influence on the hydrology of whole basin, and also reduce the unforeseen ecological loss to a great extent. If according to the traditional coal mining layout and the timing, it may be resulted in changes of the subbasin in the downstream firstly, meanwhile, coal mining in the upstream may have impact on the water allocation and the course of the runoff of the downstream, which can greatly increase the uncertainty of water allocation and the difficulty of the management and utilization of water resources. Secondly, if coal mining be carried out from upstream to downstream, the contaminants produced from the coal mining process are more likely to pollute the subbasin in the downstream, and the technologies and the cost needed for governance would increase. In addition, the coal mining be carried out from upstream to downstream would lead to instability of drought and flood, it may cause cutoff of downstream during the dry years(runoff filling depression), on the contrary, it can also cause potential risk of floods to downstream in a wet year, because the study shows that mining subsidence increases runoff yield in the basin.It's difficult to restore the runoff generated mechanism and runoff yield of the basin fundamentally with reclamation only on the coal mining subsidence area. Aiming to achieve restoration fundamentally, it's recommend that the planed and step by step governance in advance should be made on the catchment area with great slope of coal mining subsidence area. Strip shrubs or small trees can cover steep slope along the contour, it's also recommend that the seeds of native herb could sow between trees and shrubs. By increasing the surface roughness to decrease water flow speed, thus reduce current flow scour and soil and water loss radically. It can keep and store soil fertility of catchment area in the coal mining subsidence basin.The feasibility of SWAT model construction in this region is discussed and laid foundation for the simulation of surface runoff and soil erosion, the mutual recharging relationships among waterlogged area caused by coal mining, Sihe collection area and surface runoff are quantitative studied, providing reference to the study on the collection of Sihe basin and the influence on surface runoff caused by coal mining subsidence, in addition, it provides reference for the water resources allocation of the basin, reasonable land use planning and the management of coal mine subsidence land, aiming to reduce the effects of coal mining on the ecological environment of Sihe basin. |
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