大沽河流域土壤水-地下水流耦合模拟及补给量估算

Numerical Simulation Coupling Soil Water/Groundwater and Estimation of Groundwater Recharge in Dagu River Basin

青岛大沽河流域的含水层主要分布在大沽河中下游沿线的狭长地带内,构成了地下水库,是青岛市主要的水源地之一。由于大沽河流域地下水超采严重,为加强该地区地下水资源的综合管理,亟需准确计算地下水补给量。土壤水和地下水耦合模拟研究是准确计算地下水补给量的重要保障。本文以HYDRUS package for MODFLOW软件的原理为基础,结合GIS技术,建立了流域尺度(4 781 km^2)土壤水-地下水流耦合模型;在综合考虑研究区大气降水、蒸发、植物吸水、土壤质地、含水层分布、土壤水和地下水相关参数、地下水开采量、土壤水分含量及地下水埋深等资料情况下,利用本模型对大沽河流域土壤水和地下水流的运动过程进行模拟。经过模型校正和实例验证表明:耦合模拟所得的土壤剖面含水量和地下水位与实测数据的拟合结果较好,土壤水-地下水流耦合模型能够较好地模拟大沽河流域土壤水和地下水的时空变化;通过模拟计算,2013年夏玉米生长期内大沽河流域地下水补给量为3.15×10~9 m^3,2012年6月16日至2013年6月16日期间内地下水的补给量为4.77×10~9 m^3,计算所得的地下水的垂向入渗补给量具有较高的可信度和准确度,可以为制定合理的流域水资源优化配置方案提供科学依据。

【Objective】The water-bearing layers in the Dagu River Basin are distributed mainly in a long and narrow strip of area alongside the middle and lower reaches of the Dagu River, forming a huge underground reservoir, which is one of the major water sources supplying water to Qingdao. Currently, the groundwater resource in the Dagu River Basin has been over-exploited causing a series of environmental problems, like seawater intrusion, surface subsidence, etc. With rapid development of the economy and population in the region, the problem of water resource shortage significantly constrain sustainable development of Qingdao. To enhance comprehensive management of the water resources in the region, it is essential to accurately assess groundwater recharge. Although much work has been done on groundwater recharge, few of the relevant researches have taken into account the transformational relationship between soil water and groundwater, which has resulted in big gaps between the calculation of groundwater recharge and the reality. This study aimed to establish a soil water and groundwater coupled model, with which to analyze movements of soil water and groundwater in the studied region, and further to calculate accurately groundwater recharge in the Dagu River Basin under the current precipitation and irrigation conditions.【Method】Based on the principle of the software of HYDRUS package for MODFLOW coupled with the GIS technology, a basin-scaled(4 781 km^2) soil water and groundwater coupled model(SWGCM)was developed. The model was used to simulate moisture movement between vadose and saturated zone,flux at the bottom of a profile was worked out through multi iteration of the Richards-1 D equation, using the HYDRUS sub-model within each time step of the MODFLOW model, and through calculation of the acquired flux, the MODFLOW model yielded a new groundwater table, which was used as water head value at the bottom boundary for the next step of calculation. With all the relevant data put under comprehensive consideration, including precipitation, evaporation, plant water absorption, soil texture, aquifer distribution,and soil water and groundwater related parameters, such as groundwater exploitation capacity, soil moisture content, and groundwater table, the SWGCM model was used to simulate movement of soil water and groundwater in the Dagu River Basin, and accuracy of the model was varied through fitting of soil water content and groundwater in the soil profiles.【Result】Results show that the simulated soil profile moisture content and groundwater table well agree with the measured data. In the calibration phase, the determination coefficient of the soil profile moisture content in the soil profile varies in the range of 0.65~0.91, and the RMSE in the range of 0.005~0.01;and that of the groundwater table does in the range of 0.52~0.68, and of 0.12~0.38, respectively, while in the validation phase the determination coefficient of the soil profile moisture content does in the range of 0.53~0.86, and the RMSE in the range of 0.006~0.011 and that of the groundwater table in the range of 0.56~0.81 and of 0.17~0.19, respectively. The fitting of soil profile moisture content and groundwater table indicates that the SWGCM model is good enough to simulate water movement in variably-saturated porous media and groundwater recharge. In the calibration phase,the groundwater recharge is 315 million m^3, and in the validation phase, 477 million m^3.【Conclusion】To accurately assess groundwater recharge, it is necessary to take into account the relationship between soil water and groundwater in movement. The SWGCM model can be used to accurately simulate spatiotemporal variation of the soil water and groundwater in the Dagu River Basin, and hence to calculate vertical infiltration supplementary capacity of groundwater in the region with quite high reliability and accuracy. Therefore, the SWGCM model may provide certain scientific basis for development of a rational allocation of water resources of the basin and an effective approach, too, to accurate assessment of groundwater recharge.