波涌灌间歇入渗饱和-非饱和土壤水分运动数值模拟及试验

考虑到地下水浅埋对上层包气带水分分布造成一定影响,该研究结合波涌灌技术,对地下水浅埋下间歇入渗的土壤水分分布特征和运动规律进行了分析,建立了基于饱和-非饱和土壤条件下一维间歇入渗水分运动模型,根据试验实测资料采用Hydrus-1D软件反推土壤水分运动参数,并对入渗过程进行了模拟。在此基础上,确定了饱和导水率的估算模型。结果表明:所建参数估算模型较好地反映了饱和导水率与间歇周期数、循环率以及周期时间之间的相关关系,所建水分运动模型模拟值与实测值比较,累计入渗量、土壤含水率以及湿润体运移距离总体相关系数高于0.96,均方差在0.5以内,吻合度较高,能够较好地描述了地下水影响条件下波涌灌间歇入渗饱和-非饱和土壤水分运动特征。该研究为波涌灌技术进一步发展奠定了科学基础。

Numerical simulation and experiment of soil moisture movement in saturated-unsaturated soil under surge irrigation

Abstract: Studies have showed that the high water-saving rate and optimal irrigation can be realized by using the surge flow irrigation. However, the investigation is not sufficient in the rule of water movement under intermittent infiltration in saturated-unsaturated soil condition. In order to explore the effect rule of surge irrigation on the water distribution in the aeration zone under the condition of shallow groundwater, we studied the water distribution and the effect law of infiltration through the combinations of intermittent infiltration and indoor water infiltration with model simulation. The laboratory test aimed to observe and analyze the data about soil moisture, cumulative infiltration and wetting front by irrigation with shallow groundwater during surge flow irrigation (step1). The part of numerical simulation mainly included the determination of the design parameters of soil water movement (step2) and the establishment of the mathematical modeling for intermittent infiltration (step3). Finally, a case was used to test the rationality and validity of the numerical simulation results (step4). The laboratory test included three cycling rate levels of treatment (1/2, 1/3 and 1/4, respectively) and each level of treatment included 3 repeats. Based on the theory of soil water dynamics and the technology of surge flow irrigation method, the one-dimensional water movement model in the intermittent infiltration under the saturated-unsaturated soil condition was established. With the application of the measured data for three treatment levels in the Hydrus-1D software based on finite element modeling for the analysis of water flow and solute transport in variably saturated porous media, the water movement parameters (the parameters of Van-Genuchten model) were calculated with the Hydrus-1D software. Furthermore, according to determined optimal parameters, we established a saturated hydraulic conductivity estimation model under different circulation rates and different intermittent cycles in the process of the intermittent infiltration. At last, by comparing the data of simulation and the data measured in the experiment, the validity of the proposed model was tested by the analysis of the relationship among the soil moisture content, cumulative infiltration and wetting front respectively. The results showed that after the simulation, the estimation model could reflect the correlativity among the three parameters (circulation rate, cycle number and cycle time), and the saturated hydraulic conductivity coefficient was calculated with these parameters. To verify the estimation model, the result of calculated values was compared with the simulated ones, and the relative deviation of saturated hydraulic conductivity was as small as -7.7%-8.9%, showing good consistency between the calculated and simulated data. From the comparison between the spot observation and the simulation data of soil moisture, cumulative infiltration and wetting front, we drew the conclusion that the calculated and simulated data had a similar change tendency by using two commonly-used criteria: root mean square error and coefficient of determination. The coefficient of determination of soil moisture was upon 0.952 and the one of cumulative infiltration was up to 0.953. The coefficient of determination of wetting front was 0.93 at least; root mean square error above was all less than 0.5, reaching an extremely remarkable level. The result of simulation could describe the characteristics of the saturated-unsaturated soil water movement in intermittent infiltration well under the condition of shallow groundwater level. The study of the paper laid a scientific basis for the further development of the surge irrigation.