(1) According to the measured negative pressure H and solution concentration C of soil profile, the initial values of negative pressure and solution concentration of each node on the profile are given by cubic spline interpolation method.
(2) Calculate evaporation e according to meteorological data and surface soil water content.
(3) Calculate the root water absorption rate Sr by using the root water absorption model.
(4) Solve the moisture equation and give the distribution of soil negative pressure at each node at the end of the period.
(5) Calculate the pore water velocity v of soil from the negative pressure distribution at the end of the period.
(6) Solve the salt equation and give the concentration distribution of soil solution at each node at the end of the period.
(7) Test the model with measured data.
The data used in this model verification is1April 30th, 998 to1September 30th, 999, that is, 5 18 days. The time step is 1h and the space step is 1cm. A large number of data needed in the calculation, such as initial negative pressure and initial concentration of nodes, rainfall in each period, water surface evaporation, buried depth of groundwater level, groundwater salinity and other information, are provided in the form of data files. Because the negative pressure of the three monitoring sections is monitored by vacuum gauge tensiometer and expressed in kPa, it is converted into cm first when calculating; The concentration of soil solution is monitored by a salt sensor, expressed as conductivity (mS/cm), and must also be converted into solute concentration (g/L). The dynamic boundary of the lower boundary changes with the change of groundwater level. According to the literature and model debugging, the relevant parameters of immobile water body are determined as follows: Yin and Yang 1 #, Daxing 2 #: f = 0.975, α=0.005, Xinglongsha 1 #: f = 0.6, α=0.005.
The numerical calculation program is written in VB5.0 and calculated on Pentium computer. The whole calculation program consists of four program modules: the first module is a data input module, the second module is a water equation solving module, the third module is a salt equation solving module, and the fourth module is a data output module. Among them, the solution of salt equation module is divided into two modules: the solution of movable water body module and the solution of immobile water body module.
According to the definite solution problem describing soil water and salt transport, the dynamic process of soil salt transport can be obtained through numerical simulation. If the mathematical model can describe the actual physical process, the numerical method is reliable, and the simulated soil salt dynamic process (simulated value) should be completely consistent with the actually observed soil salt dynamic process (measured value).
Fig. 2.5.3 shows the comparison between the simulated value of Yin-Yang 1 # and the measured value. As can be seen from the figure, the measured values are in good agreement with the simulated values. It shows that the mathematical model and numerical method proposed in this paper are feasible. The simulation time of this model verification is longer than 5 18 days. Looking at the whole simulation process, from a macro point of view, the dynamic change trend of the simulated value and the measured value is consistent, and there is no obvious trend of error accumulation and expansion in the simulation process. Therefore, the established model can be used to predict the dynamic change of soil salinity.
Fig. 2.5.3 Comparison diagram between simulated and measured values of Yin and Yang 1 #.