The Mechanism Of Solute Transport In Soil And Its Application In Drip-Irrigation With Fertigation

The shortage of fresh water is a main factor that hampers sustainable agricultural development in China, and drip-irrigation, as an advanced irrigation technology, has been widely applied in China. At the meantime, drip fertigation has also received increasingly attention in modern agriculture in which the reaction between water, soil and the soil solution affects the use of soil nutrients by plants. The objective of this thesis is to use a combination of numerical modeling and experiments to investigate the fundamentals of solute transport in pore space of soil with a view to improve modeling at macroscopic scales, which can be used to investigate the movement of nutrients under drip fertigation. The main conclusions of this thesis are:(1) Assuming that water flow is laminar and that the medium is homogenous and all pores in it are hydraulically connected, water flow and solute transport in the pore scale were simulated using the lattice Boltzmann method by treating the water-solid interface as non-slip boundary. The result reveals that water flow is mainly concentrated along a few channels. As a result, the movement of tracers is anomalous, which cannot be accurately described by the classical advection-dispersion equation. Comparing the results with the continuous time random walk (CTRW) reveals that the solute movement is well described by CTRW with a modified exponentially distributed transition time.(2) A spatial fractional advection-dispersion equation model (FADE) is investigated, which is shown to be an effective model to simulate solute movement in natural soils with big pores. The FADE currently available in literature is based on Riemann-Lioville equation, limiting its application to infinite domains. A modified method based on the Caputo derivatives is presented to overcome this problem. A finite volume approach is given to numerically solve the modified FADE and its associated boundaries. The model is then compared with experimental data, and the results show good agreement.(3) Study on the movement of adsorptive solute reveals that its movement can be simulated using the similar framework as for non-adsorptive solute, using a probability distribution function to describe the distance between the pore and the time it takes a solute particle to move from one pore to another. The difference between the adsorptive and non-adsorptive solutes is the time for solute particle to move one pore to another. Given the inherent heterogeneity of natural soils, this thesis used a power-law function to describe the time it takes the adsorptive solute to move from one pore to another, making the CTRW model into a time-fractional advection-dispersion equation. Based on the principle of mass balance, a finite volume method is proposed to solve the equation and then compares it with experimental data. The results indicate that the model accurately captures the persistent long tails in the breakthrough curves, and that the transport parameters are independent of water flow rate.(4) Experimental study of the movement of nitrate under drip fertigation was carried out in the tank filled with sandy soil. The measurement of water content and nitrate concentration reveals the water content is symmetrical vertically around the emitter and after the termination irrigation, the gradient of the water potential changes. Also, the peak nitrate concentration is affected applied nitration concentration, decreasing with the distance from the emitter as the applied concentration increases.(5) A two-dimensional model is proposed to simulate the movement of water in soil under drip irrigation. To accurately describe water movement in heterogeneous soil and variably saturated condition, the mixed-form Richards' equation was used. To improve the convergence of the numerical model, a chord Newton method was used to solve the gravity term so as to make the final matrix diagonally dominant. This stabilizes the solution, thereby increasing the convergence rate.(6) Using the above models for both water flow and solute transport, the nitrate movement under the drip irrigation is modeled. Since the experiments were conducted using repacked soil, there are no obvious big pores. I thus used the time-fractional CTRW model to simulate nitrate movement in which the distance between the adjacent pores is assumed to be normally distributed. Comparing with the commonly used the mobile-immobile model, the proposed model improves the description of the solute movement in soils. Comparison of the model with experiment shows fair agreement.The movement of water and solute in soil is dictated by the pore geometry. The advent and development of imaging technology and computer power over the last two decades has made it feasible to investigate pore-scale water and solute movement. The CTRW model presented in this paper can overcome some limitations of the commonly used advection-dispersion model, and is used in this work to investigate nutrient transport under drip fertigation. However, due to the restriction of experiments, such as the difficulty of sampling, this work is restricted to nitrate. The simulations show similar trend as the measurements, but this is still some discrepancy. Further research is needed.