This thesis is part of the National Natural Science Foundation project "Research on water infiltration and retention in unsaturated freeze-thaw soil", the Shanxi Provincial Water Resources Department rural scientific research fund project "Research on surface irrigation effects and irrigation technology parameters under freeze-thaw conditions" and Taiyuan

Comprehensive research results of the Polytechnic University Youth Fund Project "Numerical Simulation and Application Research on Coupled Migration of Soil Water and Heat During Seasonal Freezing and Thawing Processes".

The project selected the Fenhe Irrigation District of Shanxi Province as the test base. Based on a large number of field soil moisture infiltration and irrigation experiments, combined with the numerical simulation method of coupled migration of heat and salt in freeze-thaw soil, the movement patterns of soil moisture and its effects under freeze-thaw conditions were analyzed.

The application was systematically studied.

The main contents are as follows.

(1) Field experiment.

Field tests include infiltration tests, winter irrigation tests and soil water, heat and salt migration tests.

The infiltration test soil includes 3 types of soil textures and 8 types of soil structures.

The winter irrigation test was conducted on 1 soil texture and 3 soil structures.

All experiments were conducted in the Fenhe Irrigation District of Shanxi Province. The test locations were the Central Irrigation Experiment Station of Shanxi Province, the Fendong Irrigation District of the Fenhe Irrigation District of Shanxi Province, and the Sanba Irrigation District of the Fenhe Irrigation District of Shanxi Province.

The soil water, heat and salt migration test was conducted in the Sanba Irrigation Area of ??the Fenhe River Irrigation Area, mainly monitoring the dynamic changes in soil moisture, temperature, and salt profiles under natural conditions.

(2) Research on soil freezing and thawing laws.

Based on the indoor and outdoor experimental results, this paper discusses the changing rules of soil freezing temperature (freezing point) under three types of soil textures, five types of moisture content and salt content, and analyzes the temperature field characteristics of soil and its influencing factors during the freezing and thawing process.

(3) Study on the infiltration rules of frozen-thaw soil water.

Based on a series of infiltration tests under different surface treatment conditions, a comprehensive analysis was conducted on the basic characteristics, seepage resistance mechanism, and main influencing factors of freeze-thaw soil water infiltration.

(4) Methods and model research on determining infiltration model parameters using conventional soil physical parameters of frozen-thaw soil.

On the basis of full analysis and research on the factors affecting the infiltration characteristics of frozen-thaw soil, the dominant factors affecting the infiltration characteristics of frozen-thaw soil were identified, and an empirical model was established to determine the parameters of the infiltration model using conventional soil parameters of frozen-thaw soil.

(5) Study on the water migration rules of freeze-thaw soil system.

Based on the analysis of the characteristics of moisture and temperature changes in the snow layer and soil during the seasonal freezing and thawing period and their interrelationships, the basic differential equations of water, heat and salt movement in the snow layer and frozen and thawed soil media were established respectively.

Based on the basic equations, the water and heat migration equations in the snow layer, the coupled water, heat and salt migration equations in the frozen soil, and the connection equations reflecting the changes in the moisture content of unfrozen soil water under negative temperatures were derived.

Combined with the mathematical model of water and heat exchange between the ground and the air, a one-dimensional coupled migration mathematical model of water, heat and salt in the snow, freeze-thaw soil system is formed that comprehensively describes the water and heat exchange between the ground and the air under natural conditions.

Comparing the currently commonly used numerical calculation methods, the implicit finite difference method suitable for model solution was selected, and the corresponding numerical simulation model was established using Newton-Raphson iteration technology, and the indoor and outdoor test results were used to verify the accuracy of the model and its calculation method.

Check and correct for accuracy and reliability.

Taking the Fenhe Irrigation District of Shanxi Province as the background, the above model is used to study the seasonal freezing and thawing process and freezing rules of the soil, the changing rules of soil moisture content during the freezing and thawing process, the soil moisture migration rules under different phreatic levels, the phreatic evaporation rules and different

The law of soil water storage and moisture conservation under initial moisture content, the initial moisture content of soil during freeze-thaw period, and the impact of winter irrigation on soil moisture.

(6) Research on winter irrigation surface irrigation process and technical parameters.

The method of combining winter irrigation experiments and surface irrigation water flow numerical simulation was used to reveal the irrigation process and irrigation effect characteristics of winter irrigation and ground border irrigation, establish a winter irrigation technology parameter optimization model, and conduct research on irrigation technologies that improve the winter irrigation effect.