| The quantitative relationship among crop yield, water and nutrient is the base for rational irrigation and fertilization scheduling, which is important for the increase of water and nutrient use efficiency and the improvement of farmland environment. The relationship among crop yield, water and nutrient has been studied extensively both in China and abroad. From these studies, a large amount of valuable data and results were obtained. However, most of the models are empirical model based on statistics of experiment data with less consideration to the physiological and biological mechanism. Therefore, the applicability of these models is usually limited. Crop growth simulation model, which has a history of over 40 years, can overcome these disadvantages. However, most of the crop simulation models are developed by agronomist. Main emphasis of these models are crop growing period, plant density and plant configuration, while soil water and temperature are often neglected or over-simplified. As a result, these models can not meet the requirement of irrigation water management, and are difficult to explain the impact of water stress on crop growth. The objective of this thesis is to develop a crop simulation model suitable for irrigation and fertilization management based on available models, and to present the crop growth process and relevant processes of water, fertilizer, light and heat in graphics, so as to study the crop growth process and the impact of environmental factors to crop growth. In the study, three methods were used to determine parameters of the model. Some parameters were cited directly from literatures, some were estimated from the analysis of experiment results from literatures and experiment of this project. The rest parameters were optimized with the objective of minimum square errors between simulated and measured crop yield of experiment in 2004. Then, the crop growth simulation model developed in this thesis was validated with experiment results of soil water variations, the process of soil temperature at different depth, crop yield and dry weight of stem, leaf and grain. It shows that the model is appropriate, parameters are reliable and the simulation precision is acceptable. Main conclusions and innovations of this thesis include: 1. In this thesis a systematic crop growth simulation model was developed based on the theory of mass transfer in soil-plant-atmosphere continuum (SPAC). The model integrated the processes of photosynthesis, respiration and biomass cumulation, field evapotranspiration, root uptake, infiltration of precipitation and irrigation, as well as the spatial and temporal variations of soil water, temperature and nutrient. The model mainly concerned with the impact of water, fertilize, light and heat on crop growth, and the distribution and transfer of photosynthetic product in crop root, stem, leaf and seed, while the tillering of winter wheat, growth of stem node and leaf distribution were simplified or neglected. Therefore, the model can be used conveniently in irrigation and fertilization management with sufficient accuracy. 2. Using the method recommended in FAO Irrigation and Drainage Paper No. 56 to calculate crop evapotranspiration, hourly solar radiation in the growing period of winter wheat in 2003 and 2004 at the experiment site of Yuci was estimated. Daily variation of potential evapotranspiration of winter wheat were calculated and analyzed from the relationship between crop coefficient and leaf area index. Then, daily variation of photosynthetic product was calculated, and it was integrated with respect to canopy depth and time with trapezoidal integration method to obtain the daily potential photosynthetic product. 3. Coefficients of growth balance between root and canopy, stem and leaf, seed and stem were proposed. Based on these coefficients of growth balance, distribution coefficient and transfer coefficient of photosynthetic product were deduced. Crop growth simulation results of different water and fertilizer treatment showed that the　distribution and transfer coefficients of photosynthetic product is effective in modeling the distribution of photosynthetic product among root, stem, leaf and seed and the impact of the amount and time of irrigation and fertilization on crop economic coefficient. 4. Soil particle distribution was used to determine unsaturated soil hydraulic conductivity and diffusivity, soil water content and hydraulic conductivity at saturation. From the comparison of simulated and measured soil water, the method above is acceptable. 5. In the modeling, partial difference equations for soil moisture and soil temperature were used to describe the transfer of soil moisture and variation of temperature. Partial difference equations for ammoniacal nitrogen and nitrate nitrogen were used to describe the transfer and transformation of soil nitrogen. Therefore, it avoided some simplifications and hypothesizes in a lot of crop simulation models. 6. Coefficients for water and nutrient stress take the power function of relative daily evapotranspiration and relative plant nitrogen content, respectively. Two parameters were optimized with the objective of minimum square errors between simulated and measured cropyield of 30 experiment plots with 20 treatments in 2004, then they were validated with the experiment results of 22 treatments in 2003. The results show that the maximum and average relative errors in 2004 are 20% and 7.05%, and the relative errors in 2003 are 34% and 13.2%, respectively. These indicate that the model and parameters are reliable and the simulation precision is acceptable. 7. The simulation results, including crop growth process, the variation of soil water, nutrition, temperature and some parameters, can be displayed in graphics with Matlab software. The visualization of crop growth simulation is convenient for relevant studies. 8. Crop growth simulation model can simulate crop yield of different irrigation and fertilization with enough accuracy, and provide the base for beneficial evaluation of irrigation and fertilization. The simulation model of water and nitrogen can estimate vertical water and nitrogen flux at different soil depth, which is helpful in the evaluation of water and nitrogen use efficiency and the impact to environment. Therefore, the crop growth simulation model in this thesis considered agricultural benefit and the impact of agricultural activity on environment together, and can evaluate the agricultural benefit and environment impact as a whole. The mechanism model of photosynthetic product distribution and transfer needs further validation with field experiment data of root, stem, leaf and seed. The assumption that the biomass of root, stem and leaf changed suddenly from accumulation to decreasing needs further improvement.
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