| With the rapid development of social economy and population, resources crisis has become the world’s second biggest environmental problems, which is only behind global warming. The shortage of water resources is the main obstacle that restricts agricultural development, so developing water-saving irrigation is without a doubt an effective way to solve this problem. We can reveal the comprehensive water-saving technology and the rule of water-salt movement of the large irrigation districts, putting forward comprehensive control technology and the corresponding management mode, because large irrigated areas are important grain production bases in China. It is significant for ensuring food security, agricultural ecological environment security, improving the agricultural comprehensive production capacity, building resource-saving society and environment friendly society, achieving the strategic of sustainable development of human society and economy.The major causes of low yield of irrigated crops are high underground water table and the accumulation of surface soil salt. Simply relying on reducing irrigation water but not accompanying by the necessory underground water control measures, we not only can not achieve the purpose of water-saving, but also will increase the soil salinity due to controlling underground water table adversely in the reduction of irrigation volume. At present, the main reason of quoting more water into the irrigated field is the requirement of salt-leaching. But at the same time, on the one hand, excess water raises the local water table and aggravates secondary salinization degree of soil. On the other hand, a lot of irrigation water, farmland drainage and abandoned water by drainage system or channel discharge directly into the downstream, meanwhile, these water will take a lot of salt into the downstream and deteriorate the water quality.This paper, considering the actual situation of water-saving irrigation district of our country, relying on the international science and technology cooperation project ’the Research of Water-saving Irrigation Comprehensive Technology and Management Measures in large Irrigated District’, taking Shahaoqu irrigation area in Hetao irrigation district as the research object, investing the transportable characteristics and balance of soil water and salt under water-saving irrigation condition of large-scale irrigation, analyzing the change of irrigation water, drainage and soil salinity composition, discussing the migration characteristics of harmful soil salinity and the response of soil water-salt under the condition of water saving irrigtion, puts forward comprehensive control technology and management measures of water saving irrigation in large irrigation district.On the base of the field experiment and the analysis of water-salt monitoring, the measurement of soil basic parameters and the results of field irrigation experiment in test sites, this paper quantitatively studies the migration regularity of salinity under different autumn irrigation quotas in Hetao irrigation district. On a field scale, the author build and distinguish a numerial model of soil water flow and solute transportation by using HYDRUS model. The author analyze and evaluate how different irrigation methods influence on soil water-salt distribution of corn fields, and the dynamic change trend of farmland water-salt under different irrigation water managements, so as to determine the reasonable water management program. On a regional scale, the author quantitatively analyze the movement of groundwater flow by using Visual-MODFLOW model, and the author predict and evaluate underground drainage water of the status quo and the improved conditions, then this can provide scientific basis for making rational irrigation water management plan. Through the analysis of field experiments and models, the effects of autumn irrigation, irrigation quotas, irrigation methods, irrigation combined with drainage under the condition of water saving irrigation on crop yield and soil water-salt movement are quantitatively studied. The author hope to gain comprehensive control technology and management measures of water saving irrigation that can keep water conservation, addition to yield and sustainable agriculture, and to provide technical support for sustainable development of agricultural water conservation and the improvement of water-soil environment.Based on the above reasurch thoughts, objectives and methods, the following aspects of research results are achieved in this paper:(1) Soil salinity of Shahaoqu irrigation area in different irrigation periods and different soil depths shows strong variability:Spatial variability of soil EC value decreases with the increasing soil depth. From the spatial autocorrelation, the soil EC value is moderate referring to spatial autocorrelation intensity, and the spatial variability of soil salinity decreases gradually from upper layer to lower layer; with increasing soil depth, the spatial autocorrelation scale of soil salinity decreases gradually, and spatial correlation is weakened and the spatial distribution pattern of surface soil is continuous and uniform; soil salinity decreases with the increasing shallow groundwater depth, and their ralationship is exponential. So, one of the most important factors controlling soil salinization is to control the groundwater depth.(2) High autumn irrigation quota may not be able to bring good soil salinity. Under high irrigation quota, the surface soil salinity only with water infiltrates down to the lower soil layers, so the surface soil salinity reduces and deep soil salt content increases. Entering the frozen period, salt accumulates in top soil with moisture moving up, so salinization is more serious, and partial soil salt content is even more than the period before autumn irrigation. In Jiefangzha irigation area of Hetao irrigation district, the autumn irrigation quota is about1800-2250m3/hm2, which can not only achieve the goal of soil moisture conservation and salt-leaching, but also achieve the purpose of water conservation.(3) The analyses of water-salt distribution and wheat yield under different irrigation quotas indicate that the soil moisture of different irrigation methods are basically the same trend. At the view of different depths of soil, the soil moisture tends to increase with the increase of soil depth. In the soil whose depth is1meter, the average value of soil salinity before sowing is higher than that after harvest, and soil salinity tends to decline overall. This is mainly because of the high coverage of wheat, reducing the influence of evaporation on soil salinity rising. Wheat yield of border irrigation does not increase with increasing irrigation quota, so at the point of persuing the maximum yield of per unit area, we should reasonably determine the irrigation quota in the whole growth period of wheat.(4) On the base of the analyses of water-salt monitoring data, the soil basic parameters, the results of field experiments in test areas, relying on the regional soil water-salt numberial model HYDRUS, we analyze field water-salt transportation under differdent irrigation patterns. The results show that:irrigation methods and quotas influence soil mean salinity significantly, and the decreasing salinity trend increases with the increasing irrigation quota. When the irrigation quotas are same, the mean soil salinity of the furrow irrigation is lower than that of border irrigation. In one-meter soil, the mean soil salinity of the furrow irrigation is24.4%lower than that of the border irrigation. After harvest, the accumulatted salt of border irrigation is higher than that of furrow irrigation, which shows that the cumulative effect of furrow irrigation to control soil salinity is better than border irrigation.(5) The HYDRUS model is established to simulate the change of soil water and salt under different irrigation schemes. Through the analysis of simulation scenario, the0-60cm root layer soil salinity has the trend of increases first and then decreases from summer irrigation to autumn irrigation. That is, after the end of the growth period, the soil salt in root layer reaches to maximum, and after autumn irrigation, because of water leaching, the soil salt reaches to minimize. When irrigation quota during the growth period is3150m3/hm2, as long as autumn irrigation quota is more than2250m3/hm2, soil salinity in root zone does not increase significantly, and when the autumn irrigation quota is2250m3/hm2, the soil salinity in root layer declines fastest; When irrigation quota during the growth period is4050m3/hm2, as long as autumn irrigation quota is more than2250m3/hm2, the soil salinity in root zone does not increase significantly, and when autumn irrigation quota is2250m3/hm2, the soil salinity in root layer declines fastest; When irrigation quota during the growth period is greater than4950m3/hm2, only if autumn irrigation quota is greater than2700m3/hm2, the soil salinity does not increase significantly. Combined the research about the relation between crop yield and irrigation water with the main crop proportion measured in shahaoqu irrigation area, the optimum irrigation quota during the growth period is recommended as3500-4050m3/hm2in study area, and autumn irrigation quota is recommended as1800-2250m3/hm2.(6) Using the regional groundwater model Visual-MODFLOW, the simulation analyses of underground drainage amount for different water management schemes show that:the total amount of current irrigation scheme is122mm, accounting for16.7%of the water used for irrigation, so we suggest that the total amount of irrigation project is85mm, which accounts for12.7%of the water used for irrigation. Compared with the current scheme, the underground water table in crop growth period of the proposed scheme decline53cm, and the average groundwater level pours down20cm during autumn irrigation, and the annual average groundwater table maintains about230cm. At the same time, the drainage water of proposed project declines from122mm to85mm, which decreases30.3%. Obviously, by controlling the groundwater level and adopting water-saving irrigation measures, we can reduce deep percolation caused by irrigation, thereby reducing the leakage entering into drain. Thus, the appropriate water management strategy is that: the comprehensive irrigation quota during growth period is3500-4050m3/hm2; the autumn irrigation quota is1800-2250m3/hm2; the buried depth of groundwater controls at least200cm; meanwhile, we should combine the irrigation water measures with the farmland drainage measures and the other beneficial methods, in order to save water, increase production, reduce drainage and improve farmland water-soil environment.(7) Considering the field experiments and model simulation, on the condition of water saving irrigation, we analyze the results that reflect the influence of autumn irrigation, irrigation style, irrigation quota, irrigation systems, irrigation and drainage on soil water-salt migration and crop yields, then we recommend the comprehensive management measures for water-saving irrigation are:On irrigation methods, we use furrow irrigation; on irrigation quota, each irrigation quota is675m3/hm2; on irrigation system, we irrigate6times during the growth period, so the total irrigation quota is3500-4050m/hm, and autumn irrigation quota is1800-2250m3/hm2. In Hetao irrigation area with salinity problems, these measures are propitious to achieve the goals of water-saving, salt control, the increase of production, reduce emissions and agricultural water-soil sustainable development. |
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