The Study Of Downstream Farmland Safety Indicators In Land Management Controlling Engineering

Because of the engineering of returning farmland to forest(grass) in Loess plateau, the cultivated land resource decreased sharply, so the government started land and ditch management since 2012 in Shaanbei region,which preliminary plan to invest 4.8 billion to renovate 500000 mu of farmland and have made great progress up to now. But soil salinization and waterlogging of farmland is serious. In view of the lacking of land resources and the serious of soil salinization, through field investigation and indoor test, this study focuses on the scope of the downstream salinization and the control standards, the following results are obtained:(1)This research adopts soil column which the height is 90 cm and the diameter is 23.5 cm to simulate the process of soil salinization of downstream controlling engineering, then gain the basic rule of soil water and salt movement under four different flow groups of constant pressure: with the increase of infiltration flow, the soil moist time of horizontal and vertical direction all show a trend of decrease, the wetting front migration rate shows the tendency of increase. The moisture content of soil layers are decreases with the decrease of the distance to the surface, the closer to the surface, the soil moisture content is lower. With the rise of water,the salt concentration is high on the surface vertically, the surface soil salt content of the biggest.. The salt content of every layers reduced over time. In the infiltration flow of 0.12 L/h, 0.26 L/h, 0.3 L/h, with the increase of infiltration flow, the soil salt content are reduced in the same location of each layer.(2)The spacial influence of soil salinization under different infiltration flow at control project: vertically, under the same infiltration flow, the closer the distance to the surface is, the slower the soil moisture infiltration rate is. The water level difference is higher, the infiltration flow rate and the soil moisture infiltration rate is higher, the depth of infiltration rate attenuation of 0 is higher. The water level difference are: 2.12 m, 4.59 m, 5.30 m, 7.07 m under four groups infiltration flow. The depth of infiltration rate attenuation of 0 are: 2.0 m, 2.2 m, 4.2 m, 4.7 m. Therefore, in order to avoid the impact on crops magnetism caused by water and salt migration, the soil thickness shall be 2.0 m, 2.2 m, 4.2 m, 4.7 m in four groups will not affect the normal growth of crops and do not cause soil salinization. According to the fitting relationship between the depth of the water infiltration rate reduced to zero and the the upstream water level, when the soil thickness should be controlled in 10 m, the upstream water level should be controlled below 11.21 m, single wide infiltration flow rate is less than 3.17 L/h;When the soil thickness should be controlled below 5 m, the upstream water level should be controlled below 7.23 m, single wide infiltration flow rate is less than 2.05 L/h.(3)The spacial influence of soil salinization under different infiltration flow at control project: horizontally,the higher the water level before dam, the seepage velocity is faster, the downstream soil salinization range are more wide. Under four different infiltration rate groups, when there is no anti-seepage measures in the front of the dam, the minimum water-salt influence range after dam is 252 m, the largest is 461 m. We can use suspended cut-off wall, horizontal impervious body, seepage intercepting ditch for seepage control, compared with the other two kinds of seepage control measures, the effect of horizontal impervious body is better, the effect can be increased by 28.3%.It is recommended to build horizontal impervious body in front of the dam.(4)The security using standard of platform farmland in control engineering: combine the design and application of platform greenhouses in Ansai Majiagou river basin, then analyses its securely and effectively usage patterns.The results show that: the pattern is to build collecting, conveying,filtration and purification, storage facilities,which can regulate and control runoff and soil erosion, so as to use water and soil resources efficiently.The control standards of platform farmland are as follows: the downstream of control engineering need to build drainage facilities in certain limits, and flood draining and desalting should be considered. When the upstream water level is 2 to 10 m, the height of platform security should be greater than 2 to 5 m. The usage of platform, crop type, sunshine time, engineering cost, rainfall and irrigation water, traffic conditions and factors such as social and economic benefit must be taken into account to determine the location of the greenhouse. The width of platform for greenhouse should not be less than 15 m. The width of greenhouse should not be greater than fifty percent of the platform width.The length of the greenhouse size should be at the range of 50 m to 60 m.The platform greenhouse should build facilities to retain runoff and prevent erosion. The design of multiple platform greater than 6 should consider water storage facilities, drainage and erosion prevention measures.