| Soil salinization has always plagued the development of agricultural production in the Hetao Irrigation District(HID)in Inner Mongolia.The low efficiency of fertilizer utilization in the HID leads to serious fertilizer loss and serious non-point source pollution to the farmland environment in the HID.Soil salinization and non-point source pollution have become important factors restricting food security and environmental security in the HID.As an important ecological security barrier in northern China,Lake Ulansuhai has faced problems such as lake water environment pollution and water ecological degradation for many years.To scientifically carry out comprehensive management of lake water environment,we must first solve the problem of agricultural non-point source pollution in the HID.This study used Landsat8 OLI 30m remote sensing image combined with NDVI(Normalized Difference Vegetation Index)threshold method and supervised classification method to classify and extract arable land crops in the HID,and modified the land use type map.Combined with the multiple linear regression theory,the soil salinity in the 0-100cm soil layer was inverted,and the soil attribute database was revised.The study improved the original version of SWAT 2012 by modifying the modules of soil water balance,solute balance,groundwater balance,and plant growth.Using improved SWAT(Soil and Water Assessment Tool)model to construct a distributed hydrological model of HID.And used the measured data of the HID to calibrate and verify the model.According to the main soil types in HID are Cumulic Anthrosols and Mollic Solonchaks.In this study,4 salinity levels were set under the 2 soil types,and they were classified according to the grades of non-salinized soil(S1),lightly salinized soil(S2),moderately salinized soil(S3),and heavily salinized soil(S4).At the same time,according to the long-term farming mode of the HID,five farming treatments are selected:no tillage management(CK),no tillage(T1),less tillage(T2),conventional spring tillage(T3)and template plowing(T4).The aim is to explore the effects of tillage methods and soil salinization on water production,crop nitrogen uptake,nitrate nitrogen leaching and transportation,and crop yields in irrigation areas.Based on the current situation,three main crops(corn,sunflower and wheat)are set up to reduce irrigation,fertilization,and adjust farming method,respectively.Based on the simulation results of different scenarios of the improved SWAT model,calculated and analyzed the nitrate nitrogen and total phosphorus loads and the effects on the yield of various crops under each management scenario.The main research results are as follows:(1)The data of crop planting structure were classified and extracted,and a comprehensive inversion model of soil salinity was established.The Landsat8 OLI remote sensing images were used to classify and extract cultivated crops in the Hetao Irrigation District in 2018.The overall classification accuracy,mapping accuracy,and user accuracy are over 90%,86%,and 88%,respectively.The Kappa coefficient has reached 0.87,the classification results are in good agreement with the spatial distribution of real crop planting.The soil of 0-40cm in the study area is mainly heavily salinized soil and moderately saline soil,accounting for 42.28%and 33.74%of the total area of the study area,respectively.The soil of 40-100cm is mainly moderately saline soil and mildly saline soil,account for 37.22%and 31.56%of the total area of the study area,respectively.(2)The crop planting structure data in land use was revised and an improved distributed hydrological model of irrigation area was established.Using Landsat8 OLI 30m medium-resolution remote sensing images to classify and extract crops for SWAT model crop planting structure spatial data location correction or accuracy improvement can improve the accuracy of runoff simulation and nitrate nitrogen simulation to a certain extent.Comprehensively correcting the spatial position and improving the data accuracy of crop planting structure can effectively improve the simulation accuracy of the SWAT model in the irrigation district.The model’s simulation accuracy of hydrology,water quality and crop yield met the evaluation criteria of the SWAT model,indicating that the improved SWAT model combined with the crop planting structure data corrected by remote sensing can effectively simulate the runoff,dynamic changes of groundwater depth nutrient circulation and crop yield in the Hetao Irrigation District.(3)The effects of tillage methods and soil salinity on regional nitrogen transport and crop yield were discussed.Tillage methods and soil salinity have significant effects on the total regional water production,crop nitrogen uptake,nitrate nitrogen leaching,nitrate nitrogen transport and crop yield(P<0.05).Among them,regional water production,nitrate nitrogen leaching,nitrate nitrogen transport in different hydrological paths,and wheat yield gradually decrease with the increase of tillage mixing depth and mixing efficiency.The amount of nitrogen uptake by crops and the yield of corn and sunflower gradually increase with the increase of mixing depth and mixing efficiency.Compared with no tillage management,the nitrogen uptake of template plowing crops increased by 11.78%on average,and the nitrate nitrogen leached volume was reduced by 16.5%on average,effectively reducing soil nutrient loss and groundwater pollution.In addition,template plowing significantly reduced wheat yield by 14.67%.The increase in soil salinity significantly increases the total regional water production,nitrate nitrogen leaching(except Mollic Solonchaks)and underground nitrate nitrogen transport by reducing the effective water holding capacity of the soil layer,reducing the amount of crop nitrogen uptake and crop yield.Compared with non-salinized soil,the yield of wheat,maize,and sunflower in the treatment of heavily saline soil was significantly reduced by 19.15%,27.31%,and 26%on average(P<0.05).The increase in soil salinity has a more significant impact on regional water production,soil nutrients and crop yields than changes in tillage methods.Therefore,in order to better manage the problems of serious pollution in irrigation areas and declining crop yields,it is necessary to focus on the prevention,control and treatment of regional soil salinization.(4)The effects of different irrigation,fertilization and tillage methods on nitrogen and phosphorus nutrient loads and crop yield were discussed.For different crops,reducing the amount of summer irrigation by 5%can increase production by up to 8.41%～10.32%,and reducing the amount of autumn irrigation by 10%will significantly reduce the load of nitrate nitrogen and total phosphorus.The nutrient load of different crops showed a gradually decreasing trend with the increase of the reduction ratio of nitrogen and phosphorus fertilization,but the decline curve gradually became flat.The yield of different crops increased first and then decreased with the increase in the reduction ratio of nitrogen and phosphorus fertilization.Among them,the yield of corn and wheat began to decline when the reduction ratio of nitrogen and phosphorus fertilization reached 20%,and the yield of sunflower began to decline when the reduction ratio of nitrogen and phosphorus fertilization reached 25%.The nutrient load and wheat yield of different crops gradually decreased with the increase of the mixing depth and mixing efficiency parameters of the tillage mode,while the yield of corn and sunflower gradually increased with the increase of the tillage parameters.Comprehensive analysis shows that cut 5%of summer irrigation+reducing the proportion of nitrogen and phosphorus fertilization by 20%+the template plowing combination corn yield increased by 36.5%.Cut 10%of autumn watering+reduce the proportion of nitrogen and phosphorus fertilization by 25%+the template plowing combination sunflower nitrate nitrogen load reduction by 42.1%.Cut 5%of summer irrigation+20%reduction of nitrogen and phosphorus fertilization ratio+no-tillage combination wheat yield increase of 29.1%.However,cut 5%of autumn watering+20%reduction of nitrogen and phosphorus fertilization ratio+conventional spring ploughing combination of wheat nitrate nitrogen load reduced by up to 27.2%,and total phosphorus load reduced by up to 18.5%. |
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