Research On Movement Of Water And Salt In Continuous Cotton Cropping Field Of Drip Irrigation Under Mulch

Drip irrigation under mulch is an efficient water-saving irrigation technology combining with drip irrigation technology and plastic film mulching technology,and it is applied in the range of Xinjiang after being successful used in Xinjiang Production and Construction Corps.Because of the water-saving in drip irrigation which has few applied water and wet front can only reach to the depth of the crop root layer,it is changed in spatial distribution of water and salt soil in long term application comparing with traditional flood irrigation method.In addition,this irrigation method does not produce deep percolation during irrigation and it may pose a potential threat to the growth of crops because the salt in the soil only moves near the root layer with the water migration and does not drain out.Therefore,this study used classical statistics and geostatistics to analyze the soil water and salt transport law and temporal variability of continuous cotton cropping field of drip irrigation under mulch,in order to provide a reference for sustainable planting and management of cotton.The main conclusions of this study are as follows:(1)The study by t distribution method of sampling number was estimated,and the results show that at the same confidence level and relative error,the sampling number in the 50 × 50 m grid specification is reasonable.Under the same confidence level and relative error,the sampling estimate number increases with the increase of soil water content and the coefficient of variation of salt.Reduce the confidence level and relative error,while reducing the number of sampling required,but also reduces the expression and reliability of soil spatial variability of the field,Therefore,when sampling number is estimated,the confidence level and the relative error are critical.(2)The soil moisture content in the study area increased first and then decreased with the increase of soil depth.The water concentration is mainly concentrated at 40 cm,60 cm and 80 cm.In the same year,the moisture content in October was relatively reduced in March,and the coefficient of variation of soil moisture content in October was larger than that in March.(3)The soil salt content in the study area increased with the increase of soil depth,and the salinity was mainly distributed in 40 cm,60 cm,80 cm and 100 cm soil layers.In the same year,the salt rate in October was mostly lower than the salt rate in March.The coefficient of variation of salt content in each soil layer decreases with the increase of time.Through the analysis of salt balance,the 5 cm,20 cm and 100 cm soil layers are in the salt accumulation state,and the 40 cm,60 cm and 80 cm soil layers are in a desalting state,in total,the trend is showed that surface and the deep layer are accumulated in the salt and middle layer is desalted.(4)Through the analysis of the spatial structure of soil water content and salt content,the spatial variability of water content and salinity can be fitted by the corresponding variational function model which is different from each other and there are nugget variance in every model,and it is indicating that there is a positive base effect due to sampling error,short distance variation and random variation.Throughout the period of water content variation function of the nugget coefficient,the spatial correlation of October in the same year was significantly greater than that in March.It is obviously fluctuating at different times at the partial sill and nugget coefficient of the soil moisture content variation function of 5 cm,20 cm and 40 cm soil layer,but it is still mainly spatial correlation.In the past three years,the variation function of the water content of each soil layer gradually decreases,indicating that the effect of human activities is increasing.The variation coefficient of salt content varies greatly in different soil layers at different ages.The variation of the surface soil is most obvious due to the influence of external factors.The nugget coefficient of 20 cm and 40 cm layers increased with the depth,indicating that the spatial correlation was weakened under the combined action of soil structural factors and stochastic factors.The nugget coefficient of 20 cm and 40 cm soil layer is relatively stable over time.(5)Through the analysis of Kriging Spatial Interpolation Graph,The spatial distribution variability of soil surface salinity is larger,and after a period of irrigation and rainfall leaching,the salt rate in October is significantly lower than that in March,and the distribution is more uniform.20 cm soil layer salt distribution is more uniform with time to maintain a relatively stable.Comparing with 5 cm and 20 cm soil layer,salinity of 40 cm soil layer increased significantly,and there are high and low value area that spatial distribution with time did not change greatly and maintain a relatively stable state.The soil salinity in the study area is mainly distributed in the southwest and central regions.(6)Based on the analysis of the spatial trend of the salinity of different soil layers in the study area,the spatial distribution of the salinity of each soil layer shows a similar trend,namely that the salinity of the southwestern region is significantly larger than that of other regions,and the salt content is mostly above 3 g/kg,and the salt range is relatively stable.In addition,the salt content of the surface layer and 40 cm soil layer is larger than that of 20 cm soil layer.Based on the time stability analysis of the spatial distribution of salinity in the study area,the distribution of salt in the study area showed a high degree of temporal instability in the three soil layers because of the inhomogeneity of farmers' drip fertilization.The results in soil deeps shows that the overall instability of the surface layer and 40 cm soil layer is stronger,and the 20 cm soil layer is relatively weak,and in soil range shows that 0 cm and 20 cm in the southwest of the higher salinity areas showed strong instability,40 cm soil layer in the northeast regions showed a high degree of instability.