Soil Water-Salt Coupling Relationship And Spatial And Temporal Characteristics In The Yellow River Delta Coastal Area

Soil composition builds the earth’s terrestrial ecosystem and plays a significant regulatory role in the stable and sustainable development of the ecological environment,while soil salinization is one of the severe environmental problems affecting land productivity and sustainable agricultural development.Due to the proximity to the Bohai Sea,high water table and mineralization,there are large areas of saline land distributed in the Yellow River Delta near the sea,and the salinization is particularly serious due to the influence of human factors such as unreasonable farming and irrigation and drainage of agricultural land.Therefore,exploring the spatial and temporal variation characteristics of water and salt in saline soils along the sea,and mastering these coupling relationship in the soil surface layer is crucial for the planning and utilization of saline land resources and the management of ecological environment.In this paper,the typical representative areas of Kenli District and Wudi County,which are located in the Yellow River Delta,are used as research objects.Soil water and salt-related data were obtained through field surveys,and the spatial and temporal variation characteristics of the water-salt coupling relationship in the soil surface layer(0-15 cm)were categorized and analyzed in terms of different seasons,different vegetation types and distances from the Bohai Sea.To analyze the soil water-salt relationship in the process of adding water to raise salt,in the process of drenching to lower salt,and in the process of evaporation to return salt,and to estimate the irrigation parameters for salt reduction in large fields through indoor simulation tests.Then,we used a combination of field measurements and remote sensing images to screen the soil water and salt spectral parameters with high correlation,construct a model for remote sensing inversion of soil water and salt content,and analyzed the spatial and temporal variation characteristics of soil water and salt coupling relationship.The main research contents and conclusions are as follows:(1)According to the analysis of the actual survey data,the water and salt content of the soils in the study area is generally high,the distribution area of moderately salinized soils is wide,and the surface water salt content of soil in the regional area is higher in Kenli District than in Wudi County as a whole.The magnitude of soil water content by season and vegetation type is ranked as summer>spring>autumn>winter,grassland>bare land>cultivated land>forest land,soil salt content is ranked as spring>autumn>winter>summer,bare land>grassland>cultivated land>forest land,and the comparison of soil coupling degree values in each season and vegetation type is spring>autumn>winter>summer,bare land>grassland>cultivated land>forest land.In the study area,the soil water-salt content and the water-salt coupling degree decreased successively from near the sea to the inland buffer zone,with a gentle decreasing trend in Wudi County,a fluctuating increase in soil salinity in Kenli District within the buffer zone of 40～50 km from the sea,and a significant increase in soil water in the buffer zone of 30～40 km from the sea,and an increase in the value of surface water-salt coupling degree.(2)Indoor soil water and salt test results show that in the process of adding water while ascending salt,with the increase of soil surface water,the salt shows a positive linear increase trend,the change is more significant for the higher salt soil,water and salt correlation is higher in the case of water and salt double high than water and salt double low soil.In the process of descending salt,with the injection of water,the salt decreases in a negative linear direction,when the soil reaches the field water holding capacity,adding again when the volumetric water content is 22%,the soil salinity decreases the most with the downward flow of water,and then level off.When the volumetric water content reaches 20%,the soil full water content has reached the field water holding capacity,the moderate and heavily salinized soil is reduced to light when the irrigation water is converted to about 6.6 m~3 of water and 7.8 m~3 of water to 667m~2 of farmland.Besides,the rate of salt return is faster in soils with high salinity under natural evaporation.(3)Soil water and salt estimation models were established for each season based on water-sensitive spectral indices LSWI and NDII,and salinity-sensitive spectral indices SI-T and NDSI,with validation sets R~2 greater than 0.650,RMSE less than 7,and Sig=0.000.In general,the water-salt coupling is higher in the coastal area of Kenli District and the higher terrain area in the abdomen in all seasons,while the water-salt coupling tends to decrease and increase from spring to summer and from summer to autumn in the coastal and southwestern areas of the Yellow River respectively,and generally decreases from autumn to winter,except for the central part of the region where water-salt coupling increases.The water-salt coupling in the coastal area of northeastern Wudi County is high in all seasons,which has obvious decreasing and increasing trends in riverine and abdominal areas from spring to summer and from summer to autumn,from fall to winter,the water-salt coupling generally decreases except for the regional abdomen where the water-salt coupling increases,Overall,the water-salt coupling relationship shows a pattern of bare land>grassland>cultivated land>forest land.According to the results of multiple comparisons,the differences in soil water-salt coupling between arable land and forest land,and between grassland and bare land are not significant.In general,the water-salt coupling tends to decrease with increasing distance from the sea,differences in soil water-salt coupling in inland areas farther from the sea were not significant,while significant differences in soil water-salt coupling existed near the sea.This paper has better mapped the spatial and temporal variability of soil water and salt and its coupling characteristics in the Yellow River Delta coastal zone,which provides a scientific basis for the development and utilization of coastal saline land,ecological environmental protection and management.