| Northeast China is not only the main grain producing area in China,but also one of the stable seasonal snow areas.As the region with the highest latitude in China,Northeast China is particularly affected by climate warming.In recent years,the frequent spring drought in Northeast China has brought a very adverse impact on crop sowing and agricultural production.Therefore,it is particularly important to explore the relationship between soil moisture and snow cover in spring in Northeast China under the background of climate change and make rational use of snow cover resources.In view of this,based on remote sensing snow data and ESA CCI soil moisture products,this paper extracts five snow parameters: maximum snow depth,average snow depth,first day,last day of snow cover and snow days.Through linear regression analysis,climate trend coefficient,inverse distance weight and correlation analysis,according to three land use types of woodland,grassland and farmland,the temporal and spatial distribution characteristics of different snow cover parameters and spring soil moisture under different land use types were analyzed,and the correlation between snow cover and spring soil moisture was revealed.The specific conclusions are as follows:(1)From 1980 to 2020,all snow parameters of woodland in Northeast China were higher than the other two types.The maximum snow depth and average snow depth of woodland were greater,the first day of snow cover was earlier,the last day of snow cover was later,and the number of snow days was longer.The time change trend of different snow parameters under different land use types was significantly different.The maximum snow depth and average snow depth of forest land types decreased,the first day of snow delayed,the whole day of snow advanced,snow days decreased,and the snow depth and duration decreased continuously;The maximum snow depth and average snow depth of grassland types decreased,the first day of snow cover was advanced,the last day of snow cover was advanced,and snow days decreased.Except that the first day of snow cover did not show the delay trend corresponding to other snow parameters,the change trend of four snow parameters showed that the snow depth and duration of grassland in Northeast China were decreasing;For farmland types,the maximum snow depth and average snow depth increase,the first day of snow cover advance,the last day of snow delay,snow days increase,and the depth and duration of snow continue to increase.(2)From 1980 to 2020,the spatial distribution and change trend of snow days under different land use types in Northeast China are significantly different.The five snow parameters of woodland are the highest among the three land use types,especially in Lesser Khingan Mountains and Great Khingan Mountains.At the same time,the five snow parameters of woodland types have an obvious decreasing trend;The five snow parameters of grassland vary greatly in the South and north of Inner Mongolia.The snow parameters in the south of Inner Mongolia are small.The snow parameters in the north of Inner Mongolia are higher than those in the south.The trend coefficient of grassland type increases and decreases,and the overall trend is not obvious;The increasing trend of the five snow parameters of farmland is the most obvious,especially in the northeast of Songnen Plain,the grid of the increasing trend is more concentrated,and there is less snow in the southernmost Liaohe plain,but it still shows an increasing trend as a whole.(3)The interannual variation trend of soil moisture in spring under different land use types in Northeast China is similar,and the order from high to low is forestland > farmland > grassland.There are two trends of decreasing and increasing forestland soil moisture in spring.The decreasing trend of soil moisture in April is more obvious,and the increasing trend of soil moisture in May is more obvious.There is no significant difference between the average value of soil moisture in April and May;There are two trends of decreasing and increasing grassland soil moisture in spring.Compared with May,the two trends of soil moisture in April are more obvious in different stages.The average value of soil moisture in April is higher than that in May.In April of the 1990 s,the soil moisture was lower than the multi-year average,and the soil was very dry;The soil moisture of farmland in spring has two trends of decreasing and increasing.The decreasing trend of soil moisture in April is more obvious,the increasing trend of soil moisture in May is more obvious,and the average value of soil moisture of farmland in April is higher than that in May.In the 1990 s and 2000 s,the soil moisture was low and the soil was very dry in spring.(4)There are significant differences in the spatial distribution and change trend of soil moisture in April under different land use types in Northeast China.The soil moisture of woodland in April is the highest among the three land use types.At the same time,the soil moisture of woodland types in the north of Lesser Khingan Mountains and Great Khingan Mountains increased significantly in April,and the soil moisture of woodland types in the south of Changbai Mountain and Lesser Khingan Mountains decreased in April;The soil moisture of grassland in April is the lowest among the three land use types.The trend coefficient of grassland types increases and decreases,and the overall trend is not obvious;The increasing trend of soil moisture of farmland in April is the most obvious,especially in the northeast of Songnen Plain,the trend coefficient is the highest,and the soil moisture of Liaohe Plain in the southernmost is small in April,so the overall trend is not obvious.(5)The spatial distribution and change trend of soil moisture in May under different land use types in Northeast China are significantly different.The soil moisture in wooland in May is the highest among the three land use types,and the overall increase trend is obvious.Most grids passed the significance test;The soil moisture of grassland in May is the lowest among the three land use types.The trend coefficient of grassland types increases and decreases,and the overall trend is not obvious;The increasing trend of soil moisture in farmland in May is the most obvious,especially in Songnen Plain and Sanjiang Plain.Most grids have passed the significance test.The soil moisture in Liaohe Plain in the southernmost part is small in May,and the overall trend is also increasing.(6)Different snow parameters have different effects on soil moisture in spring under different land use types in Northeast China.From the correlation analysis of time series,the correlation between the first day of snow cover and soil moisture is the weakest,and it has not passed the significance test.Snow cover has the greatest impact on soil moisture of farmland types,and continues to affect from April to May;The daily and average snow depth significantly affected the soil moisture of grassland in April,but this effect did not last until May,and the snow did not significantly affect the soil moisture of woodland in spring.From the spatial distribution of correlation coefficient,the correlation coefficient of woodland is the highest among the three land use types.Especially in Songnen Plain,the impact of snow days on soil moisture lasted from April to May.Most grids passed the significance test,and the correlation of Liaohe Plain in the southernmost part is not strong;Grassland has a strong correlation in northern Inner Mongolia.The influence of snow days on soil moisture lasts from April to May,and the correlation is not strong in Southern Inner Mongolia;The number of snow days in woodland has little correlation with soil moisture in April and has a great impact on soil moisture in May.(7)From 1980 to 2020,different snow cover parameters under different land use types in Northeast China have a significant positive impact on soil moisture in spring,that is,the higher the values of the four snow cover parameters,the greater the soil moisture in spring.The fitting coefficients of different snow cover parameters and soil moisture in April are higher than that in May,and the correlation between snow cover and soil moisture in April is higher than that in May.Among the five snow cover parameters,the first day of snow cover does not show correlation with soil moisture,so it is not suitable to be used as the snow cover parameter for predicting soil moisture.Other snow cover parameters have different applicability with soil moisture in different periods.The correlation between farmland soil moisture and snow cover parameters is the most close,and snow cover can be used as a decisive factor to predict soil moisture.Snow days is more suitable for predicting soil moisture in April,snow days and maximum snow cover depth are more suitable for predicting soil moisture in May,and the average snow cover depth has no difference in predicting soil moisture in different periods;The correlation between grassland grid and snow parameters in northern Inner Mongolia is higher.The last day of snow cover,maximum snow depth and average snow depth are suitable for predicting soil moisture in April.The comprehensive performance of snow days is the best,which is suitable for the prediction of soil moisture in all grassland grids in May;Although the change trend of forestland snow cover and soil moisture is not consistent,and the correlation is mostly negative,snow days is excellent in the scatter density map,which can be used as the basis for judging forestland soil moisture in May. |
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