Study On The Temporal And Spatial Changes Of Different Types Of Drought And Their Relationship

Drought is a universal climate phenomenon,and it is also a kind of natural disaster that will cause great loss and influence to human society.Drought can lead not only to widespread food loss,water shortages and environmental degradation,but also to famine,economic dislocation,war,social unrest and even a change of government.In addition,the occurrence of drought will also cause the deterioration of social natural resources,and then lead to a series of socio-economic environmental and ecological problems.Therefore,drought is not only related to the ecological environment,but also to the social economy,and is closely related to the development of human beings.Drought has become a key issue of concern to all countries.Based on the global land assimilation data and remote sensing data,this paper analyzes the temporal and spatial changes of water cycle related factors,remote sensing monitoring of drought and the characteristics of different types of drought in China.Firstly,the spatial-temporal evolution characteristics of water cycle elements,which are directly related to drought,are analyzed by using Mann Kendall trend test and wavelet analysis.Secondly,the temporal and spatial characteristics of soil moisture,temperature,surface temperature and vegetation index of each layer are analyzed.Two kinds of Temperature Vegetation Drought Index?TVDI?are calculated by fitting the dry and wet edge equations,and the applicability of the two kinds of TVDI in soil moisture monitoring of different regions and depths is analyzed.The distribution characteristics of various probability density functions of precipitation,soil moisture and groundwater runoff are analyzed,the optimal probability density function is selected,and the parameters are estimated.On this basis,the standardized precipitation index?SPI?,the standardized soil moisture index?SSI?and the standardized runoff index?SRI?are calculated,and the spatial and temporal characteristics of SPI,SSI and Sri and the relationship between drought development is analyzed by Pearson correlation analysis and run theory.Finally,based on SPI,SSI and Sri,the corresponding drought variables,i.e.drought frequency,drought duration and drought magnitude,which are obtained by using the run theory,and the change rules of each drought variable are compared.The main conclusions are as follows:?1?Among different climatic zones,the temporal variation characteristics of these climatic variables are quite different.In arid and semi-arid regions,the annual precipitation is very close to the evapotranspiration.In addition,there is a lag relationship between annual precipitation,soil water storage and groundwater runoff.In terms of the spatial distribution characteristics of annual value,the average annual precipitation,evapotranspiration,soil water storage and groundwater runoff all showed a gradual increase from northwest to southeast in space.Based on the meteorological drought index AWD,the effects of meteorological conditions on soil and hydrology were preliminarily analyzed.The results show that the influence is regional and the meteorological conditions have a great influence on the soil water storage and groundwater runoff in the humid areas of China.?2?In the soil moisture of each layer,temperature,surface temperature,vegetation index on the change of time and space,in central and southern regions,between the layers of soil moisture change process is consistent,unified style,first increase then decrease variability and can see obvious with the increase of depth,soil moisture is also characterized by increased state.Soil surface temperature was significantly higher than that of other layers,and the soil temperature of each layer in each climatic region showed a peak curve.In the cold season,the temperature of the deep soil is higher than that of the shallow soil,and in the warm season,the temperature of the shallow soil is higher than that of the deep soil.According to the daily sequence changes of Vegetation Index,the changes of NDVI?Normalized Differential Vegetation Index?and EVI?Enhanced Vegetation Index?show an obvious uniformity single-peak curve in the northwest and Inner Mongolia,and the peak occurs about 225 days in a year,approximately in the middle of July of a year.The pattern of change in the qinghai-tibet plateau and northeast China was similar to that in the northwest and Inner Mongolia,but the peak was slightly earlier.The characteristics of monthly value change are as follows:in northwest China,Inner Mongolia and northeast China,the time series changes of NDVI and EVI are highly consistent,and the peak value of NDVI is higher than that of EVI.?3?The results of the applicability analysis of soil moisture monitoring index TVDI showed that,in terms of the monitoring effect of appropriate soil moisture depth,NDVI-based TVDI(TVDI_NDVI)and EVI-based TVDI(TVDI_EVI)were the best in the applicability of 0-10 cm soil moisture monitoring.The applicability of TVDI_EVI under soil moisture conditions of 0-10cm is better than that of TVDI_NDVI,and it has a significant effect in northwest China,while the performance of TVDI_NDVI in other regions is relatively better.The correlation coefficients between soil moisture and TVDI_NDVI(or TVDI_EVI)in the range of 0-10 cm nationwide were mostly negative,but there were significant differences between regions.In terms of soil moisture monitoring at different depths,the deeper the soil layer is,the worse the correlation is,and the worse the monitoring effect is.In a word,it is better to select the temperature of the corresponding soil layer as the calculation of TVDI parameter for the monitoring of soil moisture content.?4?The comparison results of different types of drought showed that the meteorological drought was mainly concentrated in the southwest region,while the agricultural and hydrological drought was mainly concentrated in the northwest region.The main cycle of agricultural drought and hydrological drought is larger than that of meteorological drought,and the spatial distribution characteristics of agricultural drought and hydrological drought are similar.There is a low correlation between SPI and SSI?or SRI??Standardized Precipitation/Soil Moisture/Runoff index?in northwest China,indicating that there is a low correlation between short-term meteorological drought and agricultural/hydrological drought in arid and semi-arid regions in northwest China.In addition,the lag time between SSI and SRI in northwest China was longer than that in south China,indicating the slow transmission of meteorological drought and hydrological drought in arid and semi-arid regions.The rapid spread of meteorological drought in south China means that drought prevention and control should attach importance not only to meteorological drought,but also to agricultural drought and hydrological drought.?5?Through the comparative analysis of drought indicators at different time scales,it was found that the drought frequency and duration of SPI,SSI and SRI increased significantly with the increase of time scale.However,with the increase of time scale,the average drought grade of SPI,SSI and SRI gradually decreases,which indicates that when drought occurs,although long-term drought lasts for a long time and has a high frequency,its drought grade will also decrease accordingly.In terms of the cumulative frequency analysis of drought variables,the cumulative frequency of SPI was the highest under the same drought duration in any partition at any time scale.In addition,in terms of the relationship between drought duration and drought duration in each climate zone,the correlation between drought duration and drought duration in Inner Mongolia?namely,the determination coefficient R²?was lower than that in other climate zones,but all of them were larger than 0.8.