Spatial And Temporal Characteristics Of Soil Drought Change In China During The Past 60 Years

Soil moisture is an important variable of the climate system and provides information about surface hydro-climatic processes.Soil moisture directly or indirectly affects the exchange of heat,moisture,and materials between the surface and the atmosphere by changing the soil physical properties such as surface evaporation,albedo,and soil heat capacity as well as vegetation growth conditions;it also affects local and regional climates,thereby making it an important variable in measuring dry–wet changes.In this paper,we have studied the characteristics of changes in soil moisture and soil drought in China between 1961–2017,and quantitatively evaluated the contribution changes of precipitation and potential evapotranspiration in soil moisture in different periods.The main conclusions are as follows:(1)We calculated the Standardized Soil Moisture Index(SSI)based on the monthly soil moisture data of China(GLDAS-2-adj)which were corrected and merged by mathematical statistics method,and compared it with the Palmer Drought Severity Index(PDSI),modified PDSI(PDSI?CN),self-calibrating PDSI(sc PDSI),Standardized Precipitation Index(SPI),Standardized Precipitation Evapotranspiration Index(SPEI),Multivariate Standardized Drought Index(MSDI).The results showed that on the 12-month time scale,the interdecadal variations of 12-month SSI and other five drought indices in China and their sub-regions during1961-2017 were basically the same,but after 2010,but after 2010,the results of 12-month SSI were drier than those of other indices in the regional average of China,North China and Northwest China.The consistency of each drought indices in spring was better in the south than in the north,but it was opposite in summer and autumn.1-month SSI and 3-month SPI and SPEI were the closest to the real situation in the monitoring process of the development and change of regional seasonal drought.Therefore,1-month SSI can be used to monitor seasonal drought.(2)From 1961 to 2017,the annual soil moisture in China showed a significant overall decreasing trend.In spring,there was a slight increasing trend.There were significant decreasing trend in summer and autumn,and the range of soil moisture drying in autumn was smaller than that in summer.The frequency,duration and severity of soil drought at 12-month and 1-month timescales increased significantly in China from 1961 to 2017,and the rate of increase at 1-month timescale was faster than that at 12-month timescale.After 1998,drought events with longer duration and more serious increased significantly,and the severity of 1-month timescale drought events in Northeast China,North China and Western Northwest China was more serious than that in other regions.Since 1998,the different return levels of drought duration have been increasing in most areas of China,but the increase rate was not as obvious as the severity of drought.The joint occurrence probability of drought with both variables exceeding the 50-yr and 100-yr return levels exhibited relative increases of 3%and 2%,across China,respectively.(3)Although the Northwest has been warmer and wetter in recent decades,but in the past60 years,the frequency of concurrent droughts and hot extremes exhibited an increasing trend over most parts of Northwest China,especially in the eastern part of Xinjiang,the northern part of Golmud,Qinghai Province,the Hexi Corridor,Ningxia to Longnan,and the northern part of Shaanxi Province,and the increasing trend were more significant.The frequency of concurrent droughts and hot extremes was the highest in mid-summer(July to August).In the past 20 years,the average concurrent droughts and hot extremes the whole region has grew 2–5 times.(4)During 1961-2017,the change of annual water surplus and deficit in China were not completely consistent with soil moisture.In recent 20 years,the decreasing rate of soil moisture in North China is obviously faster than before,and the influence of potential evapotranspiration on soil moisture is gradually increasing.The percent contribution of annual potential evapotranspiration to soil moisture increased by 26% during 1998–2017 relative to that in1961–1997,and the percent contribution of summer potential evapotranspiration even increased by 45%.The response of soil drought anomaly to each climatic factor was not completely synchronous.The soil moisture started to respond to the increase of sunshine hours first,and the increase of sunshine hours leaded to the abnormal rise of air temperature.Then the relative humidity decreases rapidly,and the pressure difference of saturated water vapor begins to increase rapidly,and the drought was further aggravated.