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Soil moisture is one of the most important indicators for the exchange of energy and water in the landing gas. The resulting surface evapotranspiration, water distribution, and soil heat capacity can be changed under the local, regional, and even global climate. Therefore, soil moisture can be measured to control the dry and wet state of the soil, as well as the climate. This study aims to explore the spatiotemporal distribution characteristics and evolution trends of soil moisture memory at multiple time scales. The soil moisture data was collected from the European central reanalysis products (ERA5-Land) in China from 1979-2018. The lag autocorrelation was then combined to calculate the soil moisture memory. Finally, a systematic investigation was implemented to determine the spatiotemporal distribution and evolution trend of soil moisture memory under different time scales (season, year, and inter-year) and dry/wet conditions. The results show that: 1) There was no autocorrelation coefficient in the 0.01 significance test after the lag time over 300 d from the perspective of the annual average state, indicating the average regional memory time of 300 d. Spatially, the memory time presented a spatial pattern of high in the north and low in the south. Among them, the memory time was less than 20 d in the southwest, less than 40d in the central, the East, and South China, whereas, there was over 80 d in the northwest, and North China, with a maximum of 240 d. 2) In season, the memory time of soil humidity in most areas was characterized by short summer, long winter, as well as the second autumn and spring. There were small seasonal differences in the Qinghai-Tibet Plateau, whereas, the large seasonal differences were found in North China, with the winter exceeding 70, 20 d in summer, and 30-50 d in spring and autumn. There was also the spatial distribution pattern of high in the north and low in the south under different dry and wet conditions. But the memory time was much longer in the dry period than the wet one. For example, the Qinghai-Tibet Plateau, North China, and northeast China were recorded as more than 90 d during the dry period, whereas, the wet period was less than 60 d. 3) The evolution trends of soil moisture memory and duration were relatively consistent in the northeast, central, South China, and the northern part of the Qinghai-Tibet Plateau. The memory time of soil moisture decreased in the northern, especially in the north, and Qinghai-Tibet Plateau. There was an increase in the southern regions. In addition, the spatial distribution of the memory evolution trend was changed with the increase in lag time, indicating a lag time of 30 d in most parts of Chinese mainland. [ABSTRACT FROM AUTHOR]

Flash droughts are extreme events characterized by a short duration, strong intensity, and rapid onset. Owing to their unusually rapid occurrence, the use of current drought monitoring tools to capture them is still challenging. In this study, using ERA-Interim soil moisture reanalysis products from the European Center for Medium-Range Weather Forecast, an approach that considers the rate of decrease in soil moisture content during the onset stage of flash droughts was proposed for the identification of the flash droughts that occurred during the 1979-2018 period. Further, the anomalies associated with meteorological variables during the drought onset stage of these events were analyzed, and the feasibility of using precedent meteorological driving forces to realize flash drought simulation based on a multivariate linear regression model was investigated. The results obtained showed that the rate of soil moisture intensification during the onset state of flash droughts showed significant spatial discrepancy. Specifically, during this stage, soil moisture content declined much faster in the south of the Yangtze River than in Northwestern China. Additionally, the mean, peak, and variation ranges of the anomalies of the different meteorological variables exhibited more significant spatial discrepancies during this stage. Particularly, the peak values showed the most significant variation, and were half the standard deviation higher than the slowly-evolving drought. Furthermore, by integrating the anomalies associated with multiple meteorological variables during the early and onset stages of the flash drought events, the regression model provided satisfying simulations of the soil moisture intensification rate, suggesting that it is possible to simulate flash droughts from a meteorological perspective. [ABSTRACT FROM AUTHOR]