Spatio-temporal characteristics and trend analysis of soil moisture memory at multiple time scales in China

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.