多时间尺度下中国区域土壤湿度记忆性时空特征及趋势分析

土壤湿度是大气陆面异常的最敏感指示要素，在土壤和植被作用下土壤湿度变化具有一定持续性，能够不同程度地保留前期土壤湿度的状态，对干旱、洪水等极端事件具有显著的影响和调节作用，开展土壤湿度记忆性研究对农业、水利部门具有重要意义。为探究多个时间尺度下土壤湿度记忆性时空特征及演变趋势，该研究基于欧洲中心再分析产品（ERA5-Land）中国区域1979—2018年的土壤湿度数据，结合滞后自相关分析，计算中国区域1979—2018年土壤湿度记忆性（soil moisture memory，SMM），探究不同时间尺度（季节、年、年际）及干湿条件下SMM的时空分布特征及演变趋势。结果表明：1）从多年平均状态来看，土壤湿度在滞后时间超过300 d后无显著相关；空间上北方地区SMM远高于南方地区，其中西南地区记忆时长不足20 d，华中、华东及华南地区小于40 d，西北和华北地区则超过80 d。2）季节上，大部分区域表现为冬季长、夏季短、秋春季次之的规律，其中华北地区季节性差异较大，冬季记忆时长超过70 d，夏季则低于20 d；同时在不同干湿条件下，土壤湿度在干燥期的记忆性明显强于湿润期，干燥期青藏高原、华北和东北地区记忆时长超过90 d，湿润期则小于60 d。3）从演变趋势来看，北方地区土壤湿度记忆时长呈下降趋势，其中华北和青藏高原地区较为显著，南方地区则呈增长趋势，2010s记忆时长比1980s记忆时长增加30 d以上；此外，大部分地区记忆时长的变化趋势与滞后30 d记忆性的变化趋势较为一致。研究可为气候预测和农业干旱监测提供参考。

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. Specifically, more than 30d increased in the 2010s, compared with the 1980s. In addition, the spatial distribution of the memory evolution trend was changed with the increase in lag time, indicating a lag time of 30d in most parts of mainland China.