Detecting a Declining Trend of Multidepth Soil Moisture Over the Mongolian Plateau From 1950 to 2020 Using ERA5-Land Reanalysis Datasets

Soil moisture (SM) is a pivotal element in surface hydrological processes, energy transfer, and mass exchange over the Mongolian Plateau (MP). However, spatial and temporal SM variability on the MP has remained unclear over previous decades due to global warming. Therefore, we conducted a spatio-temporal investigation of SM in the MP from 1950 to 2020 using ERA5-Land reanalysis datasets. Our research detected a declining trend of SM (from −0.003
$\text{m}^{3} / \text{m}^{3}$
per decade to −0.005
$\text{m}^{3} / \text{m}^{3}$
per decade) as well as an increasing trend of soil temperature (ST) (from 0.247 °C per decade to 0.267 °C per decade) along with an increasing soil depth on the MP during the past 70 years. Meanwhile, as the depth increased, the fluctuation degrees of SM and ST gradually declined, and the month for the maximum value emergence was delayed. Statistical analyses, including correlation and Granger causality analysis, suggest that precipitation is the dominant driver of SM dynamics in the MP over the warm season (ST
$ > 0~^{\circ }\text{C}$
). The proportion that precipitation being the cause of the SM variation was above 80% across different depths. Additionally, evaporation is a leading factor in triggering SM fluctuations. The percentage of evaporation being the cause of the SM variation was maintained above 60% among different soil layers over the warm season (ST
$ > 0~^{\circ }\text{C}$
). Meanwhile, multilayer SM, except for the 100–289 cm layer ones, expressed effective feedback to both precipitation (regional proportions varying between 22.64% and 40.28%) and evaporation (regional proportion varying between 36.76% and 64.72%).