A study of the dependence between soil moisture and precipitation in different ecoregions of the Northern Hemisphere

Abstract. Soil moisture plays a critical role in the land–atmosphere coupling system. It is replenished by precipitation and transported back to the atmosphere through land surface evaporation and vegetation transpiration. Soil moisture is, therefore, influenced by both precipitation and evapotranspiration, with spatial heterogeneities and seasonal variations across different ecological zones. The relationship between soil moisture and precipitation was found to be nonlinear and negative in Northern Hemisphere ecosystems. However, the driving mechanisms of these negative correlations, especially how soil moisture is influenced by precipitation and evapotranspiration, still remain unclear. This study quantified the spatiotemporal distribution of the nonlinear dependence of soil moisture to precipitation, and identify the dominant factors in different ecoregions to explore the driving mechanisms and regional patterns. The joint distributions of precipitation and soil moisture were analyzed at monthly and annual scales, using soil moisture and precipitation data from ERA5-Land and Global Precipitation Climatology Project, respectively. The nonlinear negative dependences reached to 19.2 %, 0.7 %, and 2.3 % at monthly scale, while were 3.0 %, 4.0 %, and 8.6 % at annual scale, respectively, for the three soil layers. These negative dependences were shown to be most prominent in temperate grasslands, savannas, shrublands, deserts, xeric shrublands, and tundra regions, where driven by the land surface temperature and by the air temperature–gross primary production relationship at the monthly scale based on Ridge regression models and Bayesian models. Additionally, the negative dependence is also linked to freeze–thaw cycles, precipitation seasonality, and temperature fluctuations, which lead to asynchronous changes between soil moisture and precipitation at the seasonal scale. At the annual scale, the negative dependence was associated with long-term changes in precipitation and temperature that affect vegetation and surface properties, by altering soil water capacity. These findings enhance the understanding of land–atmosphere interactions providing a valuable basis for future research on drought, hydrometeorology, and ecological conservation.