Hydrological links in Southeastern South America: soil moisture memory and coupling within a hot spot

Abstract

Southeastern South America has been identified as a hot spot of soil moisture and evapotranspiration coupling efficiency during austral summer in a previous study. Here, hydrological processes such as coupling and memory of soil moisture, evapotranspiration and precipitation and the links between these variables are discussed on the daily time scale over this region. The correlations between surface variables, rainfall persistence and soil moisture memory are discussed over three subregions selected on basis of their coupling efficiency and mean daily intensity of precipitation. The relationship between surface climate and land cover is qualitatively assessed. The memory, or statistical persistence, is longer and has a more robust spatial pattern for the root zone than for the top soil moisture. Where the coupling efficiency between soil moisture and evapotranspiration is high, the evapotranspiration is regulated by soil moisture conditions independently on the intensity of precipitation, whereas in a region with low coupling efficiency and high intensity, the evapotranspiration is regulated by the atmosphere. The coupling efficiency is in general related to the memory of the root-zone layer, since the soil state is modified when the soil moisture and the atmosphere interact, resulting in an anticorrelation between these metrics. The persistence of rainfall is another factor that modulates the memory. Nevertheless, there are some areas around the La Plata River where both the coupling efficiency and the memory are relatively high, such as Uruguay and the northeast of Argentina, where an improvement of soil moisture initial conditions could improve predictability of surface variables on a monthly timescale.