Characterizing precipitation and soil moisture drydowns in Finland using SMAP satellite data

Precipitation (P) and soil moisture (SM) are critical components of the global water, energy, and biogeochemical cycles, yet their patterns and interrelations in the Arctic are poorly understood. Due to the sparse in situ measurement network, satellites are the only way to observe P and SM in high-latitude regions. This study uses NASA's SMAP satellite to analyze the relationship between SM and P, assess the feasibility of estimating P from SM, and examine SM drydown patterns in Finland from April to September over 2018–2019. The analysis reveals a notable spatial and temporal variability in SM, with a weaker correlation between P and SM in spring due to snowmelt and a stronger relationship in summer and fall. Water bodies complicate the SM retrieval causing the SM retrievals to saturate. Using the SM2RAIN algorithm, we estimated P from SM data. The algorithm shows promising results, detecting the area of rainfall accurately in most cases but estimating the intensity of the rainfall is more challenging, particularly for light and very heavy rain. We analyzed SM drydown patterns by fitting an exponential model to each SM drydown period, from which we estimated the exponential decay time scale (τ) and the lower bound of SM (SMmin). τ does not show much spatial or temporal variability. The distribution of τ is positively skewed, with a mode of 1.6 d and a median of 4.0 d, consistent with other studies. The distribution of SMmin is also positively skewed, with a mode of 0.14 m3 m−3 and a median of 0.17 m3 m−3. SMmin exhibits another lower peak at 0.02 m3 m−3, the lower limit of SMAP SM retrievals, possibly causing an artifact in the results. SMmin shows spatial variability, with the lower bound being slightly higher near water bodies but also showing a more prominent peak at 0.02 m3 m−3. Grid cells with dense vegetation and low vegetation agree better with each other, indicating that water bodies particularly affect and complicate SM retrieval. The promising results suggest that the method could be applied across the entire Arctic region.