Ground observation-based analysis of soil moisture spatiotemporal variability across a humid to semi-humid transitional zone in China

Little measurement-based soil moisture information is available for the humid to semi-humid transitional zones in China. Anhui Province is a representative area of these zones and spans several hydroclimatic and geomorphological zones. To overcome shortage of observational soil moisture information in this area, we collected and compiled a multi-layer daily soil moisture record from 2012 to 2016 across Anhui Province based on the observations of a soil moisture-observing network composed of 85 sites. We then analyzed the vertical and spatiotemporal variability of soil moisture across this region. Primary findings are: (a) soil moisture in this region has apparent vertical gradients and increases with soil depth; (b) substantial soil moisture spatial variability appears in the shallow soil layers (depth <= 40 cm) with the highest values in the middle part of this region, but soil moisture in the deep layers (depth > 40 cm) shows low spatial variability; (c) seasonality of soil moisture across this region is complicated and has fluctuations with July as the wettest month in most of the province; (d) inter-monthly fluctuation of soil moisture is larger in the upper soil layers than in the lower layers; and (e) antecedent accumulated rainfall is a major factor influencing soil moisture throughout this region with soil moisture in the upper layers more sensitive to immediate weather conditions and soil moisture in the lower layers showing lagged responses to the weather conditions. This study can not only reveal the spatiotemporal and vertical characteristics of soil moisture across this region, but also provide a useful reference dataset for the other soil moisture modeling and retrieval studies.