QLB-NET: A Dense Soil Moisture and Freeze/Thaw Monitoring Network in the Qinghai Lake Basin on the Qinghai-Tibetan Plateau

Abstract Soil moisture (SM) and soil freeze/thaw (FT) are two relatively active surface parameters that are significant to the sustainable development of the water-land-air-plant-human nexus. Over time, regional or global SM and FT datasets with different spatial resolutions have been developed. In response to the requirements of multiscale product validation and multisource uncertainty tracking, a soil moisture and soil temperature (ST) monitoring network in the Qinghai Lake Basin (QLB-NET) was established in September 2019. The QLB-NET is characterized by densely distributed in situ sites (82 sites) measuring SM and ST at 5-, 10- and 30-cm depths, with 60 sites in a large-scale network covering an area of 36 km×40 km and 22 sites evenly distributed across two small-scale 1 km×1 km networks. Quantitative analyses of the in situ measurements show that the QLB-NET can provide stable and reliable ground truth for SM and FT over coarse grid scales, e.g., 36 km×36 km, 25 km×25 km, and 0.25°×0.25°. When statistics are correspondingly performed over 50 out of 54, 25 out of 29, and 25 out of 28 sites, the results are described as follows: 1) the STD of the mean SM varies between 0.0127 and 0.0196 m3/m3, with the corresponding difference between the upper and lower quartiles being less than 0.02 m3/m3; 2) the ground freeze/thaw state can be correctly identified with high probabilities ranging from 85.3% to 100% on two freeze/thaw transitional dates. The QLB-NET observed datasets are distributed online and will be continuously updated through cooperation with the National Tibetan Plateau Data Center (http://data.tpdc.ac.cn), facilitating product validation and uncertainty tracking, spatiotemporal analysis of SM change and FT transition, optimization of the SM and FT retrieving algorithms and scaling methods and development of the mountainous microwave radiative transfer model.