Evaluation of relative soil moisture from CMA Land Data Assimilation System at different spatiotemporal scales in China

Based on daily relative soil moisture from the CMA Land Data Assimilation System (CLDAS) and hourly relative soil moisture from automatic soil moisture stations during 2020−2021, CLDAS relative soil moisture was evaluated in terms of accuracy and suitability by using multiple statistic indices at temporal scales of day and month as well as spatial scales of station and region. The results showed consistent daily variation in CLDAS relative soil moisture and observed relative soil moisture. CLDAS relative soil moisture at depths of 0−10 cm and 0−20 cm was close to the observed relative soil moisture, whereas CLDAS relative soil moisture at a depth of 0−50 cm was smaller than the observed relative soil moisture. The correlation coefficients between CLDAS relative soil moisture and observed relative soil moisture at the three depths were generally greater than 0.6, between which the root mean square errors (RMSE) were smaller than 30%. At the regional scale, the correlation coefficient between CLDAS relative soil moisture and observed relative soil moisture at a depth of 0−10 cm was 0.78−0.95, with the largest value in South China. The RMSE between CLDAS relative soil moisture and observed relative soil moisture at a depth of 0−10 cm was 5.70%−17.26%, with the smallest value in East China. The bias between CLDAS relative soil moisture and observed relative soil moisture at a depth of 0−10 cm was −6.63%−15.80%, with the smallest absolute value in Central China. At a depth of 0−20 cm, the correlation coefficient between CLDAS relative soil moisture and observed relative soil moisture was 0.78−0.95, with the largest value in Northeast China and Inner Mongolia. The RMSE between CLDAS relative soil moisture and observed relative soil moisture at a depth of 0−20 cm was 4.45%−14.03%, with the smallest value in East China. The bias between CLDAS relative soil moisture and observed relative soil moisture at a depth of 0−20 cm was −5.36%−12.56%, with the smallest absolute value in East China. At a depth of 0−50 cm, the correlation coefficient between CLDAS relative soil moisture and observed relative soil moisture was 0.68−0.97, with the largest value in Northeast China. The RMSE between CLDAS relative soil moisture and observed relative soil moisture at a depth of 0−50 cm was 4.00%−15.83%, with the smallest value in East China. The bias between CLDAS relative soil moisture and observed relative soil moisture at a depth of 0−50 cm was −9.83%−9.62%, with the smallest absolute value in Northeast China. The correlation coefficients between monthly CLDAS relative soil moisture and observed relative soil moisture were generally high, with larger values between June and October. The RMSE between monthly CLDAS relative soil moisture and observed relative soil moisture was smaller than 15%, with the smallest value in East China. Overall, CLDAS relative soil moisture performed well in East China, Central China, and Northeast China at the spatial scale and during June−October at the temporal scale.