Evaluation of multiple gridded root zone soil moisture products in Europe using in-situ measurements from ICOS

Abstract

Soil moisture is an essential climate variable, with applications in i.e. climate modelling and drought- and flood forecasting. The soil moisture in the rootzone (RZSM), 0-100cm depth, is especially important, compared to soil moisture at the surface (SSM), 0-5cm in depth. Large data gaps still exist for RZSM, as it cannot be directly observed through remote sensing. As such, modelling, and therefore the evaluation of models becomes increasingly important. In this study, the performance of seven gridded RZSM products – ERA5-Land, GLDAS-NOAH, GLEAM 4.2a MERRA2, NCEP-R2, SMAP-L4 and SMOS-L4 – were evaluated in Europe using ICOS in-situ measurements over the time-period 2022-2023.
Performance of each product was assessed using the performance metrics bias, root mean square error (RMSE), Pearson correlation coefficient and unbiased RMSE (ubRMSE).
Similarly, these metrics were used to assess the performance of products in different climates, land covers and seasons. The influence of the climatic variables temperature, precipitation and relative humidity was assessed using the Pearson correlation coefficient.
Products generally showed high correlations, with ERA5-Land, GLDAS-NOAH, MERRA-2, and SMAP-L4 showing correlations over 0.75. Additionally, SMAP-L4 and MERRA-2
achieve the lowest ubRMSE (below 0.03 m3 m−3). All products overestimated RZSM, resulting in a wet bias, apart from SMOS-L4, which showed a slight dry bias.
Seasonal trends were overall captured reasonably well, with the exception of SMOS-L4. MERRA-2 showed consistent performances across all seasons, whilst most products showed a
decreased performance in summer. Performance varied by land cover, with decreased performances of products in savannas and evergreen needleleaf forests, whilst the RZSM in croplands tends to be underestimated. Negative correlations with RZSM were shown for temperature, while precipitation showed a negligible influence. Overall SMAP-L4 performed well across all metrics, while NCEP-R2 and SMOS-L4 showed a comparatively poor performance.
While land cover, climatic variables and seasonality can explain some of the differences between products, there are other factors that influence soil moisture. As a result, this study highlights the need for more standardized in-situ measurements and further evaluations of gridded RZSM products.

Popular Abstract

Soil moisture, the amount of water held in the soil, is an important factor in the climate cycle as it affects temperature and precipitation. It is commonly used in flood and drought prediction and water management. Despite its importance, there is still a large gap in available data. Specifically soil moisture in the rootzone, considered to be 0-100cm in depth, cannot be measured with all methods.
Estimation of soil moisture is generally done through measurements from local stations, satellites, or modelling. Estimations from satellites are limited to the first five cm of the soil, whilst local stations only capture a small area. As these limitations do not apply to modelling, root zone soil moisture models (products) become more important. However, such products require evaluation for which satellite retrievals and local measurements are required.
In this study, seven different products were evaluated against the measurements from 25 stations from the Integrated Carbon Observation Portal (ICOS). For these products – ERA5-Land, GLDAS-NOAH, GLEAM 4.2a, MERRA-2, NCEP-R2, SMAP-L4 and SMOS-L4 – common statistics were used to evaluate their performance over the time-period 2022-2023.
Most products performed well for all metrics, with the exception of NCEP-R2 and SMOS-L4, who performed relatively poor compared to other products. Overall SMAP-L4 showed the best performance. Seasonal variations of root zone soil moisture were also captured well, with all products showing a similar trend. Once again, SMOS-L4 forms an exception, with a decrease in soil moisture in summer as opposed to the increase shown by the ICOS stations.
Changes in root zone soil moisture were largely driven by temperature, and to a lesser extent relative humidity. When temperatures increased, a decrease in root zone soil moisture occured. Surprisingly, precipitation did not have any impact on the root zone soil moisture. Products performed worse in cold climates compared to temperate climates, again indicating that temperature rather than precipitation is the main driver of root zone soil moisture in products. The performance of products was also affected by landcover, with different products showing varying performances for each landcover type. Generally, products performed worse in savannas and evergreen needleleaf forests.
This study provided an overview of which products could be useful in certain situations. E.g. when looking at soil moisture in a location with a certain landcover type or climate, some products might provide a better fit depending on their performance there. Aside from this, the study also shows a need for more standardised measurements, to improve performances in areas that show worse performances in this study.

• author: Otten, Bram
• supervisor: Zheng Duan
• organization: Dept of Physical Geography and Ecosystem Science
• course: NGEM01 20251
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: Physical Geography, Evaluation, ICOS, Root zone soil moisture, Europe
• publication/series: Student thesis series INES
• report number: 740
• language: English
• id: 9207325
• date added to LUP: 2025-07-31 12:59:14
• date last changed: 2025-07-31 12:59:14