{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,4,30]],"date-time":"2026-04-30T00:50:15Z","timestamp":1777510215587,"version":"3.51.4"},"reference-count":51,"publisher":"MDPI AG","issue":"12","license":[{"start":{"date-parts":[[2019,6,17]],"date-time":"2019-06-17T00:00:00Z","timestamp":1560729600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100001809","name":"National Natural Science Foundation of China","doi-asserted-by":"publisher","award":["41801232"],"award-info":[{"award-number":["41801232"]}],"id":[{"id":"10.13039\/501100001809","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100012226","name":"Fundamental Research Funds for the Central Universities","doi-asserted-by":"publisher","award":["2018QNA6011"],"award-info":[{"award-number":["2018QNA6011"]}],"id":[{"id":"10.13039\/501100012226","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>The European Space Agency (ESA) Climate Change Initiative (CCI) project combines multi-sensors at different microwave frequencies to derive three harmonized soil moisture products using active, passive and combined approaches. These long-term soil moisture products assist in understanding the global water and carbon cycles. However, extensive validations are a prerequisite before applying the retrieved soil moisture into climatic or hydrological models. To fulfill this objective, we assess the performances of three CCI soil moisture products (active, passive and combined) with respect to in-situ soil moisture networks located in China, Spain and Canada. In order to compensate the scale differences between ground stations and the CCI product\u2019s coarse resolution, we adopted two upscaling approaches of Inverse Distance Weighting (IDW) interpolation and simple Arithmetic Mean (AM). The temporal agreements between the satellite retrieved and ground-measured soil moisture were quantified using the unbiased root mean square error (ubRMSE), RMSE, correlation coefficients (R) and bias. Furthermore, the temporal variability of the CCI soil moisture is interpreted and verified with respect to the Tropical Rainfall Measuring Mission (TRMM) precipitation observations. The results show that the temporal variations of CCI soil moisture agreed with the in-situ ground measurements and the precipitation observations over the China and Spain test sites. In contrast, a significant overestimation was observed over the Canada test sites, which may be due to the strong heterogeneity in soil and vegetation characteristics in accordance with the reported poor performance of soil moisture retrieval there. However, despite a retrieval bias, the relatively temporal variation of the CCI soil moisture also followed the ground measurements. For all the three test sites, the soil moisture retrieved from the combined approach outperformed the active-only and passive-only methods, with ubRMSE of 0.034, 0.050, and 0.050\u20130.054 m3\/m3 over the test sites in China, Spain and Canada, respectively. Thus, the CCI combined soil moisture product is suggested to drive the climatic and hydrological studies.<\/jats:p>","DOI":"10.3390\/s19122718","type":"journal-article","created":{"date-parts":[[2019,6,17]],"date-time":"2019-06-17T11:24:45Z","timestamp":1560770685000},"page":"2718","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":23,"title":["Evaluation of ESA Active, Passive and Combined Soil Moisture Products Using Upscaled Ground Measurements"],"prefix":"10.3390","volume":"19","author":[{"given":"Luyao","family":"Zhu","sequence":"first","affiliation":[{"name":"Institute of Agricultural Remote Sensing and Information Technology Application, College of Environmental and Resource Sciences, Zhejiang University, Hangzhou 310058, China"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Hongquan","family":"Wang","sequence":"additional","affiliation":[{"name":"Institute of Agricultural Remote Sensing and Information Technology Application, College of Environmental and Resource Sciences, Zhejiang University, Hangzhou 310058, China"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Cheng","family":"Tong","sequence":"additional","affiliation":[{"name":"Institute of Agricultural Remote Sensing and Information Technology Application, College of Environmental and Resource Sciences, Zhejiang University, Hangzhou 310058, China"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Wenbin","family":"Liu","sequence":"additional","affiliation":[{"name":"Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Benxu","family":"Du","sequence":"additional","affiliation":[{"name":"Natural Resources Service Center, Dalian 116021, China"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2019,6,17]]},"reference":[{"key":"ref_1","unstructured":"Srivastava, P., Petropoulos, G., and Kerr, Y.H. (2016). Satellite Soil Moisture Retrieval: Techniques and Applications, Elsevier."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"666","DOI":"10.1109\/JPROC.2010.2043032","article-title":"The SMOS mission: New tool for monitoring key elements of the global water cycle","volume":"98","author":"Kerr","year":"2010","journal-title":"Proc. IEEE"},{"key":"ref_3","first-page":"244","article-title":"Optimization of a coupled hydrology\u2013crop growth model through the assimilation of observed soil moisture and leaf area index values using an ensemble kalman filter","volume":"430","author":"Pauwels","year":"2007","journal-title":"Water Resour. Res."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"D17","DOI":"10.1029\/2006JD008288","article-title":"Characteristics of global and regional drought, 1950\u20132000: Analysis of soil moisture data from off-line simulation of the terrestrial hydrologic cycle","volume":"112","author":"Sheffield","year":"2007","journal-title":"J. Geophys. Res. Atmos."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"139","DOI":"10.1002\/hyp.3360070205","article-title":"Measuring surface soil moisture using passive microwave remote sensing","volume":"7","author":"Jackson","year":"1993","journal-title":"Hydrol. Process."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"125","DOI":"10.1016\/j.earscirev.2010.02.004","article-title":"Investigating soil moisture\u2013climate interactions in a changing climate: A review","volume":"99","author":"Seneviratne","year":"2010","journal-title":"Earth-Sci. Rev."},{"key":"ref_7","unstructured":"Ulaby, F.T., Moore, R.K., and Fung, A.K. (1982). Microwave Remote Sensing: Active and Passive, Vol. II-Radar Remote Sensing and Surface Scattering and Emission Theory, Addison Wesley Publishing Company, World Science Division."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"1729","DOI":"10.1109\/36.942551","article-title":"Soil moisture retrieval from space: The Soil Moisture and Ocean Salinity (SMOS) mission","volume":"39","author":"Kerr","year":"2001","journal-title":"IEEE Trans. Geosci. Remote Sens."},{"key":"ref_9","doi-asserted-by":"crossref","unstructured":"Entekhabi, D., Njoku, E., Neill, P.O., Spencer, M., Jackson, T., Entin, J., Im, E., and Kellogg, K. (2008, January 7\u201311). The Soil Moisture Active\/Passive Mission (SMAP). Proceedings of the IEEE International Geoscience and Remote Sensing Symposium, Boston, MA, USA.","DOI":"10.1109\/IGARSS.2008.4779267"},{"key":"ref_10","unstructured":"Entekhabi, D., Yueh, S., Neill, P.E.O., Kellogg, K.H., Allen, A., Bindlish, R., Brown, M., Chan, S., Colliander, A., and Crow, W.T. (2014). SMAP Handbook Soil Moisture Active Passive, Jet Propulsion Laboratory, California Institute of Technology."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"215","DOI":"10.1016\/j.rse.2017.01.021","article-title":"Validation of SMAP surface soil moisture products with core validation sites","volume":"191","author":"Colliander","year":"2017","journal-title":"Remote Sens. Environ."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"9","DOI":"10.1016\/j.rse.2011.05.028","article-title":"GMES Sentinel-1 mission","volume":"120","author":"Torres","year":"2012","journal-title":"Remote Sens. Environ."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"6145","DOI":"10.1002\/2017GL073904","article-title":"Joint Sentinel-1 and SMAP data assimilation to improve soil moisture estimates","volume":"44","author":"Lievens","year":"2017","journal-title":"Geophys. Res. Lett."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"191","DOI":"10.1016\/S0034-4257(99)00036-X","article-title":"A method for estimating soil moisture from ERS scatterometer and soil data","volume":"70","author":"Wagner","year":"1999","journal-title":"Remote Sens. Environ."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"20","DOI":"10.1029\/2007GL031088","article-title":"Initial soil moisture retrievals from the METOP-A advanced scatterometer (ASCAT)","volume":"34","author":"Bartalis","year":"2007","journal-title":"Geophys. Res. Lett."},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"1999","DOI":"10.1109\/TGRS.2008.2011617","article-title":"An improved soil moisture retrieval algorithm for ERS and METOP scatterometer observations","volume":"47","author":"Naeimi","year":"2009","journal-title":"IEEE Trans. Geosci. Remote Sens."},{"key":"ref_17","doi-asserted-by":"crossref","unstructured":"Gloersen, P. (1981). Summary of the Status of the Nimbus-7 SMMR, Springer.","DOI":"10.1007\/978-1-4613-3315-9_77"},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"1655","DOI":"10.1109\/36.942543","article-title":"A multifrequency algorithm for the retrieval of soil moisture on a large scale using microwave data from SMMR and SSM\/I satellites","volume":"39","author":"Paloscia","year":"2001","journal-title":"IEEE Trans. Geosci. Remote Sens."},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"507","DOI":"10.1016\/S0034-4257(03)00052-X","article-title":"Soil moisture estimates from TRMM microwave imager observations over the southern United States","volume":"85","author":"Bindlish","year":"2003","journal-title":"Remote Sens. Environ."},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"184","DOI":"10.1109\/TGRS.2002.808331","article-title":"The Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E), NASDA\u2019s contribution to the EOS for global energy and water cycle studies","volume":"41","author":"Kawanishi","year":"2003","journal-title":"IEEE Trans. Geosci. Remote Sens."},{"key":"ref_21","unstructured":"Gaiser, P.W., Twarog, E.M., Li, L., St Germain, K.M., Poe, G.A., Purdy, W., Jelenak, Z., Chang, P.S., and Connor, L. (2004, January 20\u201324). The WindSat space borne polarimetric microwave radiometer: Sensor description and mission overview. Proceedings of the IEEE International Geoscience & Remote Sensing Symposium, Anchorage, AK, USA."},{"key":"ref_22","unstructured":"Imaoka, K., Maeda, T., Kachi, M., Kasahara, M., Ito, N., and Nakagawa, K. (2012). Status of AMSR2 instrument on GCOM-W1. Proceedings Volume 8528, Earth Observing Missions and Sensors: Development, Implementation, and Characterization II, Proceedings of the SPIE Asia-Pacific Remote Sensing, Kyoto, Japan, 29 October\u20131 November 2012, International Society for Optics and Photonics."},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"315","DOI":"10.5194\/isprsannals-I-7-315-2012","article-title":"Fusion of active and passive microwave observations to create an Essential Climate Variable data record for soil moisture","volume":"I-7","author":"Wagner","year":"2012","journal-title":"ISPRS Ann. Photogramm. Remote Sens. Spat. Inf. Sci."},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"1675","DOI":"10.5194\/hess-15-1675-2011","article-title":"The International Soil Moisture Network: A data hosting facility for global in situ soil moisture measurements","volume":"15","author":"Dorigo","year":"2011","journal-title":"Hydrol. Earth Syst. Sci."},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"185","DOI":"10.1016\/j.rse.2017.07.001","article-title":"ESA CCI soil moisture for improved earth system understanding: State-of-the art and future directions","volume":"203","author":"Dorigo","year":"2017","journal-title":"Remote Sens. Environ."},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"280","DOI":"10.1016\/j.rse.2012.03.014","article-title":"Trend-preserving blending of passive and active microwave soil moisture retrievals","volume":"123","author":"Liu","year":"2012","journal-title":"Remote Sens. Environ."},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"425","DOI":"10.5194\/hess-15-425-2011","article-title":"Developing an improved soil moisture dataset by blending passive and active microwave satellite-based retrievals","volume":"15","author":"Liu","year":"2011","journal-title":"Hydrol. Earth Syst. Sci."},{"key":"ref_28","first-page":"469","article-title":"CCI soil moisture assessment with SMOS soil moisture and in situ data under different environmental conditions and spatial scales in Spain","volume":"225","author":"Pablos","year":"2018","journal-title":"Remote Sens. Environ."},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"15388","DOI":"10.3390\/rs71115388","article-title":"Quality assessment of the CCI ECV soil moisture product using ENVISAT ASAR wide swath data over Spain, Ireland and Finland","volume":"7","author":"Pratola","year":"2015","journal-title":"Remote Sens."},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"95","DOI":"10.5194\/gi-5-95-2016","article-title":"The Sodankyl\u00e4 in situ soil moisture observation network: An example application of ESA CCI soil moisture product evaluation","volume":"5","author":"Ikonen","year":"2016","journal-title":"Geosci. Instrum. Methods Data Syst."},{"key":"ref_31","first-page":"28","article-title":"Validation of the ESA CCI soil moisture product in China","volume":"48","author":"An","year":"2016","journal-title":"Int. J. Appl. Earth Obs. Geoinf."},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"15729","DOI":"10.3390\/rs71115729","article-title":"Evaluation of satellite and reanalysis soil moisture products over southwest China using ground-based measurements","volume":"7","author":"Peng","year":"2015","journal-title":"Remote Sens."},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"91","DOI":"10.1016\/j.rse.2015.03.008","article-title":"Evaluation of remotely sensed and reanalysis soil moisture products over the Tibetan Plateau using in-situ observations","volume":"163","author":"Zeng","year":"2015","journal-title":"Remote Sens. Environ."},{"key":"ref_34","first-page":"96","article-title":"Evaluating ESA CCI soil moisture in East Africa","volume":"48","author":"McNally","year":"2016","journal-title":"Int. J. Appl. Earth Obs. Geoinf."},{"key":"ref_35","doi-asserted-by":"crossref","first-page":"1530","DOI":"10.1109\/TGRS.2011.2168533","article-title":"Validation of Soil Moisture and Ocean Salinity (SMOS) soil moisture over watershed networks in the U.S","volume":"50","author":"Jackson","year":"2012","journal-title":"IEEE Trans. Geosci. Remote Sens."},{"key":"ref_36","doi-asserted-by":"crossref","first-page":"1562","DOI":"10.1109\/TGRS.2013.2252468","article-title":"Comparison between SMOS, VUA, ASCAT, and ECMWF soil moisture products over four watersheds in U.S.","volume":"52","author":"Leroux","year":"2014","journal-title":"IEEE Trans. Geosci. Remote Sens."},{"key":"ref_37","doi-asserted-by":"crossref","unstructured":"Crow, W.T., Berg, A.A., Cosh, M.H., Loew, A., Mohanty, B.P., Panciera, R., de Rosnay, P., Ryu, D., and Walker, J.P. (2012). Upscaling sparse ground-based soil moisture observations for the validation of coarse-resolution satellite soil moisture products. Rev. Geophys., 50.","DOI":"10.1029\/2011RG000372"},{"key":"ref_38","doi-asserted-by":"crossref","first-page":"170","DOI":"10.1016\/j.jhydrol.2015.01.061","article-title":"Inter-comparison of spatial upscaling methods for evaluation of satellite-based soil moisture","volume":"523","author":"Qin","year":"2015","journal-title":"J. Hydrol."},{"key":"ref_39","doi-asserted-by":"crossref","first-page":"1907","DOI":"10.1175\/BAMS-D-12-00203.1","article-title":"A multiscale soil moisture and freeze\u2013thaw monitoring network on The Third Pole","volume":"94","author":"Yang","year":"2013","journal-title":"Bull. Am. Meteorol. Soc."},{"key":"ref_40","doi-asserted-by":"crossref","unstructured":"Xiao, Z., Jiang, L., Zhu, Z., Wang, J., and Du, J. (2016). Spatially and temporally complete satellite soil moisture data based on a data assimilation method. Remote Sens., 8.","DOI":"10.3390\/rs8010049"},{"key":"ref_41","doi-asserted-by":"crossref","first-page":"1602","DOI":"10.1109\/TGRS.2012.2186971","article-title":"Validation of the SMOS L2 soil moisture data in the REMEDHUS network (Spain)","volume":"50","author":"Sanchez","year":"2012","journal-title":"IEEE Trans. Geosci. Remote Sens."},{"key":"ref_42","doi-asserted-by":"crossref","first-page":"1","DOI":"10.2136\/vzj2014.08.0114","article-title":"Calibration and evaluation of a frequency domain reflectometry sensor for real-time soil moisture monitoring","volume":"14","author":"Ojo","year":"2015","journal-title":"Vadose Zone J."},{"key":"ref_43","doi-asserted-by":"crossref","first-page":"1","DOI":"10.1029\/2007JF000769","article-title":"Multisensor historical climatology of satellite-derived global land surface moisture","volume":"113","author":"Owe","year":"2008","journal-title":"J. Geophys. Res. Earth Surf."},{"key":"ref_44","unstructured":"Gruber, A., and Scanlon, T. (2018). ESA Climate Change Initiative Phase II-Soil Moisture Algorithm Theoretical Baseline Document (ATBD), Earth Observation Data Centre for Water Resources Monitoring."},{"key":"ref_45","doi-asserted-by":"crossref","first-page":"38","DOI":"10.1175\/JHM560.1","article-title":"The TRMM Multisatellite Precipitation Analysis (TMPA): Quasi-global, multiyear, combined-sensor precipitation estimates at fine scales","volume":"8","author":"Huffman","year":"2007","journal-title":"J. Hydrometeorol."},{"key":"ref_46","doi-asserted-by":"crossref","first-page":"207","DOI":"10.1016\/j.rse.2013.12.002","article-title":"A new approach for validating satellite estimates of soil moisture using large-scale precipitation: Comparing AMSR-E products","volume":"142","author":"Tuttle","year":"2014","journal-title":"Remote Sens. Environ."},{"key":"ref_47","doi-asserted-by":"crossref","first-page":"1569","DOI":"10.2136\/sssaj2005.0117","article-title":"Soil water characteristic estimates by texture and organic matter for hydrologic solutions","volume":"70","author":"Saxton","year":"2006","journal-title":"Soil Sci. Soc. Am. J."},{"key":"ref_48","doi-asserted-by":"crossref","first-page":"358","DOI":"10.2136\/vzj2007.0143","article-title":"Soil moisture measurement for ecological and hydrological watershed-scale observatories: A review","volume":"7","author":"Robinson","year":"2008","journal-title":"Vadose Zone J."},{"key":"ref_49","doi-asserted-by":"crossref","first-page":"667","DOI":"10.1175\/JHM-D-17-0176.1","article-title":"Spatial distribution of diurnal rainfall variation in summer over China","volume":"19","author":"Zhu","year":"2018","journal-title":"J. Hydrometeorol."},{"key":"ref_50","doi-asserted-by":"crossref","first-page":"2784","DOI":"10.1109\/TGRS.2014.2364913","article-title":"The Soil Moisture Active Passive Validation Experiment 2012 (SMAPVEX12): Prelaunch calibration and validation of the SMAP soil moisture algorithms","volume":"53","author":"McNairn","year":"2015","journal-title":"IEEE Trans. Geosci. Remote Sens."},{"key":"ref_51","doi-asserted-by":"crossref","first-page":"1","DOI":"10.2136\/vzj2018.07.0132","article-title":"Assessing SMAP soil moisture scaling and retrieval in the Carman (Canada) study site","volume":"17","author":"Bhuiyan","year":"2018","journal-title":"Vadose Zone J."}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/19\/12\/2718\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T12:59:03Z","timestamp":1760187543000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/19\/12\/2718"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2019,6,17]]},"references-count":51,"journal-issue":{"issue":"12","published-online":{"date-parts":[[2019,6]]}},"alternative-id":["s19122718"],"URL":"https:\/\/doi.org\/10.3390\/s19122718","relation":{},"ISSN":["1424-8220"],"issn-type":[{"value":"1424-8220","type":"electronic"}],"subject":[],"published":{"date-parts":[[2019,6,17]]}}}