Four decades of microwave satellite soil moisture observations: Part 2. Product validation and inter-satellite comparisons

• Published in: Advances in water resources 2017-11, Vol.109, p.236-252
• Main Authors: Karthikeyan, L., Pan, Ming, Wanders, Niko, Kumar, D. Nagesh, Wood, Eric F.
• Format: Article
• Language: English
• Subjects: Accuracy; Active microwave; Algorithms; Climate change; Climate science; Computer simulation; Dynamics; Evolution; Evolutionary algorithms; Hydrometeorology; Infiltration; Infiltration capacity; Interactions; ISMN; Land surface models; Microwave sensors; Missions; Moisture; Passive microwave; Products; Remote sensing; Retrieval; Satellite observation; Satellites; Scatterometers; Sensors; Series & special reports; Soil; Soil dynamics; Soil investigations; Soil moisture; Special Sensor Microwave Imager; Validation; VIC
• ISSN: 0309-1708; 1872-9657
• DOI: 10.1016/j.advwatres.2017.09.010

•Review of evolution of sensors and accuracy of microwave soil moisture retrievals.•Eight passive and two active soil moisture products are validated over the CONUS.•ISMN station data and VIC soil moisture simulations are considered as reference.•Coverage, spatio-temporal performance and inter satellite comparisons are discussed.•Observed significant growth in coverage and performance over past four decades. Soil moisture is widely recognized as an important land surface variable that provides a deeper knowledge of land-atmosphere interactions and climate change. Space-borne passive and active microwave sensors have become valuable and essential sources of soil moisture observations at global scales. Over the past four decades, several active and passive microwave sensors have been deployed, along with the recent launch of two fully dedicated missions (SMOS and SMAP). Signifying the four decades of microwave remote sensing of soil moisture, this Part 2 of the two-part review series aims to present an overview of how our knowledge in this field has improved in terms of the design of sensors and their accuracy for retrieving soil moisture. The first part discusses the developments made in active and passive microwave soil moisture retrieval algorithms. We assess the evolution of the products of various sensors over the last four decades, in terms of daily coverage, temporal performance, and spatial performance, by comparing the products of eight passive sensors (SMMR, SSM/I, TMI, AMSR-E, WindSAT, AMSR2, SMOS and SMAP), two active sensors (ERS-Scatterometer, MetOp-ASCAT), and one active/passive merged soil moisture product (ESA-CCI combined product) with the International Soil Moisture Network (ISMN) in-situ stations and the Variable Infiltration Capacity (VIC) land surface model simulations over the Contiguous United States (CONUS). In the process, the regional impacts of vegetation conditions on the spatial and temporal performance of soil moisture products are investigated. We also carried out inter-satellite comparisons to study the roles of sensor design and algorithms on the retrieval accuracy. We find that substantial improvements have been made over recent years in this field in terms of daily coverage, retrieval accuracy, and temporal dynamics. We conclude that the microwave soil moisture products have significantly evolved in the last four decades and will continue to make key contributions to the progress of hydro-meteorological and climate science