Modelling soil moisture at SMOS scale by use of a SVAT model over the Valencia Anchor Station

Hydrol. Earth Syst. Sci., 14, 831–846, 2010 www.hydrol-earth-syst-sci.net/14/831/2010/ doi:10.5194/hess-14-831-2010 © Author(s) 2010. CC Attribution 3.0 License. Hydrology and Earth System Sciences Modelling soil moisture at SMOS scale by use of a SVAT model over the Valencia Anchor Station S. Juglea1 , Y. Kerr1 , A. Mialon1 , J.-P. Wigneron2 , E. Lopez-Baeza3 , A. Cano3 , A. Albitar1 , C. Millan-Scheiding3,4 , M. Carmen Antolin4 , and S. Delwart5 1 Centre d’Études Spatiales de la BIOsphère, UMR 5126 (CNRS, CNES, IRD, UPS), Toulouse, France 2 Ecologie fonctionnelle et PHYSique de l’Environnement (INRA/EPHYSE), Bordeaux, France 3 Universitat de Valencia, Departament de Termodinamica i Fisica de la Terra, Valencia, Spain 4 Center for Desertification Research (CIDE), Department of Territorial Planning, Valencia, Spain 5 European Space Research and Technology Centre (ESA/ESTEC), Noordwijk, The Netherlands Received: 21 December 2009 – Published in Hydrol. Earth Syst. Sci. Discuss.: 27 January 2010 Revised: 26 April 2010 – Accepted: 19 May 2010 – Published: 28 May 2010 Abstract. The main goal of the SMOS (Soil Moisture and Ocean Salinity) mission is to deliver global fields of surface soil moisture and sea surface salinity using L-band (1.4 GHz) radiometry. Within the context of the Science preparation for SMOS, the Valencia Anchor Station (VAS) experimental site, in Spain, was chosen to be one of the main test sites in Europe for Calibration/Validation (Cal/Val) activities. In this framework, the paper presents an approach consisting in accurately simulating a whole SMOS pixel by representing the spatial and temporal heterogeneity of the soil moisture fields over the wide VAS surface (50×50 km2 ). Ground and meteorological measurements over the area are used as the input of a Soil-Vegetation-Atmosphere-Transfer (SVAT) model, SURFEX (Externalized Surface) - module ISBA (Interactions between Soil-Biosphere-Atmosphere) to simulate the spatial and temporal distribution of surface soil moisture. The calibration as well as the validation of the ISBA model are performed using in situ soil moisture measurements. It is shown that a good consistency is reached when point comparisons between simulated and in situ soil moisture measurements are made. Actually, an important challenge in remote sensing approaches concerns product validation. In order to obtain an representative soil moisture mapping over the Valencia Anchor Station (50×50 km2 area), a spatialization method is Correspondence to: S. Juglea () applied. For verification, a comparison between the simulated spatialized soil moisture and remote sensing data from the Advanced Microwave Scanning Radiometer on Earth observing System (AMSR-E) and from the European Remote Sensing Satellites (ERS-SCAT) is performed. Despite the fact that AMSR-E surface soil moisture product is not reproducing accurately the absolute values, it provides trustworthy information on surface soil moisture temporal variability. However, during the vegetation growing season the signal is perturbed. By using the polarization ratio a better agreement is obtained. ERS-SCAT soil moisture products are also used to be compared with the simulated spatialized soil moisture. However, the lack of soil moisture data from the ERS-SCAT sensor over the area (45 observations for one year) prevented capturing the soil moisture variability. 1 Introduction Soil moisture is a key variable controlling the exchanges of water and energy at the surface/atmosphere interface (Betts et al., 1996; Entekhabi et al., 1996). It is highly variable both spatially and temporally as the result of the spatial heterogeneity of soil and vegetation properties, topography, land cover, rainfall and evapo-transpiration (Bosch et al., 2006; Entekhabi and Rodrigues-Iturbe, 1994). Observing the spatial distribution of soil moisture at the catchment scale is a Published by Copernicus Publications on behalf of the European Geosciences Union. 832 difficult task requiring intensive field instrumentation for accurate spatial and temporal representation. Nowadays, remote sensing technology has matured to the point that surface soil moisture can be estimated at global scale from space (Wigneron et al., 2003; Wagner et al., 2006). Microwave remote sensing at low frequencies have been found to produce the best results (Kerr, 2007; Wagner et al., 2006; Njoku and Entekhabi, 1996; Jones et al., 2004). In spite of the importance of soil moisture observations, the instruments that have been or are currently operating are not adapted to soil moisture monitoring. Nevertheless, there are a number of soil moisture products available from different sensors. The Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) (Njoku et al., 2003) on board the National Aeronautics and Space Administration’s (NASA) Aqua satellite and the scatterometers (SCAT) on board the European Remote Sensing Satellites 1 and 2 (ERS-1 and ERS-2) (Wagner et al., 1999a) provide soil moisture products. Both instruments use frequencies above 5 GHz. The SMOS (Soil Moisture and Ocean Salinity) (Kerr et al., 2001) mission was designed to measure soil moisture over continental surfaces as well as ocean salinity using a low microwave frequency – L-band (1.4 GHz). At this frequency, microwave observations are sensitive to soil moisture through the effects of moisture (water) on the dielectric constant and hence on the emissivity of the soil. The soil emission is integrated over a soil depth of a few centimeters, giving a more representative measurement of soil moisture conditions over this layer. Consequently, the SMOS mission benchmark is to provide global maps of soil moisture with an accuracy better than 0.04 m3 /m3 (Kerr et al., 2001). SMOS will achieve a maximum spatial resolution of 50 km over land (43 km on average over the field of view), providing multi-angular dual polarized (or fully polarized) brightness temperatures over the globe (Kerr et al., 2001). Launched in November 2009, SMOS will deliver, for the first time, global surface soil moisture measurement twice a day (06:00 a.m. and 06:00 p.m. LT – local time) in less than 3 days. L-band passive microwave radiometry is a very useful tool for soil moisture monitoring, allowing nearly all weather observation and surface vegetation cover information. Numerous field experiments using ground based and airborne Lband observations indicated a soil moisture retrieval capability of better than 0.04 m3 /m3 accuracy (Wang et al., 1990a; Schmugge et al., 1992; Jackson et al., 1995, 1999). In this context, the strategy adapted by ESA for its Soil Moisture and Ocean Salinity mission was to develop specific land product validation activities over well equipped monitoring sites. The Valencia Anchor Station (Lopez-Baeza et al., 2005a), in eastern Spain, and the Upper Danube Catchment (Delwart et al., 2007), in southern Germany, are chosen as the two main test sites in Eu (...truncated)