Abstract
The study presents the validation of SMOS soil moisture satellite products for Romania. The validation was performed with in-situ measurements spatially distributed over the country and with in-situ measurements concentrated in on small area. For country level a number of 20 stations from the national meteorological observations network in Romania were selected. These stations have in-situ measurements for soil moisture in the first 5 cm of the soil surface. The stations are more or less distributed in one pixel of SMOS, but it has the advantage that covers almost all the country with a wide range of environmental conditions. Additionally 10 mobile soil moisture measurements stations were acquired and installed. These are spatially concentrated in one SMOS pixel in order to have a more detailed validation against the soil type, soil texture, land surface temperature and vegetation type inside one pixel. The results were compared and analyzed for each day, week, season, soil type, and soil texture and vegetation type. Minimum, maximum, mean and standard deviation were extracted and analyzed for each validation criteria and a hierarchy of those were performed. An upscaling method based on the relations between soil moisture, land surface temperature and vegetation indices was tested and implemented. The study was financed by the Romanian Space Agency within the framework of ASSIMO project <a href="http://assimo.meteoromania.ro"target="_blank">http://assimo.meteoromania.ro</a>.
Highlights
Soil moisture is a pivotal variable in land surface, atmosphere, hydrological and climate models
Spaceborne microwave remote sensing has proven to be a valuable tool to fulfil those needs by quantitatively measuring soil moisture on a global scale under a variety of conditions (Clement Albergel et al, 2012; Clément Albergel et al, 2011; Dorigo et al, 2015; Draper et al, 2013; Louvet et al, 2015; Panciera et al, 2009; Petropoulos, Ireland, & Barrett, 2015; Petropoulos, Ireland, & Srivastava, 2015; Pierdicca et al, 2015; Piles, Petropoulos, Sánchez, GonzálezZamora, & Ireland, 2016; Rossato, De Jeu, Alvalá, & Souza, 2011) It monitors soil moisture on a spatial and temporal scale, which cannot be achieved with in situ sampling
Other studies have focused on downscaling SMOS soil moistures products using either deterministic (Merlin, WALKER, Chehbouni, & Kerr, 2008) either by fitting a regression model between other vegetation products obtained from MODIS, SEVIRI etc and the SMOS soil moisture product (Piles et al, 2011, 2016; Yu, Di, & Yang, 2008) The approach presented in the current study was based on the evaluation of SMOS L2 and L3 soil moisture products over Romania using in-situ stations evenly distributed over the entire country and ins-situ station concentrated in one SMOS pixel The in-situ stations evenly distributed over Romania are part of the network for meteorological observations from The National
Summary
Soil moisture is a pivotal variable in land surface, atmosphere, hydrological and climate models. With the changing climate there is a need for long term spatially distributed soil moisture data. In situ soil moisture measurements play an important role in the calibration and validation of satellite-based soil moisture retrievals and landsurface models. In most cases soil moisture networks provide one single observation within a satellite footprint, which impedes the upscaling of in situ soil moisture to the footprint level. In Romania, soil moisture data is available from the agrometeorological network with a temporal resolution of 10 days. Validation requires hourly in-situ measurements from a dense network, whose nodes locations have to observe a number of criteria. It has been decided to complement observations at a 22 weather station locations with soil moisture and temperature measurements
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