VALIDATION OF SMOS L2 AND L3 SOIL MOISTURE PRODUCTS OVER THE DUERO BASIN AT DIFFERENT SPATIAL SCALES
A. González-Zamora
a
, N. Sánchez
a,
, A. Gumuzzio
a
, M. Piles
b
, E. Olmedo
c
, J. Martínez-Fernández
a a
Universidad de SalamancaCIALE, Duero 12, 37185 Villamayor, Spain - aglezzamora, nilda, angela.gumuzzio, jmfusal.es
b
Universitat Politècnica de Catalunya, UPCIEEC, and SMOS Barcelona Expert Center, Jordi Girona 08034, Barcelona, Spain – maria.pilestsc.upc.edu
c
Institut de Ciències del Mar, ICMCSIC and SMOS Barcelona Expert Center, Pg. Marítim 37-49, 08003 Barcelona, Spain - olmedoicm.csic.es
KEY WORDS:
Soil moisture, SMOS, Validation, Remote Sensing, L-Band
ABSTRACT:
An increasing number of permanent soil moisture measurement networks are nowadays providing the means for validating new remotely sensed soil moisture estimates such as those provided by the ESA’s Soil Moisture and Ocean Salinity SMOS mission.
Two types of in situ measurement networks can be found: small-scale 100-10000 km
2
, which provide multiple ground measurements within a single satellite footprint, and large-scale 10000 km
2
, which contain a single point observation per satellite footprint. This work presents the results of a comprehensive spatial and temporal validation of a long-term January, 2010 to June,
2014 dataset of SMOS-derived soil moisture estimates using two in situ networks within the Duero basin Spain. The first one is the Soil Moisture Measurement Stations Network of the University of Salamanca REMEDHUS, which has been extensively
applied for validation of soil moisture remote sensing observations, including SMOS. REMEDHUS can be considered within the small-scale network group 1300 km
2
. The other network started from an existing meteorological network from the Castilla y León region, where soil moisture probes were incorporated in 2012. This network can be considered within the large-scale group 65000
km
2
. Results from comparison to in situ show that the new reprocessed L2 product v5.51 improves the accuracy of former soil moisture retrievals, making them suitable for developing new L3 products. Validation based on comparisons between densesparse
networks showed that temporal patterns on soil moisture are well reproduced, whereas spatial patterns are difficult to depict given the different spatial representativeness of ground and satellite observations.
Corresponding author
1. INTRODUCTION
Recently, the World Meteorological Organization WMO, the Global Climate Observing System and the Committee on Earth
Observation Satellites included soil moisture as one of the Essential Climate Variables WMO, 2010, highlighting its
importance in meteorology and climatology, taking a central role in the land surface-atmosphere interface, and in the
interactions between soil, vegetation and climate forcing. It is also very important for agricultural applications, as it represents
the reservoir of the plant available water.
In the last few years, several satellites have been launched dedicated to globally measuring the Earth’s surface soil
moisture based on passive microwave techniques at the frequency band from 1 to 2 GHz L-band Jackson et al.,
2012. The Soil Moisture and Ocean Salinity SMOS satellite was launched by the European Space Agency ESA and is
providing the first global maps of soil moisture since January 2010 Kerr et al., 2010. The second dedicated mission, the
National Aeronautics and Space Administration NASA’s Soil Moisture Active Passive SMAP mission, was launched in
January 2015. It aims at retrieving soil moisture and the freezethaw state using the combination of active and passive
technologies Entekhabi et al., 2010. A third instrument, Aquarius, was launched in June 2011 on board the international
AquariusSAC-D mission by NASA and Argentinas space to primarily measure sea surface salinity. This mission is also
providing soil moisture products Lagerloef et al., 2008. Remotely sensed soil moisture measurements have the
advantage of covering large areas and identify synoptic events that are difficult to characterize using in situ measurements, but
their averaged and coarse-resolution nature renders validation of these products
difficult. A key issue is to bridge the gap between point scale measurements with footprint-scale
estimates. Crow et al. 2012 distinguishes between two types of networks, small-scale and large-scale. The small scale networks
between 100 km
2
and 10000 km
2
have the advantage of providing multiple measurements within a single footprint, and
therefore allows for the examination of the sub-footprint scale. The large scale networks 10000 km
2
, in turn, have the advantage of covering large areas and a larger range of land
cover soil types, but they typically lack of multiple measurements per footprint.
In this work, the last version 5.51 of SMOS L2 soil moisture product and a new SMOS BEC L3 v.001 soil moisture product
were tested over an area of more than 65000 km
2
located at the Duero Basin, Spain, for the period January 2010 to June 2014.
Two nested in situ networks were used: the Soil Moisture Measurement Stations Network of the University of Salamanca
REMEDHUS, small-scale and the recent Inforiego network large-scale. Spatio-temporal agreements of the L2 and L3 soil
moisture temporal series were analysed through a comparison with four different strategies: point-scale measurements at the
two networks, area-averaged, land-use-averaged and soil- texture-averaged data.
This contribution has been peer-reviewed. doi:10.5194isprsarchives-XL-7-W3-1183-2015
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2. IN SITU NETWORKS 2.1
