34 YEARS OF REMOTELY SENSED SOIL MOISTURE: WHAT CLIMATE SIGNALS DO WE (NOT) SEE? Wouter Dorigo 1 , Clement Albergel 2 , Alexander Loew 3 , Tobias Stacke 3 , Alexander Gruber 1 , Wolfgang Wagner 1 , Robert Parinussa 4 , Richard de Jeu 4 , Luca Brocca 5 , Bernhard Bauer-Marschallinger 1 ,Daniel Chung 1 , Christoph Paulik 1 1 Department of Geodesy and Geoinformation, Vienna University of Technology, Vienna, Austria 2 European Centre for Medium-Range Weather Forecasts (ECMWF), Reading, United Kingdom 3 Max-Planck-Institute for Meteorology, KlimaCampus, Hamburg, Germany 4 Earth and Climate Cluster, Faculty of Earth and Life Sciences, VU University Amsterdam, the Netherlands 5 Research Institute for Geo-Hydrological Protection, National Research Council, Perugia, Italy ABSTRACT Within the Climate Change Initiative of the European Space Agency a multi-satellite soil moisture product covering the period 1979-2010 was released. In this study we first assess its quality by comparing it with soil moisture from ground- based stations and several land surface model estimates. Secondly, the dynamics in the dataset were assessed using trend analysis and comparisons with ancillary data sets of precipitation and vegetation. Significant changes over time were found that largely correspond to changes in precipitation and vegetation vigorousness. However, the influence of changing observation density and data set quality over time need to be better understood for a more precise interpretation of the observed trends. Index Terms— Soil Moisture, Essential Climate Variable, Climate Data Record, Climate Change 1. INTRODUCTION Soil moisture is one of the main controllers of the exchange of water, energy, and carbon between the land surface and the atmosphere. Thus, dynamics in soil moisture are expected to strongly correlate with short-term and long-term variations of several hydrological and biological variables. During the last few years, various products from past and present satellite microwave missions have become publicly available. However, none of the single missions covers a period long enough to study climate dynamics in a robust manner. To bridge this gap, within the Climate Change Initiative of the European Space Agency a harmonized soil moisture product was released which combines six of the freely available products from active and passive satellite sensors for the period 1979 to present [1, 2]. The harmonized product, referred to as ECV_SM, constitutes the first purely observation-based soil moisture dataset that is long enough to study systematic changes over time [3]. The scope of this study is twofold: first we provide a thorough quality assessment of the multi-satellite dataset using ground-based observations and model data. This is necessary to see whether the signals encountered in the dataset are true signals or related to artifacts. Second, we study the soil moisture dynamics contained within the dataset and confront them with the dynamics encountered in other biogeophysical variables. 2. ECV_SM VALIDATION First, we assessed the quality of ECV_SM using over 600 in-situ data sets from almost 30 networks worldwide taken from the International Soil Moisture Network [4]. The performance was assessed using conventional error metrics like Spearman correlation and unbiased root mean square difference and the recently introduced triple collocation technique. In general a good agreement was found between the in-situ sites and ECV_SM although results over densely vegetated areas and areas of complex topography lagged behind those encountered for semi-arid areas [5, 6]. Results for the Pearson correlation statistics per network are found in coeff 1809 978-1-4799-1114-1/13/$31.00 ©2013 IEEE IGARSS 2013