Comparison of four soil moisture sensor types under field conditions in Switzerland Heidi Mittelbach ⇑ , Irene Lehner, Sonia I. Seneviratne ⇑ Institute for Atmospheric and Climate Science, ETH Zurich, Universitätstrasse 16, CHN N12.2, 8092 Zurich, Switzerland article info Article history: Received 9 September 2011 Received in revised form 27 January 2012 Accepted 30 January 2012 Available online 8 February 2012 This manuscript was handled by Konstantine P. Georgakakos, Editor-in-Chief, with the assistance of V. Lakshmi, Associate Editor Keywords: Soil moisture Sensor comparison Low-cost Evapotranspiration Measurement networks summary Many environmental and hydrological applications require knowledge about soil moisture. Its measure- ment accuracy is known to depend on the sensor technique, which is sensitive to soil characteristics such as texture, temperature, bulk density and salinity. However, the calibration functions provided by instru- ment manufacturers are generally developed under laboratory conditions, and their accuracy for field applications is rarely investigated, in particular over long time periods and in comparison with other sen- sors types. In this paper, four side-by-side profile soil moisture measurements down to 110 cm using three low-cost sensors and one high-accuracy and high-cost time domain reflectometry (TDR) sensor are compared over a 2-year period at a clay loam site in Switzerland. The low-cost instruments include the (1) 10HS (Decagon Devices, United States), (2) CS616 (Campbell Scientific, United States), and (3) SISOMOP (SMG University of Karlsruhe, Germany) sensors, which are evaluated against the (4) TDR- based TRIME-IT/-EZ (IMKO GmbH, Germany) sensors. For the comparison, the calibration functions pro- vided by the manufacturers are applied for each sensor type. The sensors are evaluated based on daily data regarding their representation of the volumetric water content (VWC) and its anomalies, as well as the respective temperature dependency of the measurements. Furthermore, for each sensor type the actual evapotranspiration is estimated using the soil water balance approach and compared with mea- surements from a weighing lysimeter. It is shown that the root mean square difference (RMSD) of VWC for the low-cost sensors compared to the TDR measurements are up to 0.3 m 3 /m 3 , with highest val- ues in near-surface layers. However, the RMSD for the VWC anomalies are lower compared to those for absolute values. We conclude that under the studied conditions none of the evaluated low-cost sensors has a level of performance consistent with the respective manufacturer specifications. Hence the deriva- tion of site-specific calibration functions is vital for the interpretation of measurements with low-cost soil moisture sensors. Furthermore, some weaknesses of the tested low-cost sensors such as the lack of sen- sitivity in certain soil moisture regimes or spurious dependency on soil temperature, imply intrinsic issues with the measurements derived with this type of instruments. This is particularly critical for a number of environmental and hydrological applications, including the assessment of remote sensing measurements. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Soil moisture is an essential environmental, hydrological and climate variable. In particular, it strongly affects the land surface fluxes of the water and energy balances with consequent impacts on temperature, evapotranspiration, planetary boundary layer sta- bility or runoff generation (see Seneviratne et al., 2010 for a re- view). In recent years, soil moisture–atmosphere interactions received increasing attention in climate research. In particular, both numerical and observational studies highlighted their poten- tial role for climate variability and extremes, including heat waves (Seneviratne et al., 2006; Diffenbaugh et al., 2007; Fischer et al., 2007; Vautard et al., 2007; Lorenz et al., 2010; Teuling et al., 2010; Hirschi et al., 2011; Jaeger and Seneviratne, 2011). Moreover, potential (positive or negative) feedbacks with precipitation have also been suggested in various regions (Koster et al., 2004; Hohe- negger et al., 2009; Seneviratne et al., 2010; van den Hurk and van Meijgaard, 2010; Findell et al., 2011; Taylor et al., 2011). Global effects of soil moisture variability on climate have been proposed as well (Jung et al., 2010), and recent studies have further highlighted the potential role of soil moisture for sub-seasonal to seasonal fore- casting (e.g. Koster et al., 2010a,b; Weisheimer et al., 2011). Ground truth data for soil moisture are essential to analyze the processes underlying land–atmosphere interactions and to evaluate their role in land surface and climate models (e.g. Vinnikov et al., 1996; Dirmeyer et al., 2006). Thereby, information about root-zone 0022-1694/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2012.01.041 ⇑ Corresponding authors. Addresses: Universitätsstrasse 16, CHN N12.3, 8092 Zurich, Switzerland. Tel.: +41 44 632 80 77; fax: +41 44 633 10 58 (H. Mittelbach), Universitätsstrasse 16, CHN N11, 8092 Zurich, Switzerland. Tel.: +41 44 632 80 76; fax: +41 44 633 10 58 (S.I. Seneviratne). E-mail addresses: heidi.mittelbach@env.ethz.ch (H. Mittelbach), sonia.senevir- atne@env.ethz.ch (S.I. Seneviratne). Journal of Hydrology 430–431 (2012) 39–49 Contents lists available at SciVerse ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol