## 1 Introduction  
Volumetric soil water content (\(\theta_{v}\)) is a key variable in hydrological, climatological, and environmental studies and its monitoring is essential to investigate soil-water-plant relationships and nutrient fluxes. Under a changing climate, the need for accurate spatio-temporal soil moisture data in all types of soils and land-use is even increasing. Inaccurate soil moisture data has several potential implications: it may lead to the development of unreliable indicators for soil water availability or drought, it will introduce noise and systematic errors in water balance models at various spatial scales; and it could lead to incorrect nutrient budget calculations. Hence, there is a great need for affordable, highly accurate soil water measurement systems operating at a low cost for a long time. For decades, soil moisture sensors have been widely used for their ability to provide nondestructive continuous data [1] and their predictive performance has been evaluated in many studies [1; 2; 3; 4; 5; 6; 7; 8; 9; 10]. Sensors are often grouped in electromagnetic (EM) methods, such as time domain reflectometry (TDR) and frequency domain reflectometry (FDR), time domain transmission (TDT), capacitance and impedance sensors [6], and also neutron moisture meters (NMN) [11] and heat-pulse probes (thermal sensors) [12] have been applied for soil water content measurements at the point scale. The conventional reference method both in the field and in the lab is destructive gravimetric sampling, preferably in steel cylinders in order to determine gravimetric water content and bulk density on the same undisturbed soil sample. The Campbell CS616 sensor, we evaluated in this study, is an FDR type sensor widely recognized as a robust probe suitable for long-term monitoring,less accurate than TDR but more cost-effective [13]. FDR differs from conventional TDR in that it uses a specific voltage level of the signal reflected from the end of the waveguide to trigger the next pulse instead of analyzing the entire waveform as in TDR. Its installation, operation and performance are well described by [PERSON] and [PERSON] [13]. The rise time of the reflected pulse of FDR sensors changes with the soil bulk electrical conductivity, clay type (mineralogy and specific surface area) and content, soil temperature, and organic matter content [10; 11]. This limitation makes site-specific calibrations of such sensors necessary [14]. Therefore, efforts have been made for soil specific calibration of CS616 sensors: from coarse textured soils [1], clay-loam soils [5], silty clays [15] to soils with high-charge clays [3; 7] or a range of soil textures [10] including saline soils [14]. Various ways of sensor calibrations are performed. The most straightforward one-step comparison is relating the FDR output period as an indirect method with a direct measure of \(\theta_{\mathrm{V}}\) (standard gravimetric method). FDR response may also be compared with the response of other indirect methods assumed to be more accurate and precise, for instance NMN [3] or TDR [10]. Alternatively, a two-step calibration procedure can be used. In a first step, the relationship between sensor response and permittivity is determined for each sensor (i.e., a sensor-specific calibration). In a second step, site-specific relationships between permittivity and soil water content can be established using empirical or semi-empirical models [16] with a limited number of measurements on soil samples, preferably using the highly accurate TDR method (soil specific calibration) [6]. Lab calibration is often carried out on artificial or real volumetric soil samples in order to control most factors affecting the readings. Only few calibration studies are conducted directly in the field, since taking field samples is destructive, labor intensive, slow, not timely, and costly [11; 17].