A soil moisture and temperature network for SMOS validation in Western Denmark

Abstract

Abstract. The Soil Moisture and Ocean Salinity Mission (SMOS) acquires surface soil moisture data of global coverage every three days. Product validation for a range of climate and environmental conditions across continents is a crucial step. For this purpose, a soil moisture and soil temperature sensor network was established in the Skjern River Catchment, Denmark. The objectives of this article are to describe a method to implement a network suited for SMOS validation, and to present sample data collected by the network to verify the approach. The design phase included (1) selection of a single SMOS pixel (44 × 44 km), which is representative of the land surface conditions of the catchment and with minimal impact from open water (2) arrangement of three network clusters along the precipitation gradient, and (3) distribution of the stations according to respective fractions of classes representing the prevailing environmental conditions. Overall, measured moisture and temperature patterns could be related to the respective land cover and soil conditions. Texture-dependency of the 0-5 cm soil moisture measurements was demonstrated. Regional differences in 0-5 cm soil moisture, temperature and precipitation between the north-east and south-west were found to be small. A first comparison between the 0-5 cm network averages and the SMOS soil moisture (level 2) product is in range with worldwide validation results, showing comparable trends for SMOS retrieved soil moisture (R 2 of 0.49) as well as initial soil moisture and temperature from ECMWF used in the retrieval algorithm (R 2 of 0.67 and 0.97, respectively). While retrieved/initial SMOS soil moisture indicate significant under-/overestimation of the network data (biases of −0.092/0.057 m 3 m −3 ), the initial temperature is in good agreement (bias of −0.2 • C). Based on these findings, the network performs according to expectations and proves to be well-suited for its purpose. The discrepancies between network and SMOS soil moisture will be subject of subsequent studies.

Soil moisture sensor calibration for organic soil surface layers

“…In the framework of HOBE, the Decagon 5TE sensor has been previously evaluated for near-surface sandy soil layers in the Skjern River catchment. Using Topp's equation, both Vasquez and Thomsen (2010) and Bircher et al (2012a) independently found the sensor to be accurate within ±0.02-0.03 cm 3 cm −3 under coniferous forest, heathland, as well as in agricultural fields.…”

“…Therefore, a joined effort aimed at calibrating the used soil moisture sensors, namely, the capacitance Decagon ECH2O 5TE sensor (Decagon 5TE) 1 and the impedance Delta-T ThetaProbe ML2X (ThetaProbe) 1 , for organic substrate. At both sites, the Decagon 5TE sensors are installed at permanent network stations (Bircher et al, 2012a;Ikonen et al, 2016) providing data to the International Soil Moisture Network (ISMN, Dorigo et al, 2011) -a global in situ soil moisture database to support validation and improvement of satellite observations and land surface models. Meanwhile, ThetaProbes are used for hand-held measurement campaigns (e.g., Bircher et al, 2012b), a current method for spatial variation studies of soil water content at different scales (e.g., Baggaley et al, 2009;Lopez-Vicente et al, 2009) and thus, frequently applied in the scope of satellite validation (e.g., Cosh et al, 2005;Kurum et al, 2012).…”

“…The naturally occurring soil type is a podsol of coarse sandy texture with pronounced organic surface layers. Soil moisture and soil temperature measurement stations, part of a spatially distributed network (Bircher et al, 2012a;Andreasen et al, 2016), are installed in the forest with Decagon 5TE sensors at 5, 25, and 55 cm depths of the mineral soil as well as in the overlying organic layer. The samples used for laboratory calibration were taken from organic surface layers in the vicinity of two forest network stations.…”

The Raam regional soil moisture monitoring network in the Netherlands

“…We deploy the Decagon 5TM capacitance sensor in the Raam network. The 5TM and other Decagon sensors that use the same technique and frequency have been widely used for in situ soil moisture networks and have proved to fulfil the performance requirements (Bircher et al, 2012;Bogena et al, 2010;Dente et al, 2009Dente et al, , 2011Kizito et al, 2008;Matula et al, 2016;Varble and Chávez, 2011;Vaz et al, 2013). 5TM sensors use an oscillator operating at 70 MHz to measure the capacitance of the soil, which is affected by the soil's relative dielectric permittivity.…”

“…The Natural Resources Conservation ServiceSoil Climate Analysis Network, consisting of 218 stations in agricultural areas across the United States of America, is operationally used for monitoring drought development, developing mitigation policies, predicting the long-term sustainability of cropping systems and watershed health, predicting regional shifts in irrigation water requirements, and predicting changes in runoff (U.S. Department of Agriculture, 2016). Examples of regional-scale networks in a temperate climate are the Little Washita (Cosh et al, 2006) and Little River (Bosch et al, 2007) networks in North America, REMEDHUS in Spain (Martínez-Fernández and Ceballos, 2005), Twente in the Netherlands (Dente et al, , 2012Van der Velde et al, 2014), HOBE's network in Denmark (Bircher et al, 2012), SMOSMANIA in France (Albergel et al, 2008;Calvet et al, 2007), TERENO in Germany (Zacharias et al, 2011), andKyeamba (Smith et al, 2012) in Australia. This paper presents the soil moisture and soil temperature profile monitoring network in the Raam region, in the southeast of the Netherlands, established in April 2016.…”