Evaluation of near-surface soil moisture data from an AAFC monitoring network in Manitoba, Canada: Implications for L-band satellite validation

•Near-surface soil moisture network data are assessed over 2years.•Significant differences are observed between 3.5–6.5 and 0–5.7cm measurement depths.•The network is suitable for coarse resolution L-band satellite soil moisture product validation. This paper examines near-surface soil moisture data (top ∼6cm) from an Agriculture and Agri-food Canada (AAFC) monitoring network in southern Manitoba, Canada, over the 2012 and 2013 growing seasons. This network is intended for use (in part) as a core validation site for the Soil Moisture Active Passive (SMAP) mission. The network currently consists of 9 stations sited within agricultural fields. Each station uses a triplicate configuration of horizontally installed Hydra Probe soil moisture sensors at near-surface (∼5cm) to root-zone soil depths, in addition to permanently installed vertical surface sensors (0–5.7cm). This domain was studied intensively over the 6.5week SMAPVEX12 ground campaign in June/July 2012, providing coincident soil moisture datasets to assess network up-scaling over the domain. Results demonstrate that statistical differences exist between the network-level data from the horizontal (θ3.5–6.5cm) and vertical (θ0–5.7cm) surface Hydra Probes over 2012–13. Bootstrap resampling shows that a mean difference of 0.018m3m−3 (at 99% confidence) is observed between θ3.5–6.5cm and θ0–5.7cm measurement depths (θ0–5.7cm sensors exhibit dry bias). Individual station analysis shows that the largest differences between θ3.5–6.5cm and θ0–5.7cm measurement depth occur at sites with higher clay content, whereas sensors located in sandy soils exhibit negligible differences. Up-scaling comparisons of the AAFC network to field-sampled data and a dense temporary network over SMAPVEX12 demonstrate that it is representative of the domain within 0.04m3m−3 RMSE. This suggests that the AAFC network is suitable for coarse resolution L-band microwave soil moisture validation. Comparison of Soil Moisture Ocean Salinity (SMOS) Level 2 soil moisture product data to the up-scaled AAFC network data over 2012–13 shows that superior temporal correlation is found with the θ0–5.7cm dataset. The difference in RMSEs between the SMOS comparisons to θ3.5–6.5cm and θ0–5.7cm network datasets is approximately 0.018m3m−3. These results suggest that differences of near-surface measurement depth and sensor configuration should be considered when conducting L-band satellite validation.